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Synergistic and antagonistic interactions of fungicides against Pythium aphanidermatum on perennial ryegrass
386
I
U
Synergistic and antagonistic interactions of fungicides against Pythium aphanidermatumon perennial ryegrass Houston B. Couch* and Brian D. Smith Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
Abstract
Keywords
Mancozeb, fosetyl-Al, propamocarb, metalaxyl, ethazole and chtoroneb were tested singly at fractional and full label rates, and in two-way combinations at fractional label rates, for their potential to control the colonization of perennial ryegrass foliage by metalaxyt-sensitive and metalaxyl-resistant strains of Pythium aphanidermatum. The theoretical additive control level was computed for each combination of fractional rates, and this value used as the point of reference for determining if the actual degree of control given by the mixture in question was the product of either synergism or antagonism. The mancozeb + chloroneb mixture was antagonistic whereas the combinations of maneozeb+propamocarb, fosetyl-Ai+metalaxyl, and fosetyI-AI + propamocarb were synergistic in the control of the metalaxyl-sensitive strain. Mancozeb + metalaxyl was synergistic in the control of the metalaxyl-sensitive and the metalaxyl-resistant strains. The null hypothesis (additivity) was not rejected for the other combinations. The level of disease control provided by the synergistic combinations tested was equal to or greater than that provided by each of the components applied singly at their full label rates. Pythium blight; fungicide mixtures; fungicide resistance
Introduction The increasing use ofsingle-site fnngicides and the accompanying reports of resistance on the part of certain of the target organisms to these materials has heightened interest in the development of disease control programmes that utilize mixtures of fungicides with different modes of action but effective against the same pathogen. A question related to the use ofthese combinations is what the dosage level should be for each component of the mixture. Specifically, when used in a mixture, can the standard rate of each fungicide be reduced, or should it still be applied at the same level at which it would have been used as a single-component spray.'? When two or more pesticides that have different modes of action but are toxic to the same target organism are combined into a single treatment, the efficacy level of the mixture will either be synergistic, antagonistic, or additive. Synergism is the combined action o f two or more pesticides in which the control provided by their joint application is greater than the control predicted by an appropriate reference model. Antagonism is a level o f control provided by the joint application o f two or more pesticides which is less than that of the predicted control. Additivity is the level o f disease control provided by the co-operative action of two or more pesticides which is equal to the response predicted by taking into account the response o f each ft, ngicide applied singly (Hatzios and Penner, 1985). The *To whom correspondence should be addressed 0261-2194/91/05/0386-05 ~ 1991 Butterworth-Heinemann Ltd
additive control level of the pesticide mixtnre in question is commonly used as the reference fo r determining synergy or antagonism (Richer, 1987). Additive control is not the direct sum of the efficacy levels of the respective components of the mixture, it is an adjusted value. Several mathematical methods have been developed for determining the theoretical additive control levels of mixtures of pesticides (Wadley, 1945; Gowing, 1960; Colby, 1967; Flint, Cornelius and Barrett, 1988). Each of these calculations takes into consideration the fact that when the individual components of a reduced-rate mixture are acting independently, there will be a certain percentage o f escapes from each component that are not acted upon by the other component(s). This means that if the reduced rate mixture of fungicides is not synergistic, the control potential of the joint application will be less than that of the potential of the full label rate of the most efficacious component used alone. The early studies on the efficacy levels of mixtures of fungicides with different modes of action were hi vitro oniy (Scardavi, 1966). In recent years, however, the most widespread procedure for this type of research has been the use of pathogen-plant systems. Samoucha and Cohen (1984) reported synergism between metalaxyl and mancozeb when used as tank mixtures in the control o f downy mildew [incited by Pseudoperonospora cubensis (Berk. & Curt.) Ro.~tow.] on cucumbers (Cucmnis sativus L.). Samoucha and Gisi (1987a,b) found that oxadixyl-mancozeb, oxadixylcymoxanil and cymoxanil-mancozeb, applied either as
Fungicides vs
tank mixtures or sprayed sequentially as single-ingredient treatments, were synergistic in their control ofan oxadixylsensitive strain of Phytopllthora hfestans (Mont.) de Bary on tomato plants, and that a combination of cymoxanil and mancozeb was synergistic in the control of an oxadixyl-resistant form. Gisi, Binder and Rimbach (1985) tested 17 fungicides, singly and in mixtures, on the growth hi vitro of Phytophthora cactorlml (Leb. & Cohn) Schroet. and P. chlnamomi Rands, and on the activity of Phytophthora h#'stans in colonizing the foliage of tomatoes and potatoes, and Plasmopara v#icola in colonizing the leaves of grape vines. In the studies in vitro, mixtures of oxadixyi and mancozeb or cymoxanil and mancozeb showed high levels of synergism. In the tests involving P. hlfestans and P. viticola-host interactions, the highest synergism was observed in combinations of oxadixyl with either mancozeb, cymoxanil or fosetyl-Al. None of the fungicide combinations showed antagonistic interactions. In further tests with these same fungicide-pathogenplant combinations, Grabski and Gist (1987) quantified the degrees of synergism among the various fungicide mixtures. They found that although the activity of the oxadixyl-mancozeb mixture decreased significantly with increasing levels of resistance to oxadixyl, the mixture of oxadixyl, mancozeb and cymoxanil was almost equally effective on all strains. Pythium blight [incited by Pythhml aphanidernlatunl (Edson) Fitzp., P. myrioo'lum Drechs., and P. tdtimum Trow.] is one of the major diseases of turfgrasses throughout the continental United States and southern Canada. Characteristically, the disease has an explosive epidemic pattern. Under conditions of high air temperatures and extended periods of leaf wetness, it is capable of totally destroying large areas of turfgrass within 24-48 h of the first trace of visible symptoms (Couch, 1974; Saladini, 1981). During the period from 1982 to 1986, the systemic fungicides metalaxyl, propamocarb, and fosetyl-AI were registered for the control of Pythium blight in the United States. Field research had shown that these materials provide a greater degree of Pythium blight control than the currently used contact fungicides (Couch, Garber and Smith, 1980). However, soon after they were brought into commercial use, deficiencies were found in their disease control patterns. In 1984, metalaxyl failed to control Pythium blight in a bentgrass fairway on a golf course in eastern Pennsylvania. Certain isolates of P. aphanidermature recovered from diseased plants collected at this location were shown to be metalaxyl resistant (Sanders, 1984). Since this initial report, six confirmed instances of P. aphanidermatlml resistance to metalaxyl have been reported in the United States (Sanders, 1987). In addition to resistance of P. aphanidermatum to metalaxyl, it has also been observed that when the pressure from Pythium blight is extremely heavy, there can be a significant reduction in the longevity of control normally provided by the systemic fungicides. During July-August of 1987 and 1988, there were severe outbreaks of Pythium blight in several areas of the continental United States.
Pythiumaphanidermatum:H.
