Establishment of Seedling Test for Resistance to Phytophtora capsici Leonian in Cucurbita.

Cucurbit Genetics Cooperative Report 9:88-90 (article 27) 1986

Y. Kuginuki, I. Igarashi and T. Kanno
Vegetable and Ornamental Crops Research Station, M.A.F.F. Ano, Agei, Mie, Japan 514-23

In Japan, the commercial varieties of pumpkin classified into Cucurbita maxima are widely cultivated. C. maxima seems to be susceptible to Phytophtora capsici compared with C. moschata on field observation. It is necessary to transfer Phytophtora resistance gene of C. moschata to C. maxima. The purpose of this study was to establish the method of seedling test to screen Cucurbita cultivars for resistance to Phytophtora capsici.

Materials and Cultivation. Fi8ve varieties were used for this study: C. maxima: Utsugiwase-akaguri and Kurokawaguri, and C. moschata Heiankogiku, Hyuga 14 and Bizenchirimen. The seeds after forced sprouting were sown in plastic pot (9 cm diameter) willed with perlite. Experiments were designed for two replications of eight plants per plot.

Preparation of Pathogen and Inoculation. Thirty ml of liquid medium consisted of vegetable juice in 100 ml Erlenmeyer flask was sterilized. Mycelia of Phytophtora were inoculated in this medium. Mycelial mat, which was produced by 2-week incubation at 28˚C, was rinsed on a Buchner funnel with sterile distilled water and put on the filter paper moistened by sterile distilled water in the petri dish. The petri dish was covered with a Japanese paper lid and placed in a 28˚C incubator illuminated with fluorescent lamp. Numerous zoosporangia were formed on the surface of mycelial mat by 20-30 hour incubation. The zoosporangia were collected with a small brush after pouring of sterile distilled water into the petri dish (1). About 50 ml o the zoosporangia suspension was poured into each pot.

Evaluation. At 10-14 days after inoculation, results of the observation were recorded.

Effect of Inoculum Concentration (Table 1). The six kinds of inoculum were prepared as 0, 4 x 10, 2 x 10², 1 x 10³, 5 x 10³ and 1 x 10⁴, zoosporangia/ml. Two weeks after sowing, plants were inoculated. With the inoculation of suspensions above 1 x 10³, zoosporangia/ml, all of the plants in C. moschata were diseased but the percentage of died plants were low. At low inoculum concentrations (4 x 10 and 2 x 10² zoosporangia/ml), the percentage of diseased plants and the percentage of died plants were high C. maxima and low in C. moschata.

Effect of Seedling Stage (Table 2). Plants were grown to the 8-day, 15-day and 22-day seedling stage before inoculation. Inoculum concentrations were prepared for 2.5 x 10² zoosporangia/ml in C. moschata. In C. maxima, all of the plants inoculated at all of the seedling stages were diseased and died. In C. moschata, the percentage of diseased plants and the percentage of died plants were low compared with those of C. maxima, and the percentage of the 15-day seedling was higher than those of the 8-day and 22-day seedling stages.

Effect of Temperature Condition (Table 3). Plants had been kept in 30˚C – 23˚C (day temperature-night temperature), 25˚C-18˚C and 20˚C – 13˚C during 10 days from 2 days before inoculation. Plants were inoculated with 1 x 10³ zoosporangia/ml at 2 weeks. In C. maxima, the percentage of diseased and the percentage of died plants were 100% or nearly 100% at high and middle temperature conditions. In C. moschata, the percentage of diseased plants was very low and the percentage of died plants was 0%, at all of the temperature conditions.

Conclusion. It seems likely that the appropriated zoosporangia concentration may be in the range from 2 x 10² to 1 x 10³ zoosporangia/ml, because of stability of disease appearance. It was estimated that the uniform seedlings about 2 weeks old may be available to inoculate Phytophtora pathogen. Desirable temperature condition for inoculation and nursery of inoculated seedlings seems to be in the high or middle range; 30˚C – 25˚C at day and 23˚C – 18˚C at night.

Table 1. Effect of inoculum concentration of Phytophtora capsici on percentage of diseased plants and percentage of died plants.

		Inoculum Concentration (zoosporangia/ml)
Species	Variety	0	4 x 10	2 x 103	1 x 103	5 x 103	1 x 104
C. maxima	Utsugiwase-akaguri	0/0Z	100/88	100/100	100/100	100/100	100/100
	Kurokawaguri	0/0	88/88	100/100	100/100	100/100	100/100
C. moschata	Heiankogiku	0/0	0/0	63/13	100/25	100/38	100/38
	Hyuga 14	0/0	50/25	50/25	100/63	100/50	100/38

^ZLeft; Percentage of diseased plants.
Right; Percentage of died plants.

^Z Left; percentage of diseased plants.
Right; Percentage of died plants.

Table 3. Effect of temperature condition on percentage of diseased plants and percentage of died plants.

		Temperature Condition ( ˚C, Day-Night)
Species	Varieties	30-23	25-18	20-13
C. maxima	Utsugiwase-akaguri	100/100Z	100/94	88/75
	Kurokawasguri	100/100	100/100	81/75
C. moschata	Heinkogiku	19/0	19/0	6/0
	Bizenchirimen	13/0	6/0	31/0

^Z Left; Percentage of diseased plants.
Right; Percentage of died plants.

Literature cited

1. Katsura, K., Y. Miyata and T. Mitani. 1968. A new method for the numerous formation of zoosporangia in Phytophtora spp. Sci. Rep. Kyoto Pref. Univ. Agric. 20:32-36.