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Cucurbit Genetics Cooperative Report 10:80 (article 41) 1987

Inheritance of Resistance to Zucchini Yelow Mosaic Virus in Cucurbita moschata

H. M. Munger

Departments of Plant Breeding And Vegetable Crops, Cornell University, Ithaca, NY 14583

R. Provvidenti

New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456

Since Provvidenti et al (1) reported resistance to zucchini yellow mosaic virus (ZYMV) in a C. moschata from Nigeria ('Nigerian Local'), work has been underway to transfer resistance to Butternut squash, also C. moschata. Butternut has never shown the high susceptibility to cucumber mosaic virus that we see regularly in C. pepo, but it is extremely susceptible to ZYMV. Butternut seedlings in greenhouse tests are frequently killed by ZYMV, and plants infected naturally at flowering time in the field in 1985 produced almost no fruit. Consequently there seems to be some urgency to breed a resistant Butternut.

The F1 of Nigerian Local X Waltham Butternut (WBN) showed clear symptoms of ZYMV, much less severe than on WBN but far greater than on Nigerian, suggesting partial dominance that might permit selection during successive backcrossing. The F1 of the BC1 was started in the greenhouse and inoculated and selected there, with 24 classified as resistant and 23 susceptible. These were transplanted to the field, alternating individual resistant and susceptible plants. Differences between the 2 groups were maintained throughout the season, but differences in extent of growth appeared within the resistant group, suggesting modifier genes for a single basic resistance gene. As the resistant plants did not set fruit well, most backcrosses were made with their pollen and only a few obtained because of severe ZYM on uninoculated Butternut growing in another field to serve as female parent.

Six BC2 F1 progenies were grown in the greenhouse in the early months of 1986. All had approximately 50% resistant plants for a total of 73 resistant to 66 susceptible. Once again, little fruit set on the resistant plants and their male flowers were used to pollinate Waltham Butternut and Puritan Butternut. Their BC3 F1 progenies were started and inoculated in the greenhouse for the 1986 field planting, with 17 resistant plants represented in the parentage. Of these, only 2 failed to give approximately 50% resistant plants. Exact counts were not made because over 500 plants were grown and some discarded before it was clear how they should be classified. A better site than that of 1985 was chosen for growing the resistant segregates and there was no problem in getting self-pollinated fruit on them. Nine selfs giving the BC3 F2 generation planted in the greenhouse in December 1986 were chosen on the basis of their similarity to Waltham BN and Puritan in size, shape, color, and quality of cooked flesh. When symptoms appeared after ZYMV inoculation, plants were classified as follows:
40 homozygous resistant (slight mottling of older leaves but symptomless young leaves
91 heterozygous resistant (definite mottling of young leaves but little stunting)
37 susceptible (strong mottling and stunting or death).

The 9 progenies all had some apparently homozygous resistant plants and were traceable to 5 different resistant segregates in the BC2. These results indicate that a single gene when homozygous in C. moschata confers a high level of resistance to ZYMV and cast doubts on the original thought that additional modifiers were needed.

We have attempted to transfer resistance from Nigerian Local to various C. pepo summer squashes. We have found in BC2 F2 progenies infected segregates with vastly better growth than the C. pepo parents and presumably carrying the resistance gene from C. moschata. However the best of these do not approach the growth or freedom from mottling found in F2 plants of the third backcross to Butternut.

Literature Cited

  1. Provvidenti, R., D. Gonsalves, and H. S. Humaydan. 1983. Occurrence of zucchini yellow mosaic virus in the United States. Cucurbit Genetics Coop. Rept. 6:99.
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Page citation: Wehner, T.C., Cucurbit Genetics Cooperative;
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