B. Couch and B. D. Smith
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Under these conditions, it was observed that the duration of effective disease control by the systemic fungicides was shortened from the expected 14-21 days to 2-4 days per spray (Kane, 1987). Sanders et al. (1985) reported that treatments with half low label rate (i.e. half of the manufacturer's lowest recommended rate) mixtures of metalaxyl with propamocarb, mancozeb, or fosetyI-Al gave excellent control of Pythium blight of perennial ryegrass (Lolitmt perenne L.) incited by a metalaxyl-sensitive strain of P. aphanMerntattml. Although their data showed that each of the half low label rate components applied singly also controlled the disease, no attempts were made to determine if the magnitudes of control provided by these treatments were the same as those produced by the mixtures. The development of resistance in P. aphanidernlatunl to metataxyl, and the demonstrated lack of ability of any of the currently marketed fungicides to function satisfactorily under conditions of continuing heavy disease pressure, illustrate the need for more efficacious fungicides for control of Pythium blight. The purpose of this study was to determine (1) whether certain combinations of commercially available Pythium-active fungicides with different modes of action exhibit additive, antagonistic or synergistic activity against metalaxyl-sensitive and metalaxyiresistant strains of P. aphatlidenllattoll on the foliage of tur~rasses, and (2) if these combinations of fungicides at fractional rates provide the same or greater level of control of Pythium blight compared with individual components used singly at their full low label rate.
Methods
Perennial ryegrass (Lolitmtperenne L. cv. 'Pennfine') plants were grown in Styrofoam pots, 7.5 cm wide x 8.5 cm deep, with Weblite (a granular, heat-expanded shale manufactured by the Webster Brick Co., Salem, Virginia) serving as the support medium. Every 7 days after seed germination, a 20-20-20 water-soluble fertilizer was applied at a rdte of 273 p.p.m, nitrogen. The intervening daily irrigations were accomplished with tap water. The plants were clipped on a 5-day schedule to a height of 2.0cm. All tests were performed on plants at the 4-leaf stage of growth. The fungicides used in this study and the basic label rates of each (active ingredient per 93m 2) were as follows: mancozeb (136.08g), fosetyl-Al (90.72g), propamocarb (28.35g), metalaxyl (7.09g), ethazole (17.01 g) and chloroneb (73.71 g). These dosage levels are the manufacturers' lowest recommended rates for the control of Pythium blight. The individual treatments consisted of combinations of these materials at the following fractional rates: ½ x propamocarb+½ x metalaxyi; ½ × propamocarb+~ x metaIaxyl; ~ x propamocarb+ ~ x metalaxyl;~ × propamocarb+a~ x metalaxyI; ½ x mancozeb+½ x metalaxyl;½ x mancozeb+a~ x metalaxyl;½ x fosetyl-Al + ½ x metalaxyl;½ x fosetyl-Al+a~ x metalaxyl;~ x fosetyl-Al +~} x metalaxyl; ~ x fosetyl-Al+~ x metalaxy|i ½ x mancozeb+½ x propamocarb;½ x mancozeb+l x propamocarb;k x mancozeb+l x propamocarb;½ x fosetyI-A!
388
Fungicides vs
Pythium aphanidermatum: H. B. Couch and B. D. Smith
+ 3 x propamocarb;:~ x fosetyl-Al+~ x propamocarb;½ x mancozeb+½ x fosetyl-Al;~ x mancozeb+~ x fosetylAI; ½ x mancozeb+½ x chloroneb;~ x mancozeb+:~ x chloroneb; ½ x mancozeb+½ x ethazole; and ~ x mancozeb + k x ethazole. In addition to thesecombinations, the design of the individual tests also included each fungicide applied singly at its 1 x rate, and singly at the various fractional low label rates. Two series o f experiments were conducted: in one, all o f the fungicide combinations listed above were tested against a metalaxyl-sensitive isolate of P. aphanidermatum; in the other, a combination of ½x mancozeb + ½x metalaxyl, and single-component applications of ½xmancozeb, 1x mancozeb, ½x metalaxyl and 1 x metalaxyl, were tested against two isolates of metalaxyl-resistant P. aphaniderntallllH.
All fungicides were sprayed on to the foliage of the test plants in an amount of water equivalent to 15 1/93 m 2. Each treatment was replicated nine times, and each experiment was performed twice. Twenty-four hours after fungicide application, the plants were inoculated with mycelial fragments of Pythhml aphanidermatum that had been prepared in the following manner. Potato dextrose broth (PDB) cultures of P. aphanidermatum were chopped in a blender under aseptic conditions and equal portions o f the homogenate poured into sterile flasks containing 500ml PDB each. After 72 h incubation at 24°C, the contents of each o f these flasks was chopped in a blender under aseptic conditions, and equal portions poured into sterile flasks containing 500 ml PDB each. When this latter group had incubated for 72 h at 24"C, the contents of the flasks were chopped in a blender and uniform amounts of the homogenate were sprayed on to the foliage. The plants were then stored in a Percival dew chamber (Percival Manufacturing Co., Boone, Iowa) in a moisture-saturated atmosphere at 30oc. The incidence and severity of disease among the plants ofeach container was determined by use of a visual scale of 0-10 (0, no disease; 1, 10% of the foliage blighted; 10, 100% o f the foliage blighted). The data were subjected to analysis of variance and then compared by means o f protected l.s.d, values. The theoretical additive control levels of the various single-component fungicide treatments were computed by employing the method of Gowing (1960). According to this system, with a two-component pesticide mixture, where X = the actual level ofcontrol provided by one component used singly, and Y= the actual level ofcontrol provided by the other component used singly, then E (the expected control from the combined effect ofthe two components) is calculated as follows: E = X + [Y(100-X)]/100. The determination of synergism or antagonism for each mixture was then performed by applying the error factor from the protected I.s.d. (p=0.05) to the theoretical additive disease control level for each mixture and then comparing this figure with the actual level of control. For example, if the theoretical additive disease control level for a fungicide mixture was 70% and the I.s.d. at p = 0.05 was + 10%, then an actual control level > 80% would be due to synergism, a control level < 6 0 % would be due to
Table I. Elfectiveness of fractional low label rate combinations of metalaxyl and mancozeb in the control of Pythium blight of perennial ryegrass incited by a metalaxyl-sensitive strain of Pythium
aphanidermatum
Percentage disease c o n t r o l F u n g i c i d e a n d fractional label a m o u n t per 93 m 2 Check M a n c o z e b ~ x ( 6 8 . 0 4 g a.i.) M e t a l a x y l l~0 x (0.71 g a.i.) M a n c o z e b I x (136.08 g a.i.) M e t a l a x y l ]10 x (0.71 g a . i . ) + m a n c o z e b ,~ x (68.04g a.i.) M e t a l a x y l ~ x ( 1 . 4 2 g a.i.) M e t a l a x y l ] x ( 1 . 4 2 g a.i.) + m a n c o z e b ~ x (68.04 g a.i.) M e t a l a x y l 1 x ( 7 . 0 9 g a.i.)
Expected" -----
Actual h 0 5.7 17.1 45.7
a a a b
21.8 --
71.4 c ~ " 72.9 c
74.4 --
84.8 c ~da 91.4 c
"Computed according to the method ofGo~ing (1960);~protected I.s.d. (p = 0.05); confidence interval 5: 20.7; means followed by the same letter are not significantly different from each other G,~a, additive; ~>,, synergistic)
antagonism, and a control level between 60 and 80% would be considered additive.
