Cucurbit Genetics Cooperative Report 7:71-72 (article
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
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.
Number of cultivars
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.
- 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.
- 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.
- Flock, R.A. and D.E. Mayhew. 1981. Squash leaf curl, a new
disease of cucurbits in California. Plant Disease 65:75-76.