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Screening of apple germplasm for susceptibility to Pythium ultimum Trow during different seasons
Scientia Horticulturae, 40 (1989) 63-69 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
63
Screening of Apple Germplasm for Susceptibility to Pythium ultimum Trow During Different Seasons I.M. SHARMA and V.K. GUPTA
University of Horticulture and Forestry, Horticultural Regional Research Station, Sharbo, District Kinnaur, P.O. Rackong Peo, Himachel Pradesh 172107 (India) (Accepted for publication 13 December 1988)
ABSTRACT
Sharma, I.M. and Gupta, V.K., 1989. Screening of apple germplasm for susceptibility to Pythium ultimum Trow during different seasons. Scientia Hortic., 40: 63-69. Twenty-two apple rootstocks were screened against Pythium ultimum Trow in the field. Six, including M2, M4, M M l l 0 , M M l l 4 , M M l l 5 and Crab apple were highly resistant, 9 were resistant, 5 were susceptible and 2 (MM103 and MM104) were highly susceptible. In the field, maximum susceptibility of apple rootstocks was found during March (bud-swell stage) followed by July (maximum growth period) and minimum in November (leaf-fall stage). In the laboratory, these growth periods had no influence on the susceptibility of apple rootstocks and apple cultivars. Keywords: apple rootstock, collar rot, growth period, Pythium ultimum.
INTRODUCTION
In Himachal Pradesh, Rana (1981) established that in addition to Phytophthora cactorum, Pythium ultimum was also associated with collar-rot-affected plants. The presence of both pathogens in the same localities seems to aggravate the disease and increase losses. In 1975, Pythium ultimum caused a severe epidemic in Poland on 6-year-old apple trees (Bielenin et al., 1976). Apple rootstocks have been tested against P. cactorum in different countries, but unfortunately apple rootstocks considered resistant in some regions have been found susceptible in other regions, thus making the problem more complicated (McIntosh, 1975). In India, Gupta and Rana (1983) screened different rootstocks and cultivars against collar-rot pathogens by the twig inoculation method (Borecki and Millikan, 1969) and showed that in addition to P. cactorum, Pythium ultimum was also virulent on different rootstocks. However, information on this aspect is lacking under field conditions. 0304-4238/89/$03.50
© 1989 Elsevier Science Publishers B.V.
64 Growth stage and plant age have also exhibited a marked influence on the susceptibility of apple to collar rot, caused by P. cactorum. Bielenin (1977) demonstrated that susceptibility in apple trees varied during the growing season in the field, phytotron and laboratory. In the field, susceptibility was greater from the pink-tip stage to the end of the flowering season. Gupta and Singh (1979) found that maximum susceptibility of apple plants to P. cactorum in the field was observed in April followed by July and was three times less in September. In the laboratory, there were no significant effects of these different growth periods on the susceptibility of apple twigs. The present study was conducted to establish the effect of different growth periods on the susceptibility of apple rootstocks and cultivars to P y t h i u m ultimum.