Results In the tests with the metalaxyl-sensitive isolate of P.
aphanidermatum, six of the fungicide combinations were synergistic, one was antagonistic, and the rest were additive. The fractional low label rate combinations that were synergistic were as follows: ½x mancozeb + ~ x metalaxyl; ¼x mancozeb + ~ x propamocarb; ½x mancozeb + ½x propamocarb; ½x fosetyl-AI + ½x metalaxyl; and fosetyl-AI +propamocarb at mixtures of ~ x + ~ x and ½x + ½ x (Tables 1, 2, 3 and 4). The combination of ½x mancozeb +~ x metalaxyl was synergistic against both metalaxylresistant isolates of P. aphanMermatum (Table 5). Where comparison of the efficacy of these mixtures with single component sprays at full low label rates is concerned, the levels of control produced by the reduced low label rate combinations of mancozeb and metalaxy] were not significantly different from that provided by 1 x metalaxyl used singly, and greater than the control given by the single-component spray of 1 x mancozeb (Table 1). The levels of Pythium blight control provided by the reduced low label rate combinations of mancozeb + propamocarb, and fosetyl-Al+propamocarb were significantly greater than either of the components used singly at its full low label rate (Tables 2 and3). With fosetyl-A1 and metalaxyl, the disease control level of the combination was greater than that o f a single-component spray of I x fosetyl-AI and not significantly different from the control provided by a 1 x application ofmetalaxyl (Table4). The chloroneb-mancozeb mixture was antagonistic in action. The degree o f disease control provided by either of the chloroneb--mancozeb fractional-rate mixtures was significantly less than the control given by the full low label rate of mancozeb used singly. When the disease control patterns within thts m~xture were compared statistically, it was found that although the combination o f ~ x chloron e b + ~ x mancozeb was more effective than ¼x chloroneb
Fungicides vs Table 2. Effectiveness of fractional low label rate combinations of propamocarb and mancozeb in the control of Pythium blight of perennial ryegrass Percentage disease control Fungicide and fractional label a m o u n t per 93 m 2 Check P r o p a m o c a r b '.~x (7.09 g a.i.) P r o p a m o c a r b ~ x (14.17 g a.i.) P r o p a m o c a r b I x (28.35g a.i.) Mancozeb ~ x (39.69g a.i.) Mancozeb ~ x (68.04g a.i.) Mancozeb I x (136.08g a.i.) P r o p a m o c a r b t x (7.09g a.i.)+ mancozeb ~ x (39.69 g a.i.) P r o p a m o c a r b ~ x (28.35g a.i.)+ mancozeb ~ x (68.04 g a.i.)
Expected"
Pythium aphanidermaturn: H.
B. Couch and B. D. Smith
389
synergism is detected, tests should also be conducted to establish the optimum dosage of each component needed to produce a level of disease control equal to or greater than that of the most effective member of the group used singly at its full label rate.
Actual h
--
0
a
--
6.7
a
--
14.4
bc
--
15.6
bc
--
17.8
c
--
22.2c
--
41.1
Table4. Effectiveness of fractional low label rate combinations of fosetyI-AI and metalaxyl in the control of Pythium blight of perennial ryegrass incited by a metalaxyl-sensitive strain of Pythium
aphanidermatum Percentage disease control
d
23.0
50.0 d ~"
33.5
75.6 e ~"
"As in Table I; ^protected I.s.d. (p=0.05); confidence interval 4- 10.7; means follo~ed by the same letter are not significantly different from each other (,~,, synergistic)
Table 3. Effectiveness of fractional low label rate combinations of fosetyI-AI and propamocarb in the control of Pythium blight of perennial ryegrass
Fungicide and fractional label a m o u n t per 93 rn 2 Check FosetyI-Al ,~ x (22.68 g a.i.) FosetyI-AI ~ x (45.36g a.i.) Fosetyl-Al I x (90.72 g a.i.) Metalaxyl a~o x (0.71 g a.i.) Metalax)'l ~ x (1.42g a.i.) Fosetyl-A1 .~ x (22.68 g a.i.) + metalaxyl a~o x (0.72g a.i.) Mctalaxyl 1 x (7.09g a.i.) FosetyI-Al ~ x (45.36 g a.i.) + metalaxyl x (1.42 g a.i.)
Expected"
Actual b
-------
0 6.7 13.3 30.0 53.3 62.2
56.5 --
70.0 de ~aa 76.7 de
67.3
84.4 e 'r"
a a a b c cd
Percentage disease control Fungicide and fractional label a m o u n t per 93 m ~ Check Fosetyl-AI ~. x (22.68 g a.i.) Fosetyl-AI J x (45.36g a.i.) P r o p a m o c a r b .[ × (7.09g a.i.) P r o p a m o c a r b ~ x (14.17g a.i.) P r o p a m o c a r b 1 x (28.35g a.i.) Fosetyl-AI 1 x (90.72g a.i.) Fosetyl-A1 .~ x (22.68 g a.i.) + p r o p a m o c a r b .~ x (7.09g a.i.) FosetyI-AI ~, x (45.36 g a.i.) + p r o p a m o c a r b ~ x (14.17g a.i.)
Expected" --------
A ctualh 0a 1.0 a 15.0 b 19.0 b 20.0 bc 26.0 bc 32.0 c
19.8
80.0 d ~"
32.0
84.0 d ~ "
"As in Table I; hprotected 1.s.d. (.o=0.05); confidence interval 4-12.4; means followed by the same letter are not significantly different from each other (,~,, synergistic)
used singly, its control level was significantly less than that produced when the ~ x mancozeb component was used singly. Furthermore, even though the level of disease control produced by the combination of ½xchloroneb +½ x mancozeb was greater than that given by ½x chloroneb used singly, it was significantly less than the control provided by the single-component spray of ½x mancozeb
(Table6). Discussion The results of the present study are in agreement with the conclusion of Grabski and Gist (1987) that the degree of synergism of a specific fungicide mixture is determined by the proportion ofits components and the level ofactivity of each ingredient against the fungal strains in question. For example, although the combinations of ~xpropamocarb+¼ x mancozeb and ½x propamocarb+½ x mancozeb were both synergistic, the level of Pythium blight control provided by the latter mixture was significantly higher. In screening fungicide combinations, therefore, if
"As in Table I; t'protected I.s.d. (p=0.05); confidence interval • 15.8; means follo~vcd by the same letter are not significantly different from each other (,aa, additive; ~,,, synergistic) Table5. Effectiveness of fractional low label rate c o m b i n a t i o n s of m a n c o z e b and metalaxyl in the control of Pythium blight of p e r e n n i a l ryegrass incited by a metalaxyl-resisfant strain of Pythium
aphanidermatum Percentage disease control Fungicide and fractional label a m o u n t per 93 m 2 Check Metalaxyl ~ x (1.42g a.i.) Metalaxyl 1 x (7.09 g a.i.) Mancozeb ~ x (68.04g a.i.) Mancozeb 1 x (136.08 g a.i.) Mancozeb ~ x (68.04 g a.i.) + metalaxyl x (1.42g a.i.)
