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Parasitism of carnation rust (Uromyces dianthi) by Verticillium lecanii
[ 191 ]
Trans. Br. mycol. Soc. 74 (1) 191-194 (1980)
Primed in Great Britain
PARASITISM OF CARNATION RUST (UROMYCES DIANTHI) BY VERTICILLIUM LECANII By D. M. SPENCER Glasshouse Crops Research Institute, Littlehampton, W. Sussex, BN16 3PU Verticillium lecanii (Zimm.) Viegas was found infesting uredinia of Uromyces dianthi (Pers.) Niessl, the causal organism of carnation rust in a nursery. In the laboratory U. dianthi, inoculated on to carnation leaf pieces, caused typical rust symptoms, but in the presence of V. lecanii either rust infection was prevented or the formation of urediniospores was arrested, depending on the time of application of V. lecanii conidia. In a glasshouse, V. lecanii applied at the appropriate time reduced rust in carnation plants inoculated with U. dianthi.
In recent years there has been increasing attention given to the biological control of plant diseases (Mitchell, 1973; Baker & Cook, 1974), particularly of soil-borne diseases (Papavizas, 1973). The use of antagonistic micro-organisms as disease control agents has been studied (Hong & Ueyama, 1976; Ebben & Spencer, 1978) and the mode of action of some such antagonists has been investigated (Shigemitsu, Kunoh & Akai, 1978). The biological control of diseases other than those that are soil borne has received rather less attention (de Trogoff & Ricard, 1976; Jarvis & Slingsby, 1977) though Silveira & Rodrigues (1971) reported that Verticillium spp. caused bursting of the urediniospores of coffee rust (Hemileia vastatrix Berk. & Br.). Verticillium lecanii (Zimm.) Viegas has a wide host range; it parasitizes scale insects and aphids and has recently been reported by Hall, Allen & Leeming (1979) to colonize cultures of Erysiphe graminis DC., the causal organism of barley mildew, and of Uromyces appendiculatus (Pers.) Ung., the cause of dwarf bean rust. McKenzie & Hudson (1976) found V. lecanii to be a common mycoparasite of wheat stem rust caused by Puccinia graminis Pers. f.sp. tritici. In this paper the occurrence of V. lecanii on Uromyces dianthi (Pers.) Niessl is reported and observations are made on certain host-parasite interactions in laboratory and glasshouse experiments.
waxy nature of the leaf surface caused droplets to be shed by younger leaves, but discrete droplets were retained by the older lower leaves and the retention of spores on leaves was improved by addition of 0'05 % Teepol to the suspension. Immediately after inoculation plants were removed to conditions of high relative humidity where droplets of spore suspension did not evaporate. After 24 h plants were returned to the glasshouse bench where developing uredinia became visible some two weeks later. They appeared first as pale green spots, and the orange-brown urediniospores burst through the epidermis after 2-3 days. A development of this method was used in laboratory experiments when carnation leaf pieces f : 5 em long, floating on tap water in plastic drinking cups, were sprayed with a urediniospore suspension (c. 10· ml"). Polystyrene Petri dish lids were used to cover the drinking cups to maintain the requisite high humidity for 24 h. V. lecanii was found colonizing uredinia of carnation rust in a local nursery (Fig. 2). Its identity was confirmed by Dr R. A. Hall. The organism was maintained at 20°C on Czapek-Dox agar and ro-day-old cultures were used to provide conidial suspensions (c. 5 x 10· mlr! water) to study its effect on U. dianthi on carnation plants or leaf pieces. RESULTS
MATERIALS AND METHODS
Carnation rust was maintained on potted carnation plants (cv. William Sim) using the method described by Spencer (1979). Plants bearing uredinia were originally obtained from a local nursery (Fig. 1) to provide the inoculum. Stock plants were inoculated by spraying with a suspension of urediniospores (c. 10· ml- 1 ) in water. The
Spore germination Approximately 95 % of U. dianthi urediniospores kept in water droplets on glass slides at 25° germinated within 24 h. During this period the germ tubes grew to a mean length of c. 350 /lm. Under similar conditions c. 80 % of V. lecanii spores germinated and produced germ tubes of c. 20 /lm. In a mixed suspension of spores of the two fungi
0007-1536/80/2828-5630 $01.00 © 1980 The British Mycological Society
192
Parasitism of Uromyces by Verticillium
Fig. 1. Stem and leaves of carnation with normal uredinia of Uromyces dianthi. Fig. 2. Stem and leaves of carnation with Verticillium lecanii growing on coalesced uredinia of Uromyces dianthi.