MATERIALSAND METHODS F u n g u s . - The culture of P y t h i u m u l t i m u m was obtained from the Department of Mycology and Plant Pathology, Himachal Pradesh Agriculture University, Solan, India. It was isolated from apple soils of Himachal Pradesh and was maintained under active growth on potato-dextrose agar medium. Pathogenicity was shown on apple seedlings and apple twigs in the laboratory and on apple seedlings in the field. P l a n t material. - The following 22 apple rootstocks and twigs of 12 apple cul-
tivars were procured from the Department of Horticulture, Solan, and Regional Fruit Research Station, Mashobra of the University. A p p l e rootstocks. - M2, M4, M5, M7, M9, M12, M17, M20, M25, M26, MM101,
MM103, MM104, MM106, M M l l 0 , M M l l l , M M l l 2 , M M l l 4 , M M l l 5 , Crab apple and apple seedlings were studied. A p p l e cultivars. - 'Royal Delicious', 'Red Delicious', 'Richared', 'Red Gold',
'Golden Delicious', 'Tydeman Worcester', 'Early Redbird', 'Granny Smith', 'McIntosh', 'Allington Pippin', 'Baldwin' and 'Lady Sudley'. Inoculation in the field S c r e e n i n g of g e r m p l a s m . - Four plants each of the above rootstocks were screened in the field against P y t h i u m u l t i m u m by the method suggested by
Gupta and Mir (1983). The soil around the collar region was removed up to the root zone. A polythene bag (8 × 4" ) with both ends open was tied around the rootstock, in such a way that the lower end of the bag touched the root zone of the rootstock and the upper end was pushed toward the root zone and tied round the stem with a rubber band. The polythene bag was inverted in such a
65 way that the tied end remained inside the polythene bag and formed a tubelike structure around the collar portion. The bag was filled with soil up to the collar zone. The injury was given to the collar portion with a sterilized cork borer (4 mm). The soil was then drenched with water. Two mycelial parts of actively growing culture were placed in the central portion of the injured area. The injured portion was then covered with soil and the bag was covered with a polythene disc in order to maintain the moisture. The lesion developed at the collar portion below the soil level on the rootstock and was measured after 20, 40 and 60 days of inoculation. Effect of seasons on lesion development on different apple rootstocks in the field. Three plants each of 10 rootstocks, M7, M9, M12, M16, M25, MM101, MM103, MM104, MM106 and MM111, were inoculated on 24 March, 26 July and 20 November 1983 in the field by the method developed by Gupta and Mir (1983), as described above, with a view to finding out the optimum time of the year for disease development. Lesion size was recorded after 20 days of inoculation. -
Inoculation in the laboratory by the twig inoculation method The reaction of the above 10 rootstocks and 12 cultivars was seen in the laboratory during different seasons (i.e. in spring, rainy weather and winter) by the twig inoculation method given by Borecki and Millikan (1969). Sixtymm stem sections from the rootstock and cultivar from the previous year's growth were taken and bark was removed to expose the phloem cambium with a cork borer. These twigs were placed in Petri dishes (90 mm) having moist filter paper on both sides. A 4-mm disc of actively growing culture was placed in the central exposed portion of each twig. T h e n plates containing twigs were incubated at 24 + 1 ° C for 15 days. One infection point per twig was given. RESULTS Symptomatology. - Symptoms produced by P y t h i u m ultimum on the highly susceptible rootstock MM104 were observed after 40 days of infection. Rotting of the tissue started from the point of infection, increased longitudinally, and developed into dark brown to black coloured lesions. Plants showed poor growth and chlorotic foliage. Reaction of apple rootstock in the field. - Apple rootstocks showed variation in their reaction to P y t h i u m ultimum (Table 1 ). Six (M2, M4, M M l l 0 , M M l l 4 , M M l l 5 and Crab apple) showed minimum lesion size of up to 9 mm and 9 (M5, M16, M17, M20, M25, M26, MM106, M M l l 2 and apple seedling) had a lesion size from 9 to 19 mm. All these were categorized as resistant. Five were found susceptible, two (MM103 and MM104) were highly susceptible.
66 TABLE 1 Reaction of different apple rootstocks to Pythium ultimum in the field, expressed in lesion size {mm) Rootstock
M2 M4 M5 M7 M9 M12 M16 M17 M20 M25 M26 MM101 MM103 MM104 MM106 MM110 MMlll MM112 MMl14 MMll5 Crab apple Apple seedlings Mean
Days after inoculation 20
40
60
Mean
3.16 2.83 8.33 9.33 13.50 7.33 5.00 6.60 8.33 6.50 6.48 6.33 14.18 17.56 4.00 2.50 5.83 7.60 3.83 1.33 2.66 6.83 6.97
4.00 5.16 9.33 16.60 23.33 12.36 7.83 9.16 12.36 9.16 9.33 12.50 23.33 26.16 8.26 4.33 12.16 13.33 5.16 3.33 3.66 9.16 10.99
5.00 6.56 11.20 20.53 27.83 20.36 12.33 14.16 17.53 18.16 11.23 20.53 33.66 37.33 11.16 5.16 19.33 16.33 5.50 3.83 5.00 11.33 15.09
4.05 4.85 9.62 15.48 21.55 13.34 8.38 9.82 12.74 11.27 9.01 13.12 23.72 27.01 7.84 3.99 12.44 12.42 4.83 2.83 3.77 9.10 10.95
LSD (P<0.05) between rootstocks=0.45, between day=0.16, and interaction between rootstocks and season = 0.78; averages of three replicates.