Expected"
Actual ^
-~ ----
0 0 0 64.0 77.0
64.0
a a a b c
78.0 c sx"
"As in Tablel; bprotected I.s.d. (p=0.05); confidence interval ±8.8; means followed by the same letter are not significantly different from each other (,7,, s) nergistic)
Table 6. Effectiveness of fractional low label rate combinations of c h l o r o n e b and m a n c o z e b in the control of Pythium blight of p e r e n n i a l ryegrass Percentage disease control Fungicide and fractional label a m o u n t per 93 m 2 Check C h l o r o n e b .1, x (18.43 g a.i.) C h l o r o n e b ~ x (36.85 g a.i.) C h l o r o n e b ,~ x (I 8.43 g a.i.) + mancozeb .~ x (39.69g a.i.) C h l o r o n e b 1 x (73.71 g a.i.) Mancozeb I x (39.69g a.i.) C h l o r o n e b ~ x (36.85g a.i.)+ mancozeb ~, x (68.04g a.i.) Mancozeb ~ x (68.04g a.i.) Mancozeb I x (136.08g a.i.)
Expected"
Actual h
--
0a
--
1.7 a
--
20.0 b
50.0 ---
25.8 b ant 49.2 c 49.2 c
72.7 ---
61.7 d am 65.8 d 82.5,e
°As in Tablel; t~protected I.s.d. (p=0.05); confidence interval ±6.9; means follo~ved by the same letter are not significantly different from each other (a,t, antagonistic)
390
Fungicides vs
Pythiumaphanidermatum: H. B. Couch and B. D. Smith
The findings of this research also support the concept that, as synergy is the response of a designated target organism to the fungicide mixture in question, this phenomenon may or may not hold true for other species within the same taxonomic grouping. Samoucha and Cohen (1988) found that a mixture of metalaxyl and mancozeb was effective in the control of a metalaxylsensitive strain ofPhytophthora htfestans in potatoes, but it was not effective against the metalaxyl-resistant form. In the present study, however, the metalaxyl-mancozeb combination was very effective in the control of Pythium blight of perennial ryegrass incited by either metalaxyl-sensitive or metalaxyl-resistant strains of P)'thhml aphanidermatton. One of the primary advantages of synergistic fungicide combinations is that they reduce the likelihood of the development of resistance to the fungicide in question (de Waard, 1988). The results of this study show that the probability of the development of resistance to fosetyl-Al in the field can be decreased by using this material in combination with either propamocarb or metalaxyl, and that the likelihood of resistance to propamocarb can also be lowered by mixing it with mancozeb. In addition, the high level of effectiveness of the metalaxyl-mancozeb combination in the control of both metalaxyl-sensitive and metalaxyl-resistant strains ofP. aphanidermatttotpresents a strong case for using this mixture both to minimize the possibility of the development ofmetalaxyl resistance, and to provide for the effective control of this organism in situations where a high incidence of resistance already exists. In addition to alleviating disease management problems associated with resistance, use of the synergistic combinations identified in this research could also mean more effective control of P. aphanMermatttm-incited Pythium blight with less fungicide. For example, the applications of reduced low label rate combinations of mancozeb+ metalaxyl, mancozcb+propamocarb, and fosetyl-AI + propamocarb produced a level of disease control significantly greater than either component used singly at its full low label rate. In each instance, the total amount of fungicide used was appreciably less than that applied in the single-component, full label sprays. Tank mixtures of chloroneb and mancozeb have been used for some 25 years in the United States and Canada for the control of Pythium blight. The finding of this research that this combination is antagonistic in its control of P. aphanidermatum, further illustrates the importance of determining the propensity for antagonism with combinations of fungicides active against the same target organism.
Couch, I !. B. (1974). Diseases of Turfgrasses, 2nd edn. Krieger Pub. Co., Melbourne, Florida, 348 pp Couch, 11. B., Gather, J. and Smith, B. D. (1980) Resuhs of 19S0 Turfgrass Disease Control Trials. Turfgrass Pathology Field Research Report. Department of Plant Pathology and Physiology, Virginia Polytechnic Institute, Blacksburg, 18 pp de Wazrd, M. A. (1988) Interactions of fungicide combinations. In: Fungickte Resistance hl North America (Ed. by C. J. Delp) pp. 98-100, APS Press, St Paul Flint, J. L., Cornelius, P. L. and Barrett, M. (1988) Analyzing herbicide interactions: a statistical treatment of Colby's method. Weed Technol. 2, 304-309 Gisi, U., Binder, 11. and Rimbaeh, E. (1985) Synergistic interactions of fungicides with different modes of action. Trans. Br. Mycol. Soc. 85, 299-306 Gm~ing, D. P. (1960) Comments on tests of herbicide mixtures. II'eeds 8, 379-391 Grabski, C. and G]si, U. (I 987) Quantification of synergistic interactions of fungicides against Plasmopara and Phytophthora. Crop Prot. 6, 64-71 llatzios, K. K. and Penner, D. (1985) Interactions of herbicides with other agrochemicals in higher plants. Rev. It'cedSci. I, 1-63 Kane, P,. T. (1987) Disease notes from summer "87. The Bullsheet 41 (7), 4,6 Richer, D. L. (1987) Synergism - a patent view. Pesticide Sci. 19, 305-315 Saladini, J. L. (1981 ) Cool versus warm season Pythium blight and other related Pythium problems. In: Advances bt Turfgrass Pathology (Ed. by P. O. Larscn and B. G. Joyner) pp. 37-39, Harcourt Brace Jovanovich Book Co., Duluth Samoucha, Y. and Cohen, Y. (1984) Synergy between mctalaxyl and mancozeb in controlling downy mildew in cucumbers. Phytopathology 74, 1434-1437 Samoueha, Y. and Cohen, Y. (1988) Synergistic interactions of cymoxanil mixtures in the control of metalaxyl-resistant Phytophthora blfestans. Phytopathology 78, 636-640 Samoucha, Y. and Gist, U. (1987a) PossibIe explanations of synergism in fungicide mixtures against Phytophthora btfestans. Ann. AppL BioL 110, 303-31 [ Samoucha, Y. and Gisi, U. (1987b) Use of two- and three-way mixtures to prevent buildup of resistance to phenylamide fungicides in Phytophthora and Plasmopara. Phytopathology 77, 1405-1409 Sanders, P. L. (1984) Failure of naetalaxyl to control Pythium blight on turfgrass in Pennsylvania. Ph.vtopathology 68, 776-777 Sanders, P. L. (1987) Subdue resistance: is it real? Golf Course Mqngmnt 55 (1 I), 96-99 Sanders, P. L., llouser, W. J., Parish, P. J. and Cole, I1., Jr (1985) Reduced-rate fungicide mixtures to delay fungicide resistance and to control selected turfgrass diseases. Plant Dis. 69, 939-943 Scarda~i, A. (1966) Synergism among fungicides. A. Rev. PhytopathoL 4, 335-341 Wadley, F. M. (1945) The Evidence Required to Show Synergistic Action of hlsecticides and a Short Cut ht Analysis. ET-223. US Department of Agriculture, Washington, 8 pp
References Colby, S. R. (1967) Calculating synergistic and antagonistic responses of herbicide combinations. Weeds 15, 20-22
Received 29 May 1990 Revised 18 March 1991 Accepted 26 March 1991
I
U
Synergistic and antagonistic interactions of fungicides against Pythium aphanidermatumon perennial ryegrass Houston B. Couch* and Brian D. Smith Department of Plant Pathology, Physiology and Weed Science, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA
Abstract
Keywords
