Cucurbit Genetics Cooperative Report 7:71-72 (article 31) 1984

Tolerance of Cucurbita spp. to Squash Leaf Curl

J.D. Mc Creight

U.S. Department of Agriculture, Agricultural Research Service, P.O. Box 5098, Salinas, CA 93915

Squash leaf curl (SLC) is a highly infectious virus disease that reached epidemic proportions in fall squash production in the desert southwest of the U.S. and the adjoining area of Mexico in 1981 (3). SLC virus is transmitted efficiently by the sweetpotato whitefly, Bemesia tabaci (Genn.), which is virtually impossible to control with chemicals (1,2). Resistance to SLC virus or its transmission are, therefore, the most promising means of controlling SLC. My objective was to test cultivars of Cucurbita maxima Duch., C. mixta Pang, C. moschata Duch., and C. pepo L. for resistance to SLC.

Seventy-four cultivars of these 4 Cucurbita spp. were rated in greenhouse tests using controlled inoculation and in a field test at Brawley, CA using natural infection. Plants were inoculated in the greenhouse by feeding SLC viruliferous whiteflies on 5 plants of each cultivar at the 2-3 leaf stage for 48-96 hr. Plants were rated 10-14 days after inoculation. The field test was planted in a randomized complete block design. There were 2, 2-plant hills per cultivar in each of 3 replications. The field was watered on July 27, 1983, and rated for SLC 50 and 91 days later. Plants in the greenhouse and field tests were rated on a scale from 1 (dead) to 9 (symptomless). The field test was rated on a plot basis. Enzyme linked immunosorbent assays (ELISA) were done for SLC virus on every surviving cultivar in one replication at the 91 day rating using the techniques of Cohen et al. (1).

Reactions (R and range) to SLC were more severe on all 4 species in the greenhouse tests than in the field test (Table 1). The maxima cultivars were in general more severely affected than those of the other 3 species. Reaction of the mixta cultivars were only slightly better than the maxima group in the greenhouse or field tests.

Table 1. Summary of ratings of 4 Cucurbita spp. to squash leaf curl virus in greenhouse and field testsz.
Species	Number of cultivars	Greenhouse test		Field test
				50 days		91 days
		Mean	Range	Mean	Range	Mean	Range
maxima	15	2	1-3	3	1-9	2	1-8
mixta	3	3	2-3	4	2-9	4	1-9
moschata	6	3	2-6	8	5-9	7	5-9
pepo	50	2	1-5	5	1-9	5	1-9
zRated on a 1 to 9 scale: 1 = dead; 3 = very severe curling and stunting; 5 = moderate symptoms; 7 = mild symptoms; 9 = symptomless.

Reaction of the moschata cultivars was not very good in the greenhouse tests except for 'Hercules' and 'Mediterranean' which showed relatively mild symptoms. In contrast, all the moschata cultivars performed well in the field test as indicated by a R severity rating of 8 at 50 days and 7 at 91 days after initial watering (Table 1).

Many pepo cultivars were severely affected by SLC in the greenhouse and field tests. Several pepo cultivars did, however, recover in greenhouse tests. For example, 'Black Magic' had severe SLC symptoms 14 days after inoculation, but by 31 days symptoms were mild, and the inoculated plants were almost indistinguishable from the control plants. The other 2 maxima and 2 pepo cultivars in the same test with 'Black Magic' did not recover. Eighteen of the 50 pepo cultivars showed tolerance to SLC in the field test, but there was large plot-to-plot variation.

ELISA readings from the field test were positive, and indicated that there was a high SLV virus titre even in those cultivars rated symptomless for SLC at time of sampling. This indicates that some cultivars of moschata and pepo are tolerant to SLC.

Literature Cited

1. Cohen, S., J.E. Duffus, R.C. Larsen, H.Y. Liu, and R.A. Flock. 1983. Squash leaf curl virus - Purification, serology, and vector relationships of the whitefly transmitted geminivirus. Phytopathology 73:1669-1673.
2. Commonwealth Institute of Biological Control. 1981. Possibilities for the use of biotic agents in the control of the whitefly, Bemesia tabaci. Biocontrol News and Information 2:1-7.
3. Flock, R.A. and D.E. Mayhew. 1981. Squash leaf curl, a new disease of cucurbits in California. Plant Disease 65:75-76.