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Cucurbit Genetics Cooperative Report 8:36-38 (Article 14) 1985

Inheritance of Resistance to Downy Mildew in Cucumis melo PI 124111 

Cohen, Y., S. Cohen and H. Eyal

Department of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel

C. E. Thomas

U. S. Department of Agriculture, Agricultural Research Service, U. S. Vegetable Laboratory, Charleston, SC 29407

This research was supported by Grant No. US 287-81 from BARD--The United States-Israel Binational Agricultural Research & Development Fund.

Downy mildew of cucurbits incited by Pseudoperonospora cubensis (Berk. & Curt.) Rost. is a devastating foliar disease world-wide (3). Major economic hosts include Cucumis sativus L. (cucumber) and Cucumis melo L. (muskmelon), although Cucurbita sp. (squash and pumpkin), Citrullus lanatus (watermelon) and others are also affected (3, 4). In 1944, Ivanoff (2) reported that four cantaloup varieties of West Indies origin were resistant to downy mildew under south Texas conditions. In an incomplete study, he concluded that resistance was partially dominant when crossed to susceptible cultivars. Thomas (4) found that among C. melo PI's and cultivars, PI 124111 (PI) expressed the highest resistance to downy mildew under field conditions in south Texas. This paper reports the reaction to downy mildew of this PI, its F1, F2, and BC1 reciprocal crosses to the susceptible commercial cultivars 'Hemed' (H) and 'Ananas-Yokneam' (AY) under growth chamber-greenhouse and field conditions.

Methods. A high degree of resistance to both downy and powdery mildews was stabilized in the PI by five generations of selection and inbreeding. This inbred of the PI was used as the resistant parent. The commercial cultivars H and AY were used as susceptible parents.

A local race of P. cubensis, collected from cucumber cv. 'Bet-Alfa' in 1979, was used for all inoculations. Inoculations of 2-leaf stage plants were conducted in chambers by spraying the adaxial leaf surfaces with a suspension of 2000 sporangia/ml. After inoculation plants were kept in a dew chamber at 17C in the dark for 30 hr and were then placed in the greenhouse (22-27C) for 7-8 days. Inoculations of 10-leaf stage plants in the field were done in a similar manner at about 2000-2100 hr, when relative humidity was 85% and temperature 22C.

Resistance in 2-leaf stage plants was assessed according to reaction type (1) and degree of sporulation at 7-8 days after inoculation. In the field, percentage leaf area mildewed was determined at about the 20-leaf stage. Fungal sporulation was determined from potted infected plants after a 20-hr incubation period in a moist atmosphere at 18C ± 1 in the dark. Leaves were removed to a fixative solution and the number of sporangia counted with a cytometer.

Results. Reaction type and fungal sporulation in AY, the PI, and their F1 reciprocal crosses are presented in Table 1. While AY supported luxuriant sporulation of the pathogen, these were limited on the PI. In F1 plants, fungal sporulation was markedly inhibited, especially in progeny with the PI as the female parent. Intermediate resistance was also observed in F1 plants of the crosses between H and the PI (Table 2). Resistant reaction type in the PI was better expressed in leaf 2, but the maternal effect of the PI on reducing sporulation was expressed only in leaf 1. In the field, F1 plants exhibited about a 50% reduction in percent leaf area infected compared to H, while the PI showed some "pinpoint" (&Mac178; 1 mm) yellow lesions which did not support sporulation. F 2 and some BC1 populations were examined for reaction type only (Table 3). With either AY or H as susceptible parents, ratios of 6:9:1 susceptible, moderately resistant, and resistant were observed in some F2 populations. BC1 resulted in a 3:1 ratio of moderately resistant to resistant plants in the populations inoculated.

Table 1. Sporulation of Pseudoperonospora cubensis on Cucumis melo PI 12411 (PI), 'Ananas-Yokneam' (AY), and their F1 and reciprocal crosses.a

 

Feature AY AY x PI PI x AY PI

Sporangia per
cotyledon, x 103
141.6 ± 38.6 67.2 ± 47.0 6.0 ± 7.1 0.5 ± 0.8
Sporangia per cm2
leaf 2, x 103
56.4 ± 22.6 13.7 ± 7.1 3.8 ± 3.3 0

aCotyledon tests conducted at 10 days after planting and leaf-2 tests on 3- week-old plants, both at 20C. Ten plants/treatment. In crosses, first parent (on the left) served as the female parent.