Mancozeb, fosetyl-Al, propamocarb, metalaxyl, ethazole and chtoroneb were tested singly at fractional and full label rates, and in two-way combinations at fractional label rates, for their potential to control the colonization of perennial ryegrass foliage by metalaxyt-sensitive and metalaxyl-resistant strains of Pythium aphanidermatum. The theoretical additive control level was computed for each combination of fractional rates, and this value used as the point of reference for determining if the actual degree of control given by the mixture in question was the product of either synergism or antagonism. The mancozeb + chloroneb mixture was antagonistic whereas the combinations of maneozeb+propamocarb, fosetyl-Ai+metalaxyl, and fosetyI-AI + propamocarb were synergistic in the control of the metalaxyl-sensitive strain. Mancozeb + metalaxyl was synergistic in the control of the metalaxyl-sensitive and the metalaxyl-resistant strains. The null hypothesis (additivity) was not rejected for the other combinations. The level of disease control provided by the synergistic combinations tested was equal to or greater than that provided by each of the components applied singly at their full label rates. Pythium blight; fungicide mixtures; fungicide resistance
Introduction The increasing use ofsingle-site fnngicides and the accompanying reports of resistance on the part of certain of the target organisms to these materials has heightened interest in the development of disease control programmes that utilize mixtures of fungicides with different modes of action but effective against the same pathogen. A question related to the use ofthese combinations is what the dosage level should be for each component of the mixture. Specifically, when used in a mixture, can the standard rate of each fungicide be reduced, or should it still be applied at the same level at which it would have been used as a single-component spray.'? When two or more pesticides that have different modes of action but are toxic to the same target organism are combined into a single treatment, the efficacy level of the mixture will either be synergistic, antagonistic, or additive. Synergism is the combined action o f two or more pesticides in which the control provided by their joint application is greater than the control predicted by an appropriate reference model. Antagonism is a level o f control provided by the joint application o f two or more pesticides which is less than that of the predicted control. Additivity is the level o f disease control provided by the co-operative action of two or more pesticides which is equal to the response predicted by taking into account the response o f each ft, ngicide applied singly (Hatzios and Penner, 1985). The *To whom correspondence should be addressed 0261-2194/91/05/0386-05 ~ 1991 Butterworth-Heinemann Ltd
additive control level of the pesticide mixtnre in question is commonly used as the reference fo r determining synergy or antagonism (Richer, 1987). Additive control is not the direct sum of the efficacy levels of the respective components of the mixture, it is an adjusted value. Several mathematical methods have been developed for determining the theoretical additive control levels of mixtures of pesticides (Wadley, 1945; Gowing, 1960; Colby, 1967; Flint, Cornelius and Barrett, 1988). Each of these calculations takes into consideration the fact that when the individual components of a reduced-rate mixture are acting independently, there will be a certain percentage o f escapes from each component that are not acted upon by the other component(s). This means that if the reduced rate mixture of fungicides is not synergistic, the control potential of the joint application will be less than that of the potential of the full label rate of the most efficacious component used alone. The early studies on the efficacy levels of mixtures of fungicides with different modes of action were hi vitro oniy (Scardavi, 1966). In recent years, however, the most widespread procedure for this type of research has been the use of pathogen-plant systems. Samoucha and Cohen (1984) reported synergism between metalaxyl and mancozeb when used as tank mixtures in the control o f downy mildew [incited by Pseudoperonospora cubensis (Berk. & Curt.) Ro.~tow.] on cucumbers (Cucmnis sativus L.). Samoucha and Gisi (1987a,b) found that oxadixyl-mancozeb, oxadixylcymoxanil and cymoxanil-mancozeb, applied either as
Fungicides vs
tank mixtures or sprayed sequentially as single-ingredient treatments, were synergistic in their control ofan oxadixylsensitive strain of Phytopllthora hfestans (Mont.) de Bary on tomato plants, and that a combination of cymoxanil and mancozeb was synergistic in the control of an oxadixyl-resistant form. Gisi, Binder and Rimbach (1985) tested 17 fungicides, singly and in mixtures, on the growth hi vitro of Phytophthora cactorlml (Leb. & Cohn) Schroet. and P. chlnamomi Rands, and on the activity of Phytophthora h#'stans in colonizing the foliage of tomatoes and potatoes, and Plasmopara v#icola in colonizing the leaves of grape vines. In the studies in vitro, mixtures of oxadixyi and mancozeb or cymoxanil and mancozeb showed high levels of synergism. In the tests involving P. hlfestans and P. viticola-host interactions, the highest synergism was observed in combinations of oxadixyl with either mancozeb, cymoxanil or fosetyl-Al. None of the fungicide combinations showed antagonistic interactions. In further tests with these same fungicide-pathogenplant combinations, Grabski and Gist (1987) quantified the degrees of synergism among the various fungicide mixtures. They found that although the activity of the oxadixyl-mancozeb mixture decreased significantly with increasing levels of resistance to oxadixyl, the mixture of oxadixyl, mancozeb and cymoxanil was almost equally effective on all strains. Pythium blight [incited by Pythhml aphanidernlatunl (Edson) Fitzp., P. myrioo'lum Drechs., and P. tdtimum Trow.] is one of the major diseases of turfgrasses throughout the continental United States and southern Canada. Characteristically, the disease has an explosive epidemic pattern. Under conditions of high air temperatures and extended periods of leaf wetness, it is capable of totally destroying large areas of turfgrass within 24-48 h of the first trace of visible symptoms (Couch, 1974; Saladini, 1981). During the period from 1982 to 1986, the systemic fungicides metalaxyl, propamocarb, and fosetyl-AI were registered for the control of Pythium blight in the United States. Field research had shown that these materials provide a greater degree of Pythium blight control than the currently used contact fungicides (Couch, Garber and Smith, 1980). However, soon after they were brought into commercial use, deficiencies were found in their disease control patterns. In 1984, metalaxyl failed to control Pythium blight in a bentgrass fairway on a golf course in eastern Pennsylvania. Certain isolates of P. aphanidermature recovered from diseased plants collected at this location were shown to be metalaxyl resistant (Sanders, 1984). Since this initial report, six confirmed instances of P. aphanidermatlml resistance to metalaxyl have been reported in the United States (Sanders, 1987). In addition to resistance of P. aphanidermatum to metalaxyl, it has also been observed that when the pressure from Pythium blight is extremely heavy, there can be a significant reduction in the longevity of control normally provided by the systemic fungicides. During July-August of 1987 and 1988, there were severe outbreaks of Pythium blight in several areas of the continental United States.
Pythiumaphanidermatum:H.