Effect of seasonal fluctuations on the disease development. Apple rootstocks in the field. - A f t e r i n o c u l a t i o n on 24 M a r c h all apple rootst o c k s p r o d u c e d larger lesions t h a n a f t e r i n o c u l a t i o n on t h e later p l a n t s (Table 2 ). Results in t h e s p r i n g s e a s o n were closely followed by the r a i n y season, b u t r o o t s t o c k s were less susceptible d u r i n g w i n t e r w h e n M16, M25 a n d M M 1 0 6 were n o t affected. R o o t s t o c k s M M 1 0 3 a n d M M 1 0 4 h a d only 2.33 a n d 2.15 m m lesion sizes d u r i n g w i n t e r in c o m p a r i s o n w i t h 13.5 a n d 16.66 m m in t h e r a i n y season.
Rootstock and apple cultivar twigs in the laboratory. - - T h e r e were no signific a n t differences ( T a b l e s 3 a n d 4 ) in t h e susceptibility o f apple r o o t s t o c k s a n d apple cultivars d u r i n g t h e different periods of g r o w t h ( s e a s o n s ) u n d e r t h e twig inoculation method, except in M9, MM104, ' R o y a l Delicious', ' R e d Gold', ' E a r l y
67 TABLE 2 Effect of different seasonal dates of inoculation on lesion development (mm) by Pythiurn ultimum on different rootstocks in the field Rootstock
M7 M9 M12 M16 M25 MM101 MM103 MM104 MM106 MMlll Mean
Dates of inoculation 24 March
26 July
20 November
9.33 14.83 8.66 5.33 6.50 7.16 14.83 17.33 4.33 7.00 9.53
7.33 13.00 8.00 5.16 4.00 5.50 13.50 16.66 4.00 6.50 8.36
1.16 2.16 1.33 0.00 0.00 0.50 2.33 2.16 0.00 0.33 0.99
(3.21) {3.97) (3.10) (2.50) (2.73) {2.85) (3.97) (4.28) (2.30) (2.82)
(2.88) (3.73) (2.89) (2.47) (2.23) (2.54) (3.80) (4.20) (2.23) (2.73)
(1.46) (1.77) (1.52) (1.00) (1.00) (1.11) (1.82) (1.77) (1.00) {1.13)
LSD (P<0.05) between rootstocks=0.094, between season=0.051, interaction between rootstocks and season = 0.16. Values in parenthesis are x/n + 1. TABLE3 Effect of different dates of inoculation during different seasons on lesion size (mm) by Pythium ultimum on apple rootstock twigs in the laboratory Rootstock
M7 M9 M12 M16 M25 MM101 MM103 MM104 MM106 MMlll Mean
Dates of inoculation 24 March
26 July
20 November
8.56 15.20 9.16 7.33 6.96 7.33 16.66 19.33 7.66 6.33 10.45
8.16 15.03 10.16 7.16 7.16 8.00 16.50 17.70 7.83 5.83 10.35
8.16 13.40 8.33 6.66 6.66 6.53 16.00 16.00 7.83 6.16 9.60
LSD (P < 0.05) between rootstocks = 0.46, between season = 0.25, interaction between rootstocks and season = 0.80. Redbird' and 'McIntosh'. In the case of MM104, the spring season was significantly better for development of the disease than the rainy season, which in turn was superior to the winter season. In M9 apple rootstocks and 'Royal
68 TABLE 4 Effect of different dates of inoculation during different seasons on lesion size (mm) by Pythium
ultimum on apple cultivar twigs in the laboratory Rootstock
'Royal Delicious' 'Red Delicious' 'Richared' 'Red Gold' 'Golden Delicious' 'Tydeman Worcester' 'Early Redbird' 'Granny Smith' 'McIntosh' 'Allington pippin' 'Baldwin' 'Lady Sudley' Mean
Dates of inoculation 1 24 March
26 July
20 November
15.90 10.26 14.30 14.50 11.83 16.63 24.70 11.03 14.50 20.56 11.33 10.66 14.68
15.86 10.10 14.26 14.40 11.40 14.93 14.50 10.16 14.00 19.13 10.83 11.20 14.23
12.50 9.06 14.16 11.73 11.83 14.96 22.20 9.96 11.83 19.50 10.00 9.66 13.11
LSD (P < 0.05) between cultivars = 0.72, between season = 0.36, interaction between cultivar and season -- 1.25.
