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,
New York State Agricultural Experiment Station, Cornell University, Geneva,
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
91 heterozygous resistant (definite mottling of young leaves but little
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
- 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.