the percentage germination of both remained unchanged, but whereas the mean length of the V. lecanii germ tubes increased to c. 100 pm with much branching, those of U. dianthi were restricted to c. 100 pm. Many of the U. dianthi germ tubes assumed a spiral configuration where V. lecanii hyphae were in close proximity but penetration of the U. dianthi germ tubes was not seen.
Effect of V. lecanii on infection of carnation leaf pieces by U. dianthi In a series of laboratory experiments carnation leaf pieces were inoculated with U. dianthi. V. lecanii spores were applied to the leaves either at the same time as the U. dianthi or at various times before and after the appearance of uredinia. It was found that carnation rust did not develop when a mixed suspension of U. dianthi urediniospores and V. lecanii conidia was used to inoculate
D. M. Spencer
193
the leaf pieces. When V . leeanii was sprayed on to leaf pieces up to 24 h before uredinia became erumpent the further development of uredinia and production of urediniospores was arre sted. A similar arrest was observed when V. leeanii was applied after uredinia had erupted, and in all these cases a weft of sporulating mycelium of V. leeanii was to be seen at the site of the uredinia.
developed rust to the same level as controls, but rust infection was much reduced if V. leeanii conidia were applied 24 h before or after inoculation with U. dianthi (Table 1). The number of erumpent uredinia was also much reduced when V. leeanii conidia were sprayed on to the plants after uredinia had begun to erupt.
Effect of V. lecanii on inf ection of carnation plants by U. dianthi In glasshouse experiments, using carnation plants (cv. William Sim) growing in compost, a significant level of rust control was achieved (Table 1). At intervals before or after the plants were inoculated with U. dianthi (T able 1), they were sprayed with conidia of V. leeanii (5 x 10s/ml) in water containing 0'05 % Teepol, to provide a
When V. leeanii was found naturally occurring on U. dianthi in a commercial carnation crop, an examination of its potential in disease control was immediately contemplated. Newhook (1957) showed that saprophytic leaf colonizers inhibited the germination of Botrytis cinerea Pers. ex Fr. conidia and prevented colonization of the leaves by B. cinerea. Though in the present experiments the percentage germination of U. dianthi urediniospores was not depressed, in the presence of germinating conidia of V. lecanii, the growth of germ tubes was inhibited. This could be associated with the reduced level of rust found when carnation plants were sprayed with V. leeanii conidia at the same time as they were inoculated with U. dianthi. These results support the work of Fokkema & Lorbeer (1974) who found that growth of germ tubes of Alternaria porri (Ell.) Neerg. was reduced in the presence of Aureobasidium pullulans (de Bary) Am. and Sporobolomyees roseus Kluyver & v. Niel and the number of lesions caused by A. porri was reduced bye. 50 %. Hashioka (1973) classified mycoparasites as either necrotrophic, where the parasite destroys its host , or biotrophic, where the parasite acts by means of haustoria or through indirect reactions by which the host is less obviously harmed. The lack of visible damage to U. dianthi spores or to their germ tubes in these investigations suggests that V . leeanii is a biotrophic parasite of U. dianthi . Apart from the direct parasitism of U. dianthi by - V. leeanii there are two further possibilities to be considered as the basis of the reduction of carnation rust shown here. The first of these is the possibility that inhibition of germination of urediniospores of U. dianthi is brought about by phytoalexins, These substances would be induced in the carnation in response to drops of spore suspension of V. leeanii but though this might account for a reduction in level of rust when V. leeanii and U. dianthi spores were applied at or about the same time it would be less likely to account for reduction of rust achieved when V. leeanii was applied at the end of the rustincubation period. Furthermore, the very poor contact between droplets of spore suspension