Delicious', 'Red Gold', 'Early Redbird' and 'McIntosh' cultivars both the spring and rainy season were statistically equally effective and were superior to the winter season in causing disease on twigs in the laboratory. Amongst the apple cultivars 'Early Redbird' was most susceptible followed by 'Allington Pippin', 'Royal Delicious', 'Richared', 'Red Gold', 'Tydeman Worcester' and 'McIntosh'. 'Red Delicious', 'Golden Delicious', 'Granny Smith' and 'Lady Sudley' were resistant. DISCUSSION
Disease intensity in a crop depends upon three factors: host susceptibility; pathogen virulence; environmental factors. McIntosh (1975) pointed out differential susceptibility of apple rootstocks to P. cactorum in different regions of the world, which was attributed to probable variability in the pathogen and/ or screening at different plant growth stages. The susceptibility of apple under field conditions to P. cactorum (Krober and Karnartz, 1979; Gupta and Singh, 1979 ) and Pythium ultimum (Bielenin et al., 1976) is known to be affected by seasonal changes and these are most susceptible during the bud-swelling stage. Gupta and Singh (1979) further showed that 2-year-old apple twigs during different seasons did not show any difference in the susceptibility under laboratory conditions. Similar results are presented in the present case. This in-
69
dicates that, in addition to host physiology, season plays a major role in the susceptibility of apple under field conditions.
REFERENCES Bielenin, A., 1977. Collar rot of apple trees. II. Seasonal fluctuation in susceptibility of apple trees to Phytophthora cactorum (Leb and Cohn) Schroet. Fruit Sci. Rep., 4: 27-39. Bielenin, A., Borecki, Z. and Millikan, D.F., 1976. Identification of Pythium ultimum in collar rot of apple. Phytopathology, 66: 127-129. Borecki, Z. and Millikan, D.F., 1969. A rapid method for determinating pathogenicity of Phytophthora cactorum. Phytopathology, 59: 247-248. Gupta, V.K. and Singh, K., 1979. Factors affecting the development of collar rot (Phytophthora cactorum) of apple. Gartenbauwissenschaft, 44: 29-32. Gupta, V.K. and Mir, M.M., 1983. Field testing of apple root stocks against Phytophthora cacto rum - - A new technique. J. Tree Sci., 2: 81-83. Gupta, V.K. and Rana, K.S., 1983. Differential reaction of apple rootstocks and cultivars to pythiaceous fungi associated with collar rot. In: National Symposium Advance in Tree Sciences, 11-12 April, 1983. Dehra Dun. {Abstract) p. 60. Krober, H. and Karnartz, A., 1979. Susceptibility of apple cultivars to Phytophthora cactorum as influenced by different factors. Z. Pflanzenkr. Pflanzenschutz, 86: 1-11. McIntosh, D.L., 1975. Proceedings of 1974 APWD workshop on crown rot of apple trees. Can. Plant Dis. Surv., 55: 109-116. Rana, K.S., 1981. Population dynamics of Phytophthora cactorum (Leb. and Cohn ) Schroet. in relation to soil factors and fungicides. Ph.D. Thesis, H.P. Krishi Vishva Vidyalaya, College of Agriculture, Solan, India. 146 pp.