B. Couch and B. D. Smith
387
Under these conditions, it was observed that the duration of effective disease control by the systemic fungicides was shortened from the expected 14-21 days to 2-4 days per spray (Kane, 1987). Sanders et al. (1985) reported that treatments with half low label rate (i.e. half of the manufacturer's lowest recommended rate) mixtures of metalaxyl with propamocarb, mancozeb, or fosetyI-Al gave excellent control of Pythium blight of perennial ryegrass (Lolitmt perenne L.) incited by a metalaxyl-sensitive strain of P. aphanMerntattml. Although their data showed that each of the half low label rate components applied singly also controlled the disease, no attempts were made to determine if the magnitudes of control provided by these treatments were the same as those produced by the mixtures. The development of resistance in P. aphanidernlatunl to metataxyl, and the demonstrated lack of ability of any of the currently marketed fungicides to function satisfactorily under conditions of continuing heavy disease pressure, illustrate the need for more efficacious fungicides for control of Pythium blight. The purpose of this study was to determine (1) whether certain combinations of commercially available Pythium-active fungicides with different modes of action exhibit additive, antagonistic or synergistic activity against metalaxyl-sensitive and metalaxyiresistant strains of P. aphatlidenllattoll on the foliage of tur~rasses, and (2) if these combinations of fungicides at fractional rates provide the same or greater level of control of Pythium blight compared with individual components used singly at their full low label rate.
Methods
Perennial ryegrass (Lolitmtperenne L. cv. 'Pennfine') plants were grown in Styrofoam pots, 7.5 cm wide x 8.5 cm deep, with Weblite (a granular, heat-expanded shale manufactured by the Webster Brick Co., Salem, Virginia) serving as the support medium. Every 7 days after seed germination, a 20-20-20 water-soluble fertilizer was applied at a rdte of 273 p.p.m, nitrogen. The intervening daily irrigations were accomplished with tap water. The plants were clipped on a 5-day schedule to a height of 2.0cm. All tests were performed on plants at the 4-leaf stage of growth. The fungicides used in this study and the basic label rates of each (active ingredient per 93m 2) were as follows: mancozeb (136.08g), fosetyl-Al (90.72g), propamocarb (28.35g), metalaxyl (7.09g), ethazole (17.01 g) and chloroneb (73.71 g). These dosage levels are the manufacturers' lowest recommended rates for the control of Pythium blight. The individual treatments consisted of combinations of these materials at the following fractional rates: ½ x propamocarb+½ x metalaxyi; ½ × propamocarb+~ x metaIaxyl; ~ x propamocarb+ ~ x metalaxyl;~ × propamocarb+a~ x metalaxyI; ½ x mancozeb+½ x metalaxyl;½ x mancozeb+a~ x metalaxyl;½ x fosetyl-Al + ½ x metalaxyl;½ x fosetyl-Al+a~ x metalaxyl;~ x fosetyl-Al +~} x metalaxyl; ~ x fosetyl-Al+~ x metalaxy|i ½ x mancozeb+½ x propamocarb;½ x mancozeb+l x propamocarb;k x mancozeb+l x propamocarb;½ x fosetyI-A!
388
Fungicides vs
Pythium aphanidermatum: H. B. Couch and B. D. Smith
+ 3 x propamocarb;:~ x fosetyl-Al+~ x propamocarb;½ x mancozeb+½ x fosetyl-Al;~ x mancozeb+~ x fosetylAI; ½ x mancozeb+½ x chloroneb;~ x mancozeb+:~ x chloroneb; ½ x mancozeb+½ x ethazole; and ~ x mancozeb + k x ethazole. In addition to thesecombinations, the design of the individual tests also included each fungicide applied singly at its 1 x rate, and singly at the various fractional low label rates. Two series o f experiments were conducted: in one, all o f the fungicide combinations listed above were tested against a metalaxyl-sensitive isolate of P. aphanidermatum; in the other, a combination of ½x mancozeb + ½x metalaxyl, and single-component applications of ½xmancozeb, 1x mancozeb, ½x metalaxyl and 1 x metalaxyl, were tested against two isolates of metalaxyl-resistant P. aphaniderntallllH.
All fungicides were sprayed on to the foliage of the test plants in an amount of water equivalent to 15 1/93 m 2. Each treatment was replicated nine times, and each experiment was performed twice. Twenty-four hours after fungicide application, the plants were inoculated with mycelial fragments of Pythhml aphanidermatum that had been prepared in the following manner. Potato dextrose broth (PDB) cultures of P. aphanidermatum were chopped in a blender under aseptic conditions and equal portions o f the homogenate poured into sterile flasks containing 500ml PDB each. After 72 h incubation at 24°C, the contents of each o f these flasks was chopped in a blender under aseptic conditions, and equal portions poured into sterile flasks containing 500 ml PDB each. When this latter group had incubated for 72 h at 24"C, the contents of the flasks were chopped in a blender and uniform amounts of the homogenate were sprayed on to the foliage. The plants were then stored in a Percival dew chamber (Percival Manufacturing Co., Boone, Iowa) in a moisture-saturated atmosphere at 30oc. The incidence and severity of disease among the plants ofeach container was determined by use of a visual scale of 0-10 (0, no disease; 1, 10% of the foliage blighted; 10, 100% o f the foliage blighted). The data were subjected to analysis of variance and then compared by means o f protected l.s.d, values. The theoretical additive control levels of the various single-component fungicide treatments were computed by employing the method of Gowing (1960). According to this system, with a two-component pesticide mixture, where X = the actual level ofcontrol provided by one component used singly, and Y= the actual level ofcontrol provided by the other component used singly, then E (the expected control from the combined effect ofthe two components) is calculated as follows: E = X + [Y(100-X)]/100. The determination of synergism or antagonism for each mixture was then performed by applying the error factor from the protected I.s.d. (p=0.05) to the theoretical additive disease control level for each mixture and then comparing this figure with the actual level of control. For example, if the theoretical additive disease control level for a fungicide mixture was 70% and the I.s.d. at p = 0.05 was + 10%, then an actual control level > 80% would be due to synergism, a control level < 6 0 % would be due to
Table I. Elfectiveness of fractional low label rate combinations of metalaxyl and mancozeb in the control of Pythium blight of perennial ryegrass incited by a metalaxyl-sensitive strain of Pythium
aphanidermatum
Percentage disease c o n t r o l F u n g i c i d e a n d fractional label a m o u n t per 93 m 2 Check M a n c o z e b ~ x ( 6 8 . 0 4 g a.i.) M e t a l a x y l l~0 x (0.71 g a.i.) M a n c o z e b I x (136.08 g a.i.) M e t a l a x y l ]10 x (0.71 g a . i . ) + m a n c o z e b ,~ x (68.04g a.i.) M e t a l a x y l ~ x ( 1 . 4 2 g a.i.) M e t a l a x y l ] x ( 1 . 4 2 g a.i.) + m a n c o z e b ~ x (68.04 g a.i.) M e t a l a x y l 1 x ( 7 . 0 9 g a.i.)
Expected" -----
Actual h 0 5.7 17.1 45.7
a a a b
21.8 --
71.4 c ~ " 72.9 c
74.4 --
84.8 c ~da 91.4 c
"Computed according to the method ofGo~ing (1960);~protected I.s.d. (p = 0.05); confidence interval 5: 20.7; means followed by the same letter are not significantly different from each other G,~a, additive; ~>,, synergistic)
antagonism, and a control level between 60 and 80% would be considered additive.