Table 1. Effect on number of uredinia appearing on carnation plants sprayed with conidia of V. lecanii before or after inoculation with U. dianthi Number of uredinia (means of fiveplants) Experiment ~
Treatment U. dianthi only V. leeanii applied 48 h before U, dianthi inoculation V . leeanii applied 24 h before U. dianrhi inoculation V. lecanii applied 24 h after U, dianthi inoculation V . lecanii applied as first U, dianthi uredinia erupt V. lecanii applied 1 wk after U, dianrhi uredinia erupt
__
1 109'8 115'2
,
~ A
2 104'6 1°4'6 57'6
44'0
42'0
37'4
48'4
34'8
43'2
cover of discrete droplets, mainly on lower leaves. Sprayed plants were kept in conditions of high relative humidity for 24 h to allow rust infection to occur, for the conidia of V . leeanii to germinate, and possibly, to permit the establishment of V. leeanii on U. dianthi. Afterwards, plants were returned to the glasshouse to await further development of rust. The level of rust infection was assessed as the total number of erumpent uredinia which appeared on each of five replicate plants in each treatment, the final assessment being made 3 wk after eruption of the first uredinia, Plants sprayed with V. leeanii conidia 48 h before they were inoculated with U. dianthi 7
DISCUSSION
MYC
74
194
Parasitism of Uromyces by Verticillium
and the waxy surface of the carnation leaf would not be conducive to the accumulation of a fungitoxic concentration of phytoalexin. The second possibility is that substances released from the rust-infected leaf serve as nutrients for V. lecanii, the growth of which then becomes so vigorous as to form a physical barrier to further progress of the rust organism. This possibility is not supported by observations made on suspensions of germinating spores nor at the leaf surface, even though here there is often a very luxuriant growth of V. lecanii, The glasshouse experiments indicate that the use of V. lecanii can provide a measure of control of U. dianthi. The level of this control might be improved if it could be ensured that U. dianthi were more effectively exposed to attack by V. lecanii by the use of wetting agents to aid dispersion of the V. lecanii conidia. The widespread natural occurrence of V. lecanii on the nursery also indicates that careful manipulation of the glasshouse environment might be used to achieve better control of rust. If V. lecanii were to be sprayed on to the crop as a means of biological control the indications are that it would be used as a prophylactic rather than as a protectant since the conidia applied to plants more than 24 h before inoculation with rust gave no benefit. It would seem therefore that conidia are either inherently short-lived or are sensitive to environmental stresses such as desiccation. The protection of the environment is receiving ever-increasing attention and the possibility of controlling plant disease by biological means rather than with chemicals thus merits serious consideration. The protected environment of the 11 h . If . ul 1 glass h ouse 1en d s itse partie ar y we to t e use of biological control agents, and further work on the nature of the inhibition of U. dianthi by V. lecanii is justified, as is a study of the level of rust control to be achieved in commercial growing conditions. If V. lecanii were to be shown to be particularly attractive in biological control, the fact that it is of widespread occurrence in commercial carnation houses should have some bearing on deliberations concerning its clearance under existing safety regulations.