63
Screening of Apple Germplasm for Susceptibility to Pythium ultimum Trow During Different Seasons I.M. SHARMA and V.K. GUPTA
University of Horticulture and Forestry, Horticultural Regional Research Station, Sharbo, District Kinnaur, P.O. Rackong Peo, Himachel Pradesh 172107 (India) (Accepted for publication 13 December 1988)
ABSTRACT
Sharma, I.M. and Gupta, V.K., 1989. Screening of apple germplasm for susceptibility to Pythium ultimum Trow during different seasons. Scientia Hortic., 40: 63-69. Twenty-two apple rootstocks were screened against Pythium ultimum Trow in the field. Six, including M2, M4, M M l l 0 , M M l l 4 , M M l l 5 and Crab apple were highly resistant, 9 were resistant, 5 were susceptible and 2 (MM103 and MM104) were highly susceptible. In the field, maximum susceptibility of apple rootstocks was found during March (bud-swell stage) followed by July (maximum growth period) and minimum in November (leaf-fall stage). In the laboratory, these growth periods had no influence on the susceptibility of apple rootstocks and apple cultivars. Keywords: apple rootstock, collar rot, growth period, Pythium ultimum.
INTRODUCTION
In Himachal Pradesh, Rana (1981) established that in addition to Phytophthora cactorum, Pythium ultimum was also associated with collar-rot-affected plants. The presence of both pathogens in the same localities seems to aggravate the disease and increase losses. In 1975, Pythium ultimum caused a severe epidemic in Poland on 6-year-old apple trees (Bielenin et al., 1976). Apple rootstocks have been tested against P. cactorum in different countries, but unfortunately apple rootstocks considered resistant in some regions have been found susceptible in other regions, thus making the problem more complicated (McIntosh, 1975). In India, Gupta and Rana (1983) screened different rootstocks and cultivars against collar-rot pathogens by the twig inoculation method (Borecki and Millikan, 1969) and showed that in addition to P. cactorum, Pythium ultimum was also virulent on different rootstocks. However, information on this aspect is lacking under field conditions. 0304-4238/89/$03.50
© 1989 Elsevier Science Publishers B.V.
64 Growth stage and plant age have also exhibited a marked influence on the susceptibility of apple to collar rot, caused by P. cactorum. Bielenin (1977) demonstrated that susceptibility in apple trees varied during the growing season in the field, phytotron and laboratory. In the field, susceptibility was greater from the pink-tip stage to the end of the flowering season. Gupta and Singh (1979) found that maximum susceptibility of apple plants to P. cactorum in the field was observed in April followed by July and was three times less in September. In the laboratory, there were no significant effects of these different growth periods on the susceptibility of apple twigs. The present study was conducted to establish the effect of different growth periods on the susceptibility of apple rootstocks and cultivars to P y t h i u m ultimum.