Table 2. Sporulation of Pseudoperonospora cubensis and reaction type to downy mildew in Cucumis melo PI 124111 (PI), 'Hemed' (H), and their reciprocal crosses.

Feature H H x PI PI x H PI

Sporangia per
cm2, x 103
Leaf 1 78.6 ± 33.4 54.3 ± 22.3 20.3 ± 8.9 5.4 ± 5.9
Leaf 2 70.6 ± 32.0 5.7 ± 5.7 7.8 ± 5.6 0.3 ± 0.6
Reaction typea
Leaf 1 1.0 ± 0 1.0 ± 0 1.0 ± 0 1.9 ± 0.4
Leaf 2 1.0 ± 0 1.4 ± 0.7 2.9 ± 0.5 4.0 ± 0
% leaf area infected
in the field
66.0 ± 18.3 31.4 ± 11.1 33.0 ± 12.7 0b

al = highly susceptible; 4 = highly resistant (see citation 1 for details); 24-51 plants/treatment.

bPinpoint yellow lesions on lower leaves.

Table 3. Segregation of the reaction of F2 and BC1 progenies from reciprocal crosses of PI 124111 (PI) with 'Ananas-Yokneam' (AY) and 'Hemed' (H) to downy mildew (Pseudoperonospora cubensis) at the two-leaf stage.a

Generation

Pedigree

Observed
S : MR : R

Expected ratio
S : MR : R

Chi-
square

df

P


F2

(PIxAY) x (PIxAY)

31 56 8

6  9  1

1.4398

2

.3-.5

 

(AYxPI) x (AYxPI)

28 90 7

6  9  1

13.1973

2

.001-.01

 

(HxPI) x (HxPI)

67 91 10

6  9  1

0.4074

2

.8-.9

 

(PIxH) x (PIxH)

46 93 18

6  9  1

9.8960

2

.001-.01

 

Combined

172 330 43

6  9  1

9.2654

2

.001-.01

 

Homogeneity

 

 

15.6751

6

.01-.02

 

 

 

 

 

 

 

BC1

PI x (PIxAY)

50 16

3  1

0.0202

1

.8-.9

 

PI x (AYxPI)

44 22

3  1

2.4444

1

.1-.2

 

(AYxPI) x PI

52 17

3  1

0.0048

1

.9-.95

 

Combined

146 55

3  1

0.5987

1

.3-.5

 

Homogeneity

 

 

1.8707

2

.3-.5


aReaction types 11, 12, 13 grouped as susceptible - S; types 22, 23, 24, 33 grouped as moderately resistant - MR; types 34, 44 grouped as resistant - R; according to citation 1.

Discussion. PI 124111 has been used in the peat in cantaloupe breeding programs, especially as a source of resistance to powdery mildew (Bohn, personal communication and Thomas, unpublished). The high level of resistance in this PI against downy mildew, expressed as reaction type 4, has not been utilized. The evidence presented here indicates that two incompletely dominant genes condition the highly resistant reaction of this PI to the Israeli race of P. cubensis since a) F1 plants showed intermediate levels of resistance; b) some F2 populations segregated in a ratio of 6:9:1 (susceptible:moderately resistant:resistant); and c) BC1 populations segregated in a ratio of 3:1 (moderately resistant:resistant). Differences in resistance levels of the reciprocal crosses in F1 plants as expressed by pathogen sporulation, point to the possible involvement of cytoplasmic factors. These differences were not observed in the other parameters used to measure resistance in these populations. Additional studies are needed to verify the existence of such factors. At this time, we cannot fully explain the deviations from the expected ratio in some F2 populations.

Literature Cited

  1. Cohen, Y., H. Eyal, and C. E. Thomas. 1984. Evaluating downy mildew resis-
    tance in Cucumis melo L. Cucurbit Genetics Coop. Rpt. 7:38-40.
  2. Ivanoff, S. S. 1944. Resistance of cantaloupes to downy mildew and the
    melon aphid. J. Hered. 35:35-38.
  3. Palti, J., and Y. Cohen. 1980. Downy mildew of cucurbits (Pseudoperonospora
    cubensis). The fungus and its hosts, distribution, epidemiology, and
    control. Phytoparasitica 8:109-147.
  4. Thomas, C. E. 1982. Resistance to downy mildew in Cucumis melo plant
    introductions and American cultivars. Plant Dis. 66:500-502.
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