REFERENCES BAKER, K. F. & COOK, R. J. (1974). Biological Control of Plant Pathogens. W. H. Freeman & Company, San Francisco. EBBEN, M. H. & SPENCER, D. M. (1978). The use of antagonistic organisms for the control of black root rot of cucumber, Phomopsis sclerotioides. Annals of Applied Biology 89, 103-106. FOKKEMA, N. J. & LORBEER, J. W. (1974). Interactions between Alternaria porri and the saprophytic microflora of onion leaves. Phytopathology 64, 1128-1133. HALL, R. A., ALLEN, D. J. & LEEMING, A. R. (1979). Parasitism and host specificity amongst isolates of Verticillium lecanii on insects and phytopathogenic fungi. (In preparation.) HASHIOKA, Y. (1973)· Mycoparasitism in relation to phytopathogens, Shohubutsu Byoga Kenkyu 8, 179- 1 90. HONG, C. Y. & UEYAMA, A. (1976). Ecological control of soil-borne diseases with special reference to enhancement of antagonism. Agriculture and Horticulture 51, 7 29-734. JARVIS, W. R. & SLINGSBY, K. (1977). The control of powdery mildew of greenhouse cucumber by water sprays and Ampelomyces quisqualis. Plant Disease Reporter 61, 728-730. McKENZIE, E. H. C. & HUDSON, H. J. (1976). Mycoflora of rust-infected and non-infected plant material during decay. Transactionsof the British Mycological Society 66, 223-238. MITCHELL, J. E. (1973). The mechanisms of biological control of plant diseases. Soil Biology and Biochemistry 5, 721-728. NEWHOOK, F. J. (1957). The relationship of saprophytic antagonism to control of Botrytis cinerea Pers. on tomatoes. New Zealand Journal of Science and Technology A 38, 473-481. PAPAVIZAS, G. C. (1973)· Status of applied biological control of soil-borne plant pathogens. Soil Biology and Biochemistry 5, 709-720 . SHIGEMITSU, H., KUNOH, H. & AKAI, S. (1978). Scanning electron microscopic studies of Corticium rolfsii sclerotia parasitized by Aspergillus terreus. Mycologia 70, 935-943. SILVEIRA, H. L. & RODRIGUES, C. J. (1971). Bursting of rust uredospores caused by Verticillium hemileiae culture filtrates. Agronomia lusitana 33, 391-396. SPENCER, D. M. (1979)· Carnation rust and its control by systemic fungicides. Plant Pathology. (In the Press.) TROGOFF, H. DE & RICARD, J. L. (1976). Biological control of Verticillium malthousei by Trichoderma viride spray on casing soil in commercial mushroom production. Plant Disease Reporter 60, 677-680.
(Received for publication 23 March 1979)
Trans. Br. mycol. Soc. 74 (1) 191-194 (1980)
Primed in Great Britain
PARASITISM OF CARNATION RUST (UROMYCES DIANTHI) BY VERTICILLIUM LECANII By D. M. SPENCER Glasshouse Crops Research Institute, Littlehampton, W. Sussex, BN16 3PU Verticillium lecanii (Zimm.) Viegas was found infesting uredinia of Uromyces dianthi (Pers.) Niessl, the causal organism of carnation rust in a nursery. In the laboratory U. dianthi, inoculated on to carnation leaf pieces, caused typical rust symptoms, but in the presence of V. lecanii either rust infection was prevented or the formation of urediniospores was arrested, depending on the time of application of V. lecanii conidia. In a glasshouse, V. lecanii applied at the appropriate time reduced rust in carnation plants inoculated with U. dianthi.
In recent years there has been increasing attention given to the biological control of plant diseases (Mitchell, 1973; Baker & Cook, 1974), particularly of soil-borne diseases (Papavizas, 1973). The use of antagonistic micro-organisms as disease control agents has been studied (Hong & Ueyama, 1976; Ebben & Spencer, 1978) and the mode of action of some such antagonists has been investigated (Shigemitsu, Kunoh & Akai, 1978). The biological control of diseases other than those that are soil borne has received rather less attention (de Trogoff & Ricard, 1976; Jarvis & Slingsby, 1977) though Silveira & Rodrigues (1971) reported that Verticillium spp. caused bursting of the urediniospores of coffee rust (Hemileia vastatrix Berk. & Br.). Verticillium lecanii (Zimm.) Viegas has a wide host range; it parasitizes scale insects and aphids and has recently been reported by Hall, Allen & Leeming (1979) to colonize cultures of Erysiphe graminis DC., the causal organism of barley mildew, and of Uromyces appendiculatus (Pers.) Ung., the cause of dwarf bean rust. McKenzie & Hudson (1976) found V. lecanii to be a common mycoparasite of wheat stem rust caused by Puccinia graminis Pers. f.sp. tritici. In this paper the occurrence of V. lecanii on Uromyces dianthi (Pers.) Niessl is reported and observations are made on certain host-parasite interactions in laboratory and glasshouse experiments.