MATERIALSAND METHODS F u n g u s . - The culture of P y t h i u m u l t i m u m was obtained from the Department of Mycology and Plant Pathology, Himachal Pradesh Agriculture University, Solan, India. It was isolated from apple soils of Himachal Pradesh and was maintained under active growth on potato-dextrose agar medium. Pathogenicity was shown on apple seedlings and apple twigs in the laboratory and on apple seedlings in the field. P l a n t material. - The following 22 apple rootstocks and twigs of 12 apple cul-
tivars were procured from the Department of Horticulture, Solan, and Regional Fruit Research Station, Mashobra of the University. A p p l e rootstocks. - M2, M4, M5, M7, M9, M12, M17, M20, M25, M26, MM101,
MM103, MM104, MM106, M M l l 0 , M M l l l , M M l l 2 , M M l l 4 , M M l l 5 , Crab apple and apple seedlings were studied. A p p l e cultivars. - 'Royal Delicious', 'Red Delicious', 'Richared', 'Red Gold',
'Golden Delicious', 'Tydeman Worcester', 'Early Redbird', 'Granny Smith', 'McIntosh', 'Allington Pippin', 'Baldwin' and 'Lady Sudley'. Inoculation in the field S c r e e n i n g of g e r m p l a s m . - Four plants each of the above rootstocks were screened in the field against P y t h i u m u l t i m u m by the method suggested by
Gupta and Mir (1983). The soil around the collar region was removed up to the root zone. A polythene bag (8 × 4" ) with both ends open was tied around the rootstock, in such a way that the lower end of the bag touched the root zone of the rootstock and the upper end was pushed toward the root zone and tied round the stem with a rubber band. The polythene bag was inverted in such a
65 way that the tied end remained inside the polythene bag and formed a tubelike structure around the collar portion. The bag was filled with soil up to the collar zone. The injury was given to the collar portion with a sterilized cork borer (4 mm). The soil was then drenched with water. Two mycelial parts of actively growing culture were placed in the central portion of the injured area. The injured portion was then covered with soil and the bag was covered with a polythene disc in order to maintain the moisture. The lesion developed at the collar portion below the soil level on the rootstock and was measured after 20, 40 and 60 days of inoculation. Effect of seasons on lesion development on different apple rootstocks in the field. Three plants each of 10 rootstocks, M7, M9, M12, M16, M25, MM101, MM103, MM104, MM106 and MM111, were inoculated on 24 March, 26 July and 20 November 1983 in the field by the method developed by Gupta and Mir (1983), as described above, with a view to finding out the optimum time of the year for disease development. Lesion size was recorded after 20 days of inoculation. -
Inoculation in the laboratory by the twig inoculation method The reaction of the above 10 rootstocks and 12 cultivars was seen in the laboratory during different seasons (i.e. in spring, rainy weather and winter) by the twig inoculation method given by Borecki and Millikan (1969). Sixtymm stem sections from the rootstock and cultivar from the previous year's growth were taken and bark was removed to expose the phloem cambium with a cork borer. These twigs were placed in Petri dishes (90 mm) having moist filter paper on both sides. A 4-mm disc of actively growing culture was placed in the central exposed portion of each twig. T h e n plates containing twigs were incubated at 24 + 1 ° C for 15 days. One infection point per twig was given. RESULTS Symptomatology. - Symptoms produced by P y t h i u m ultimum on the highly susceptible rootstock MM104 were observed after 40 days of infection. Rotting of the tissue started from the point of infection, increased longitudinally, and developed into dark brown to black coloured lesions. Plants showed poor growth and chlorotic foliage. Reaction of apple rootstock in the field. - Apple rootstocks showed variation in their reaction to P y t h i u m ultimum (Table 1 ). Six (M2, M4, M M l l 0 , M M l l 4 , M M l l 5 and Crab apple) showed minimum lesion size of up to 9 mm and 9 (M5, M16, M17, M20, M25, M26, MM106, M M l l 2 and apple seedling) had a lesion size from 9 to 19 mm. All these were categorized as resistant. Five were found susceptible, two (MM103 and MM104) were highly susceptible.
66 TABLE 1 Reaction of different apple rootstocks to Pythium ultimum in the field, expressed in lesion size {mm) Rootstock
M2 M4 M5 M7 M9 M12 M16 M17 M20 M25 M26 MM101 MM103 MM104 MM106 MM110 MMlll MM112 MMl14 MMll5 Crab apple Apple seedlings Mean
Days after inoculation 20
40
60
Mean
3.16 2.83 8.33 9.33 13.50 7.33 5.00 6.60 8.33 6.50 6.48 6.33 14.18 17.56 4.00 2.50 5.83 7.60 3.83 1.33 2.66 6.83 6.97
4.00 5.16 9.33 16.60 23.33 12.36 7.83 9.16 12.36 9.16 9.33 12.50 23.33 26.16 8.26 4.33 12.16 13.33 5.16 3.33 3.66 9.16 10.99
5.00 6.56 11.20 20.53 27.83 20.36 12.33 14.16 17.53 18.16 11.23 20.53 33.66 37.33 11.16 5.16 19.33 16.33 5.50 3.83 5.00 11.33 15.09
4.05 4.85 9.62 15.48 21.55 13.34 8.38 9.82 12.74 11.27 9.01 13.12 23.72 27.01 7.84 3.99 12.44 12.42 4.83 2.83 3.77 9.10 10.95
LSD (P<0.05) between rootstocks=0.45, between day=0.16, and interaction between rootstocks and season = 0.78; averages of three replicates.