Results In the tests with the metalaxyl-sensitive isolate of P.
aphanidermatum, six of the fungicide combinations were synergistic, one was antagonistic, and the rest were additive. The fractional low label rate combinations that were synergistic were as follows: ½x mancozeb + ~ x metalaxyl; ¼x mancozeb + ~ x propamocarb; ½x mancozeb + ½x propamocarb; ½x fosetyl-AI + ½x metalaxyl; and fosetyl-AI +propamocarb at mixtures of ~ x + ~ x and ½x + ½ x (Tables 1, 2, 3 and 4). The combination of ½x mancozeb +~ x metalaxyl was synergistic against both metalaxylresistant isolates of P. aphanMermatum (Table 5). Where comparison of the efficacy of these mixtures with single component sprays at full low label rates is concerned, the levels of control produced by the reduced low label rate combinations of mancozeb and metalaxy] were not significantly different from that provided by 1 x metalaxyl used singly, and greater than the control given by the single-component spray of 1 x mancozeb (Table 1). The levels of Pythium blight control provided by the reduced low label rate combinations of mancozeb + propamocarb, and fosetyl-Al+propamocarb were significantly greater than either of the components used singly at its full low label rate (Tables 2 and3). With fosetyl-A1 and metalaxyl, the disease control level of the combination was greater than that o f a single-component spray of I x fosetyl-AI and not significantly different from the control provided by a 1 x application ofmetalaxyl (Table4). The chloroneb-mancozeb mixture was antagonistic in action. The degree o f disease control provided by either of the chloroneb--mancozeb fractional-rate mixtures was significantly less than the control given by the full low label rate of mancozeb used singly. When the disease control patterns within thts m~xture were compared statistically, it was found that although the combination o f ~ x chloron e b + ~ x mancozeb was more effective than ¼x chloroneb
Fungicides vs Table 2. Effectiveness of fractional low label rate combinations of propamocarb and mancozeb in the control of Pythium blight of perennial ryegrass Percentage disease control Fungicide and fractional label a m o u n t per 93 m 2 Check P r o p a m o c a r b '.~x (7.09 g a.i.) P r o p a m o c a r b ~ x (14.17 g a.i.) P r o p a m o c a r b I x (28.35g a.i.) Mancozeb ~ x (39.69g a.i.) Mancozeb ~ x (68.04g a.i.) Mancozeb I x (136.08g a.i.) P r o p a m o c a r b t x (7.09g a.i.)+ mancozeb ~ x (39.69 g a.i.) P r o p a m o c a r b ~ x (28.35g a.i.)+ mancozeb ~ x (68.04 g a.i.)
Expected"
Pythium aphanidermaturn: H.
B. Couch and B. D. Smith
389
synergism is detected, tests should also be conducted to establish the optimum dosage of each component needed to produce a level of disease control equal to or greater than that of the most effective member of the group used singly at its full label rate.
Actual h
--
0
a
--
6.7
a
--
14.4
bc
--
15.6
bc
--
17.8
c
--
22.2c
--
41.1
Table4. Effectiveness of fractional low label rate combinations of fosetyI-AI and metalaxyl in the control of Pythium blight of perennial ryegrass incited by a metalaxyl-sensitive strain of Pythium
aphanidermatum Percentage disease control
d
23.0
50.0 d ~"
33.5
75.6 e ~"
"As in Table I; ^protected I.s.d. (p=0.05); confidence interval 4- 10.7; means follo~ed by the same letter are not significantly different from each other (,~,, synergistic)
Table 3. Effectiveness of fractional low label rate combinations of fosetyI-AI and propamocarb in the control of Pythium blight of perennial ryegrass
Fungicide and fractional label a m o u n t per 93 rn 2 Check FosetyI-Al ,~ x (22.68 g a.i.) FosetyI-AI ~ x (45.36g a.i.) Fosetyl-Al I x (90.72 g a.i.) Metalaxyl a~o x (0.71 g a.i.) Metalax)'l ~ x (1.42g a.i.) Fosetyl-A1 .~ x (22.68 g a.i.) + metalaxyl a~o x (0.72g a.i.) Mctalaxyl 1 x (7.09g a.i.) FosetyI-Al ~ x (45.36 g a.i.) + metalaxyl x (1.42 g a.i.)
Expected"
Actual b
-------
0 6.7 13.3 30.0 53.3 62.2
56.5 --
70.0 de ~aa 76.7 de
67.3
84.4 e 'r"
a a a b c cd
Percentage disease control Fungicide and fractional label a m o u n t per 93 m ~ Check Fosetyl-AI ~. x (22.68 g a.i.) Fosetyl-AI J x (45.36g a.i.) P r o p a m o c a r b .[ × (7.09g a.i.) P r o p a m o c a r b ~ x (14.17g a.i.) P r o p a m o c a r b 1 x (28.35g a.i.) Fosetyl-AI 1 x (90.72g a.i.) Fosetyl-A1 .~ x (22.68 g a.i.) + p r o p a m o c a r b .~ x (7.09g a.i.) FosetyI-AI ~, x (45.36 g a.i.) + p r o p a m o c a r b ~ x (14.17g a.i.)
Expected" --------
A ctualh 0a 1.0 a 15.0 b 19.0 b 20.0 bc 26.0 bc 32.0 c
19.8
80.0 d ~"
32.0
84.0 d ~ "
"As in Table I; hprotected 1.s.d. (.o=0.05); confidence interval 4-12.4; means followed by the same letter are not significantly different from each other (,~,, synergistic)
used singly, its control level was significantly less than that produced when the ~ x mancozeb component was used singly. Furthermore, even though the level of disease control produced by the combination of ½xchloroneb +½ x mancozeb was greater than that given by ½x chloroneb used singly, it was significantly less than the control provided by the single-component spray of ½x mancozeb
(Table6). Discussion The results of the present study are in agreement with the conclusion of Grabski and Gist (1987) that the degree of synergism of a specific fungicide mixture is determined by the proportion ofits components and the level ofactivity of each ingredient against the fungal strains in question. For example, although the combinations of ~xpropamocarb+¼ x mancozeb and ½x propamocarb+½ x mancozeb were both synergistic, the level of Pythium blight control provided by the latter mixture was significantly higher. In screening fungicide combinations, therefore, if
"As in Table I; t'protected I.s.d. (p=0.05); confidence interval • 15.8; means follo~vcd by the same letter are not significantly different from each other (,aa, additive; ~,,, synergistic) Table5. Effectiveness of fractional low label rate c o m b i n a t i o n s of m a n c o z e b and metalaxyl in the control of Pythium blight of p e r e n n i a l ryegrass incited by a metalaxyl-resisfant strain of Pythium
aphanidermatum Percentage disease control Fungicide and fractional label a m o u n t per 93 m 2 Check Metalaxyl ~ x (1.42g a.i.) Metalaxyl 1 x (7.09 g a.i.) Mancozeb ~ x (68.04g a.i.) Mancozeb 1 x (136.08 g a.i.) Mancozeb ~ x (68.04 g a.i.) + metalaxyl x (1.42g a.i.)