waxy nature of the leaf surface caused droplets to be shed by younger leaves, but discrete droplets were retained by the older lower leaves and the retention of spores on leaves was improved by addition of 0'05 % Teepol to the suspension. Immediately after inoculation plants were removed to conditions of high relative humidity where droplets of spore suspension did not evaporate. After 24 h plants were returned to the glasshouse bench where developing uredinia became visible some two weeks later. They appeared first as pale green spots, and the orange-brown urediniospores burst through the epidermis after 2-3 days. A development of this method was used in laboratory experiments when carnation leaf pieces f : 5 em long, floating on tap water in plastic drinking cups, were sprayed with a urediniospore suspension (c. 10· ml"). Polystyrene Petri dish lids were used to cover the drinking cups to maintain the requisite high humidity for 24 h. V. lecanii was found colonizing uredinia of carnation rust in a local nursery (Fig. 2). Its identity was confirmed by Dr R. A. Hall. The organism was maintained at 20°C on Czapek-Dox agar and ro-day-old cultures were used to provide conidial suspensions (c. 5 x 10· mlr! water) to study its effect on U. dianthi on carnation plants or leaf pieces. RESULTS
MATERIALS AND METHODS
Carnation rust was maintained on potted carnation plants (cv. William Sim) using the method described by Spencer (1979). Plants bearing uredinia were originally obtained from a local nursery (Fig. 1) to provide the inoculum. Stock plants were inoculated by spraying with a suspension of urediniospores (c. 10· ml- 1 ) in water. The
Spore germination Approximately 95 % of U. dianthi urediniospores kept in water droplets on glass slides at 25° germinated within 24 h. During this period the germ tubes grew to a mean length of c. 350 /lm. Under similar conditions c. 80 % of V. lecanii spores germinated and produced germ tubes of c. 20 /lm. In a mixed suspension of spores of the two fungi
0007-1536/80/2828-5630 $01.00 © 1980 The British Mycological Society
192
Parasitism of Uromyces by Verticillium
Fig. 1. Stem and leaves of carnation with normal uredinia of Uromyces dianthi. Fig. 2. Stem and leaves of carnation with Verticillium lecanii growing on coalesced uredinia of Uromyces dianthi.
the percentage germination of both remained unchanged, but whereas the mean length of the V. lecanii germ tubes increased to c. 100 pm with much branching, those of U. dianthi were restricted to c. 100 pm. Many of the U. dianthi germ tubes assumed a spiral configuration where V. lecanii hyphae were in close proximity but penetration of the U. dianthi germ tubes was not seen.
Effect of V. lecanii on infection of carnation leaf pieces by U. dianthi In a series of laboratory experiments carnation leaf pieces were inoculated with U. dianthi. V. lecanii spores were applied to the leaves either at the same time as the U. dianthi or at various times before and after the appearance of uredinia. It was found that carnation rust did not develop when a mixed suspension of U. dianthi urediniospores and V. lecanii conidia was used to inoculate
D. M. Spencer
193
the leaf pieces. When V . leeanii was sprayed on to leaf pieces up to 24 h before uredinia became erumpent the further development of uredinia and production of urediniospores was arre sted. A similar arrest was observed when V. leeanii was applied after uredinia had erupted, and in all these cases a weft of sporulating mycelium of V. leeanii was to be seen at the site of the uredinia.
developed rust to the same level as controls, but rust infection was much reduced if V. leeanii conidia were applied 24 h before or after inoculation with U. dianthi (Table 1). The number of erumpent uredinia was also much reduced when V. leeanii conidia were sprayed on to the plants after uredinia had begun to erupt.