Effect of seasonal fluctuations on the disease development. Apple rootstocks in the field. - A f t e r i n o c u l a t i o n on 24 M a r c h all apple rootst o c k s p r o d u c e d larger lesions t h a n a f t e r i n o c u l a t i o n on t h e later p l a n t s (Table 2 ). Results in t h e s p r i n g s e a s o n were closely followed by the r a i n y season, b u t r o o t s t o c k s were less susceptible d u r i n g w i n t e r w h e n M16, M25 a n d M M 1 0 6 were n o t affected. R o o t s t o c k s M M 1 0 3 a n d M M 1 0 4 h a d only 2.33 a n d 2.15 m m lesion sizes d u r i n g w i n t e r in c o m p a r i s o n w i t h 13.5 a n d 16.66 m m in t h e r a i n y season.
Rootstock and apple cultivar twigs in the laboratory. - - T h e r e were no signific a n t differences ( T a b l e s 3 a n d 4 ) in t h e susceptibility o f apple r o o t s t o c k s a n d apple cultivars d u r i n g t h e different periods of g r o w t h ( s e a s o n s ) u n d e r t h e twig inoculation method, except in M9, MM104, ' R o y a l Delicious', ' R e d Gold', ' E a r l y
67 TABLE 2 Effect of different seasonal dates of inoculation on lesion development (mm) by Pythiurn ultimum on different rootstocks in the field Rootstock
M7 M9 M12 M16 M25 MM101 MM103 MM104 MM106 MMlll Mean
Dates of inoculation 24 March
26 July
20 November
9.33 14.83 8.66 5.33 6.50 7.16 14.83 17.33 4.33 7.00 9.53
7.33 13.00 8.00 5.16 4.00 5.50 13.50 16.66 4.00 6.50 8.36
1.16 2.16 1.33 0.00 0.00 0.50 2.33 2.16 0.00 0.33 0.99
(3.21) {3.97) (3.10) (2.50) (2.73) {2.85) (3.97) (4.28) (2.30) (2.82)
(2.88) (3.73) (2.89) (2.47) (2.23) (2.54) (3.80) (4.20) (2.23) (2.73)
(1.46) (1.77) (1.52) (1.00) (1.00) (1.11) (1.82) (1.77) (1.00) {1.13)
LSD (P<0.05) between rootstocks=0.094, between season=0.051, interaction between rootstocks and season = 0.16. Values in parenthesis are x/n + 1. TABLE3 Effect of different dates of inoculation during different seasons on lesion size (mm) by Pythium ultimum on apple rootstock twigs in the laboratory Rootstock
M7 M9 M12 M16 M25 MM101 MM103 MM104 MM106 MMlll Mean
Dates of inoculation 24 March
26 July
20 November
8.56 15.20 9.16 7.33 6.96 7.33 16.66 19.33 7.66 6.33 10.45
8.16 15.03 10.16 7.16 7.16 8.00 16.50 17.70 7.83 5.83 10.35
8.16 13.40 8.33 6.66 6.66 6.53 16.00 16.00 7.83 6.16 9.60
LSD (P < 0.05) between rootstocks = 0.46, between season = 0.25, interaction between rootstocks and season = 0.80. Redbird' and 'McIntosh'. In the case of MM104, the spring season was significantly better for development of the disease than the rainy season, which in turn was superior to the winter season. In M9 apple rootstocks and 'Royal
68 TABLE 4 Effect of different dates of inoculation during different seasons on lesion size (mm) by Pythium
ultimum on apple cultivar twigs in the laboratory Rootstock
'Royal Delicious' 'Red Delicious' 'Richared' 'Red Gold' 'Golden Delicious' 'Tydeman Worcester' 'Early Redbird' 'Granny Smith' 'McIntosh' 'Allington pippin' 'Baldwin' 'Lady Sudley' Mean
Dates of inoculation 1 24 March
26 July
20 November
15.90 10.26 14.30 14.50 11.83 16.63 24.70 11.03 14.50 20.56 11.33 10.66 14.68
15.86 10.10 14.26 14.40 11.40 14.93 14.50 10.16 14.00 19.13 10.83 11.20 14.23
12.50 9.06 14.16 11.73 11.83 14.96 22.20 9.96 11.83 19.50 10.00 9.66 13.11
LSD (P < 0.05) between cultivars = 0.72, between season = 0.36, interaction between cultivar and season -- 1.25.
