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Cucurbit Genetics Cooperative Report 20:56-59 (article 26) 1997

Differential Response of Cucurbita pepo Cultivars to Strains of Zucchini Yellow Mosaic Virus

R.W. Robinson and R. Provvidenti

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

Lisa and Lecoq (2) described zucchini yellow mosaic virus (ZYMV) as being remarkably variable. Different strains of ZYMV can be distinguished on the basis of the symptoms on susceptible hosts, and pathotypes have been reported (1) that differ for virulence to a resistant genotype. Two strains ZYMV are known to be present in the USA (4).  The Connecticut strain (ZYMV-CT) produces the most severe symptoms but the Florida strain (ZYMV-FL) is the most common. The purpose of this research was to test resistant germplasm of summer squash for reaction to the FL and CT strains of ZYMV.

 Three sources of ZYMV resistance were included in these tests. 'Freedom II' and 'Prelude II' from the Asgrow Seed Co. have a genetically engineered coat protein gene for resistance. 'Tigress' and 'Jaguar' (from the Harris Moran Seed Co.) and 'Revenue' and 'Dividend' (from Rogers Seed Co.) all derive their ZYMV resistance from Cucurbita moschata 'Nigerian Local'. Cucurbita ecuadorensis is the source of resistance to ZYMV for breeding line NY 247.

All resistant cultivars and lines tested had a high level of resistance of ZYMV-FL. Inoculated plants grew vigorously and their fruit were symptomless. NY 247, 'Freedom II', and 'Prelude II' were also highly resistant to ZYMV-CT, but inoculated plants of 'Tigress', 'Jaguar' and 'Dividend', developed foliar mosaic and stunning in greenhouse tests. They were less affected by ZYMV-CT than susceptible C. pepo plants, but they lacked the high level of resistance to ZYMV-CT of their 'Nigerian Local' parent.

When 'Tigress' and 'Jaguar' plants were tested in the greenhouse with other strains of ZYMV, they were susceptible to those from Australia, Brazil, Egypt, and Taiwan, but displayed tolerance to a California and a China strain. Under the same growing conditions, 'Nigerian Local' and NY 247 were resistant to all strains.

Resistant cultivars and the susceptible cultivar 'Black Jack' were inoculated with ZYMV-CT in the greenhouse and transplanted to the field. 'Black Jack' was severely stunted and produced very few fruit, all of them very malformed, but all of the resistant cultigens grew vigorously and produced many marketable fruit. The number of fruit and the % marketable fruit produced by six plants of each cultivar are given in Table 1. 'Jaguar', 'Tigress' and 'Dividend' produced a higher percentage of unmarketable fruit than the other resistant cultivars, primarily due to misshapen fruit caused by ZYMV.

 Table 1. Fruit Production to 8/25/96 and Grade by Plants Inoculated with ZYMV-CT.

Cultivar or line
No. of fruit
NY 247
Freedom II
Prelude II
Black Jack

'Nigerian Local' has a high level of resistance to both ZYMV-FL and ZYMV-CT, but some cultivars deriving their resistance from this source have high level resistance to one but not the other of these ZYMV strains. 'Nigerian Local' has a single, incompletely dominant major gene and possibly modifiers for ZYMV resistance (3), and modifying genes may be needed for a high level of resistance to ZYMV-CT. The reaction of some resistant cultivars to ZYMV-CT does not appear to be necessarily due to a different expression in a C. pepo gene background of the major gene for ZYMV resistance from Cucurbita moschata 'Nigerian Local'. C. pepo breeding lines have been developed with ZYMV resistance from 'Nigerian Local' that are highly resistant to the Connecticut as well as the Florida strain of the virus.

The lower level of resistance to ZYMV-CT of some cultivars should not be a threat to most US growers. The CT strain of the virus is of only limited distribution and these cultivars are highly resistant to the predominant strain of the virus in the country (4). Also, symptoms produced by ZYMV-CT on these cultivars were less severe in the field than in the greenhouse. Despite being inoculated in the seedling stage with a high tire of ZYMV, a more severe test of resistance than commonly occurs with natural infection, they produced many marketable fruit in the field.

There is no source of immunity for summer squash to natural infection with ZYMV. The occurrence of ZYMV in leaves of a resistant cultivar could provide an opportunity for the development of new pathotypes that could multiply in that resistant host and have a selective advantage over strains that the cultivar is resistant to. Pyramiding genes for ZYMV resistance that are derived from different sources could guard against this possibility and make possible cultivars with more stable resistance to ZYMV.

Literature Cited

  1. Lecoq, H. and M. Pitrat. 1984. Strains of zucchini yellow mosaic virus in muskmelon (Cucumis melo L.). Phytopath. Z. 111:165-173.
  2. Lisa, V. and H. Lecoq. 1984. Zucchini yellow mosaic virus. CMI/AAB Descriptions of Plant Viruses No. 282.
  3. Munger, H.M. and R. Provvidenti. 1987. Inheritance of resistance to zucchini yellow mosaic virus in Cucurbita moschata. Cucurbit Genetics Coop. Rpt. 10:80-81.
  4. Provvidenti, R. and D. Gonasalves. 1984. Occurrence of zucchini yellow mosaic virus in cucurbits from Connecticut, New York, Florida, and California. Plant Disease 68:443-446.
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