Effect of V. lecanii on inf ection of carnation plants by U. dianthi In glasshouse experiments, using carnation plants (cv. William Sim) growing in compost, a significant level of rust control was achieved (Table 1). At intervals before or after the plants were inoculated with U. dianthi (T able 1), they were sprayed with conidia of V. leeanii (5 x 10s/ml) in water containing 0'05 % Teepol, to provide a
When V. leeanii was found naturally occurring on U. dianthi in a commercial carnation crop, an examination of its potential in disease control was immediately contemplated. Newhook (1957) showed that saprophytic leaf colonizers inhibited the germination of Botrytis cinerea Pers. ex Fr. conidia and prevented colonization of the leaves by B. cinerea. Though in the present experiments the percentage germination of U. dianthi urediniospores was not depressed, in the presence of germinating conidia of V. lecanii, the growth of germ tubes was inhibited. This could be associated with the reduced level of rust found when carnation plants were sprayed with V. leeanii conidia at the same time as they were inoculated with U. dianthi. These results support the work of Fokkema & Lorbeer (1974) who found that growth of germ tubes of Alternaria porri (Ell.) Neerg. was reduced in the presence of Aureobasidium pullulans (de Bary) Am. and Sporobolomyees roseus Kluyver & v. Niel and the number of lesions caused by A. porri was reduced bye. 50 %. Hashioka (1973) classified mycoparasites as either necrotrophic, where the parasite destroys its host , or biotrophic, where the parasite acts by means of haustoria or through indirect reactions by which the host is less obviously harmed. The lack of visible damage to U. dianthi spores or to their germ tubes in these investigations suggests that V . leeanii is a biotrophic parasite of U. dianthi . Apart from the direct parasitism of U. dianthi by - V. leeanii there are two further possibilities to be considered as the basis of the reduction of carnation rust shown here. The first of these is the possibility that inhibition of germination of urediniospores of U. dianthi is brought about by phytoalexins, These substances would be induced in the carnation in response to drops of spore suspension of V. leeanii but though this might account for a reduction in level of rust when V. leeanii and U. dianthi spores were applied at or about the same time it would be less likely to account for reduction of rust achieved when V. leeanii was applied at the end of the rustincubation period. Furthermore, the very poor contact between droplets of spore suspension
Table 1. Effect on number of uredinia appearing on carnation plants sprayed with conidia of V. lecanii before or after inoculation with U. dianthi Number of uredinia (means of fiveplants) Experiment ~
Treatment U. dianthi only V. leeanii applied 48 h before U, dianthi inoculation V . leeanii applied 24 h before U. dianrhi inoculation V. lecanii applied 24 h after U, dianthi inoculation V . lecanii applied as first U, dianthi uredinia erupt V. lecanii applied 1 wk after U, dianrhi uredinia erupt
__
1 109'8 115'2
,
~ A
2 104'6 1°4'6 57'6
44'0
42'0
37'4
48'4
34'8
43'2
cover of discrete droplets, mainly on lower leaves. Sprayed plants were kept in conditions of high relative humidity for 24 h to allow rust infection to occur, for the conidia of V . leeanii to germinate, and possibly, to permit the establishment of V. leeanii on U. dianthi. Afterwards, plants were returned to the glasshouse to await further development of rust. The level of rust infection was assessed as the total number of erumpent uredinia which appeared on each of five replicate plants in each treatment, the final assessment being made 3 wk after eruption of the first uredinia, Plants sprayed with V. leeanii conidia 48 h before they were inoculated with U. dianthi 7
DISCUSSION
MYC
74
194
Parasitism of Uromyces by Verticillium