Delicious', 'Red Gold', 'Early Redbird' and 'McIntosh' cultivars both the spring and rainy season were statistically equally effective and were superior to the winter season in causing disease on twigs in the laboratory. Amongst the apple cultivars 'Early Redbird' was most susceptible followed by 'Allington Pippin', 'Royal Delicious', 'Richared', 'Red Gold', 'Tydeman Worcester' and 'McIntosh'. 'Red Delicious', 'Golden Delicious', 'Granny Smith' and 'Lady Sudley' were resistant. DISCUSSION
Disease intensity in a crop depends upon three factors: host susceptibility; pathogen virulence; environmental factors. McIntosh (1975) pointed out differential susceptibility of apple rootstocks to P. cactorum in different regions of the world, which was attributed to probable variability in the pathogen and/ or screening at different plant growth stages. The susceptibility of apple under field conditions to P. cactorum (Krober and Karnartz, 1979; Gupta and Singh, 1979 ) and Pythium ultimum (Bielenin et al., 1976) is known to be affected by seasonal changes and these are most susceptible during the bud-swelling stage. Gupta and Singh (1979) further showed that 2-year-old apple twigs during different seasons did not show any difference in the susceptibility under laboratory conditions. Similar results are presented in the present case. This in-
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dicates that, in addition to host physiology, season plays a major role in the susceptibility of apple under field conditions.
REFERENCES Bielenin, A., 1977. Collar rot of apple trees. II. Seasonal fluctuation in susceptibility of apple trees to Phytophthora cactorum (Leb and Cohn) Schroet. Fruit Sci. Rep., 4: 27-39. Bielenin, A., Borecki, Z. and Millikan, D.F., 1976. Identification of Pythium ultimum in collar rot of apple. Phytopathology, 66: 127-129. Borecki, Z. and Millikan, D.F., 1969. A rapid method for determinating pathogenicity of Phytophthora cactorum. Phytopathology, 59: 247-248. Gupta, V.K. and Singh, K., 1979. Factors affecting the development of collar rot (Phytophthora cactorum) of apple. Gartenbauwissenschaft, 44: 29-32. Gupta, V.K. and Mir, M.M., 1983. Field testing of apple root stocks against Phytophthora cacto rum - - A new technique. J. Tree Sci., 2: 81-83. Gupta, V.K. and Rana, K.S., 1983. Differential reaction of apple rootstocks and cultivars to pythiaceous fungi associated with collar rot. In: National Symposium Advance in Tree Sciences, 11-12 April, 1983. Dehra Dun. {Abstract) p. 60. Krober, H. and Karnartz, A., 1979. Susceptibility of apple cultivars to Phytophthora cactorum as influenced by different factors. Z. Pflanzenkr. Pflanzenschutz, 86: 1-11. McIntosh, D.L., 1975. Proceedings of 1974 APWD workshop on crown rot of apple trees. Can. Plant Dis. Surv., 55: 109-116. Rana, K.S., 1981. Population dynamics of Phytophthora cactorum (Leb. and Cohn ) Schroet. in relation to soil factors and fungicides. Ph.D. Thesis, H.P. Krishi Vishva Vidyalaya, College of Agriculture, Solan, India. 146 pp.