Expected"
Actual ^
-~ ----
0 0 0 64.0 77.0
64.0
a a a b c
78.0 c sx"
"As in Tablel; bprotected I.s.d. (p=0.05); confidence interval ±8.8; means followed by the same letter are not significantly different from each other (,7,, s) nergistic)
Table 6. Effectiveness of fractional low label rate combinations of c h l o r o n e b and m a n c o z e b in the control of Pythium blight of p e r e n n i a l ryegrass Percentage disease control Fungicide and fractional label a m o u n t per 93 m 2 Check C h l o r o n e b .1, x (18.43 g a.i.) C h l o r o n e b ~ x (36.85 g a.i.) C h l o r o n e b ,~ x (I 8.43 g a.i.) + mancozeb .~ x (39.69g a.i.) C h l o r o n e b 1 x (73.71 g a.i.) Mancozeb I x (39.69g a.i.) C h l o r o n e b ~ x (36.85g a.i.)+ mancozeb ~, x (68.04g a.i.) Mancozeb ~ x (68.04g a.i.) Mancozeb I x (136.08g a.i.)
Expected"
Actual h
--
0a
--
1.7 a
--
20.0 b
50.0 ---
25.8 b ant 49.2 c 49.2 c
72.7 ---
61.7 d am 65.8 d 82.5,e
°As in Tablel; t~protected I.s.d. (p=0.05); confidence interval ±6.9; means follo~ved by the same letter are not significantly different from each other (a,t, antagonistic)
390
Fungicides vs
Pythiumaphanidermatum: H. B. Couch and B. D. Smith
The findings of this research also support the concept that, as synergy is the response of a designated target organism to the fungicide mixture in question, this phenomenon may or may not hold true for other species within the same taxonomic grouping. Samoucha and Cohen (1988) found that a mixture of metalaxyl and mancozeb was effective in the control of a metalaxylsensitive strain ofPhytophthora htfestans in potatoes, but it was not effective against the metalaxyl-resistant form. In the present study, however, the metalaxyl-mancozeb combination was very effective in the control of Pythium blight of perennial ryegrass incited by either metalaxyl-sensitive or metalaxyl-resistant strains of P)'thhml aphanidermatton. One of the primary advantages of synergistic fungicide combinations is that they reduce the likelihood of the development of resistance to the fungicide in question (de Waard, 1988). The results of this study show that the probability of the development of resistance to fosetyl-Al in the field can be decreased by using this material in combination with either propamocarb or metalaxyl, and that the likelihood of resistance to propamocarb can also be lowered by mixing it with mancozeb. In addition, the high level of effectiveness of the metalaxyl-mancozeb combination in the control of both metalaxyl-sensitive and metalaxyl-resistant strains ofP. aphanidermatttotpresents a strong case for using this mixture both to minimize the possibility of the development ofmetalaxyl resistance, and to provide for the effective control of this organism in situations where a high incidence of resistance already exists. In addition to alleviating disease management problems associated with resistance, use of the synergistic combinations identified in this research could also mean more effective control of P. aphanMermatttm-incited Pythium blight with less fungicide. For example, the applications of reduced low label rate combinations of mancozeb+ metalaxyl, mancozcb+propamocarb, and fosetyl-AI + propamocarb produced a level of disease control significantly greater than either component used singly at its full low label rate. In each instance, the total amount of fungicide used was appreciably less than that applied in the single-component, full label sprays. Tank mixtures of chloroneb and mancozeb have been used for some 25 years in the United States and Canada for the control of Pythium blight. The finding of this research that this combination is antagonistic in its control of P. aphanidermatum, further illustrates the importance of determining the propensity for antagonism with combinations of fungicides active against the same target organism.
Couch, I !. B. (1974). Diseases of Turfgrasses, 2nd edn. Krieger Pub. Co., Melbourne, Florida, 348 pp Couch, 11. B., Gather, J. and Smith, B. D. (1980) Resuhs of 19S0 Turfgrass Disease Control Trials. Turfgrass Pathology Field Research Report. Department of Plant Pathology and Physiology, Virginia Polytechnic Institute, Blacksburg, 18 pp de Wazrd, M. A. (1988) Interactions of fungicide combinations. In: Fungickte Resistance hl North America (Ed. by C. J. Delp) pp. 98-100, APS Press, St Paul Flint, J. L., Cornelius, P. L. and Barrett, M. (1988) Analyzing herbicide interactions: a statistical treatment of Colby's method. Weed Technol. 2, 304-309 Gisi, U., Binder, 11. and Rimbaeh, E. (1985) Synergistic interactions of fungicides with different modes of action. Trans. Br. Mycol. Soc. 85, 299-306 Gm~ing, D. P. (1960) Comments on tests of herbicide mixtures. II'eeds 8, 379-391 Grabski, C. and G]si, U. (I 987) Quantification of synergistic interactions of fungicides against Plasmopara and Phytophthora. Crop Prot. 6, 64-71 llatzios, K. K. and Penner, D. (1985) Interactions of herbicides with other agrochemicals in higher plants. Rev. It'cedSci. I, 1-63 Kane, P,. T. (1987) Disease notes from summer "87. The Bullsheet 41 (7), 4,6 Richer, D. L. (1987) Synergism - a patent view. Pesticide Sci. 19, 305-315 Saladini, J. L. (1981 ) Cool versus warm season Pythium blight and other related Pythium problems. In: Advances bt Turfgrass Pathology (Ed. by P. O. Larscn and B. G. Joyner) pp. 37-39, Harcourt Brace Jovanovich Book Co., Duluth Samoucha, Y. and Cohen, Y. (1984) Synergy between mctalaxyl and mancozeb in controlling downy mildew in cucumbers. Phytopathology 74, 1434-1437 Samoueha, Y. and Cohen, Y. (1988) Synergistic interactions of cymoxanil mixtures in the control of metalaxyl-resistant Phytophthora blfestans. Phytopathology 78, 636-640 Samoucha, Y. and Gist, U. (1987a) PossibIe explanations of synergism in fungicide mixtures against Phytophthora btfestans. Ann. AppL BioL 110, 303-31 [ Samoucha, Y. and Gisi, U. (1987b) Use of two- and three-way mixtures to prevent buildup of resistance to phenylamide fungicides in Phytophthora and Plasmopara. Phytopathology 77, 1405-1409 Sanders, P. L. (1984) Failure of naetalaxyl to control Pythium blight on turfgrass in Pennsylvania. Ph.vtopathology 68, 776-777 Sanders, P. L. (1987) Subdue resistance: is it real? Golf Course Mqngmnt 55 (1 I), 96-99 Sanders, P. L., llouser, W. J., Parish, P. J. and Cole, I1., Jr (1985) Reduced-rate fungicide mixtures to delay fungicide resistance and to control selected turfgrass diseases. Plant Dis. 69, 939-943 Scarda~i, A. (1966) Synergism among fungicides. A. Rev. PhytopathoL 4, 335-341 Wadley, F. M. (1945) The Evidence Required to Show Synergistic Action of hlsecticides and a Short Cut ht Analysis. ET-223. US Department of Agriculture, Washington, 8 pp
References Colby, S. R. (1967) Calculating synergistic and antagonistic responses of herbicide combinations. Weeds 15, 20-22
Received 29 May 1990 Revised 18 March 1991 Accepted 26 March 1991