and the waxy surface of the carnation leaf would not be conducive to the accumulation of a fungitoxic concentration of phytoalexin. The second possibility is that substances released from the rust-infected leaf serve as nutrients for V. lecanii, the growth of which then becomes so vigorous as to form a physical barrier to further progress of the rust organism. This possibility is not supported by observations made on suspensions of germinating spores nor at the leaf surface, even though here there is often a very luxuriant growth of V. lecanii, The glasshouse experiments indicate that the use of V. lecanii can provide a measure of control of U. dianthi. The level of this control might be improved if it could be ensured that U. dianthi were more effectively exposed to attack by V. lecanii by the use of wetting agents to aid dispersion of the V. lecanii conidia. The widespread natural occurrence of V. lecanii on the nursery also indicates that careful manipulation of the glasshouse environment might be used to achieve better control of rust. If V. lecanii were to be sprayed on to the crop as a means of biological control the indications are that it would be used as a prophylactic rather than as a protectant since the conidia applied to plants more than 24 h before inoculation with rust gave no benefit. It would seem therefore that conidia are either inherently short-lived or are sensitive to environmental stresses such as desiccation. The protection of the environment is receiving ever-increasing attention and the possibility of controlling plant disease by biological means rather than with chemicals thus merits serious consideration. The protected environment of the 11 h . If . ul 1 glass h ouse 1en d s itse partie ar y we to t e use of biological control agents, and further work on the nature of the inhibition of U. dianthi by V. lecanii is justified, as is a study of the level of rust control to be achieved in commercial growing conditions. If V. lecanii were to be shown to be particularly attractive in biological control, the fact that it is of widespread occurrence in commercial carnation houses should have some bearing on deliberations concerning its clearance under existing safety regulations.
REFERENCES BAKER, K. F. & COOK, R. J. (1974). Biological Control of Plant Pathogens. W. H. Freeman & Company, San Francisco. EBBEN, M. H. & SPENCER, D. M. (1978). The use of antagonistic organisms for the control of black root rot of cucumber, Phomopsis sclerotioides. Annals of Applied Biology 89, 103-106. FOKKEMA, N. J. & LORBEER, J. W. (1974). Interactions between Alternaria porri and the saprophytic microflora of onion leaves. Phytopathology 64, 1128-1133. HALL, R. A., ALLEN, D. J. & LEEMING, A. R. (1979). Parasitism and host specificity amongst isolates of Verticillium lecanii on insects and phytopathogenic fungi. (In preparation.) HASHIOKA, Y. (1973)· Mycoparasitism in relation to phytopathogens, Shohubutsu Byoga Kenkyu 8, 179- 1 90. HONG, C. Y. & UEYAMA, A. (1976). Ecological control of soil-borne diseases with special reference to enhancement of antagonism. Agriculture and Horticulture 51, 7 29-734. JARVIS, W. R. & SLINGSBY, K. (1977). The control of powdery mildew of greenhouse cucumber by water sprays and Ampelomyces quisqualis. Plant Disease Reporter 61, 728-730. McKENZIE, E. H. C. & HUDSON, H. J. (1976). Mycoflora of rust-infected and non-infected plant material during decay. Transactionsof the British Mycological Society 66, 223-238. MITCHELL, J. E. (1973). The mechanisms of biological control of plant diseases. Soil Biology and Biochemistry 5, 721-728. NEWHOOK, F. J. (1957). The relationship of saprophytic antagonism to control of Botrytis cinerea Pers. on tomatoes. New Zealand Journal of Science and Technology A 38, 473-481. PAPAVIZAS, G. C. (1973)· Status of applied biological control of soil-borne plant pathogens. Soil Biology and Biochemistry 5, 709-720 . SHIGEMITSU, H., KUNOH, H. & AKAI, S. (1978). Scanning electron microscopic studies of Corticium rolfsii sclerotia parasitized by Aspergillus terreus. Mycologia 70, 935-943. SILVEIRA, H. L. & RODRIGUES, C. J. (1971). Bursting of rust uredospores caused by Verticillium hemileiae culture filtrates. Agronomia lusitana 33, 391-396. SPENCER, D. M. (1979)· Carnation rust and its control by systemic fungicides. Plant Pathology. (In the Press.) TROGOFF, H. DE & RICARD, J. L. (1976). Biological control of Verticillium malthousei by Trichoderma viride spray on casing soil in commercial mushroom production. Plant Disease Reporter 60, 677-680.
(Received for publication 23 March 1979)