Cucurbit Genetics Cooperative Report 3:5-6 (article 3) 1980

Screening Cucumbers for Resistance to the Vegetable Leafminer

Gwendolyn Eason, G.C. Kennedy, and R.L. Lower

North Carolina State University, Raleigh, NC 27650, University of Wisconsin, Madison, WI 53706

Effective chemical control of the pickleworm and cucumber beetle has reduced natural populations of wasp species which formerly kept the vegetable leafminer, Liriomyza sativae Blanchard, under biological control (2,3,4). Host plant resistance to the leafminer was deemed more desirable than the use of additional chemicals to control leafminers on cucumber crops.

Greenhouse and controlled environment chamber (ca. 27°C, 55% RH) experiments were used to develop seedling screening techniques and to evaluate a collection of breeding lines, cultivars, and plant introductions for resistance (1). More mature plants (ca.3 leaves) were evaluated in the greenhouse only.

Both angle and height of the cotyledons were found to affect leafminer response to cucumber seedlings. Seedlings with revolute cotyledons received fewer mines than seedlings with cotyledons which were flat and parallel to the ground plane. In seedling tests conducted directly beneath a light source, taller seedlings had more mines than short seedlings. When seedling experiments were blocked against a light source, seedling height interacted with distance to the light source to affect leafminer choice. Consistent results among seedling experiments were obtained only when all cotyledons within each experiment were adjusted to a uniform distance above the ground plane. Because light reflection and distribution among seedlings may be affected by both height and angle of the cotyledons, the importance of both of these plant factors in resistance screening may be explained by the positive phototactic nature of the leafminer.

When cotyledon heights were adjusted within seedling experiments, similar results could be obtained in the controlled chamber and in the greenhouse. Greenhouse experiments with mature plants had similar results when plant samples within the experiments were of uniform development. Seedling response in the chamber or greenhouse did not predict the response of some lines in the more mature plant stage; cotyledon resistance apparently is not contingent on the same factor(s) as mature plant resistance.

Of the screened materials, PI 200815 and PI 279465 appeared to have the most potential as sources of resistant germplasm. These plant introductions were resistant in both seedling and in later stages. 'Gy 2,' a gynoecious breeding line, was resistant only in the seedling stage. 'Addis' and 'Spartan Salad', pickling cucumber cultivars, and PI 271327 were consistently susceptible at all plant stages.

Field experiments will be necessary to evaluate the resistance levels of PI 200815 and PI 279465 outside the greenhouse.

Literature Cited

1. Eason, Gwendolyn. 1980. Procedures in Screening Cucumis sativus L. for Resistance to the Vegetable Leafminer, Liriomyza sativae Blanchard. M.S. Thesis. North Carolina State University, Raleigh, NC.
2. Hills, O.A. and E.A Taylor. 1951. Parasitization of dipterous leafminers in cantaloupe and lettuce in the Salt River Valley, Arizona. J. of Econ. Entomol. 44:759-762.
3. Michelbacher, A.E., W.W. Middlekauff, O.G. Bacon, and J.E. Swift. 1955. Controlling melon insects and spider mites. California Agric. Expt. Stn. Bul. 749, 46p.
4. Wolfenbarger, D.O. 1958. Serpentine leafminer: brief history an summary of a decade of control measures in southern Florida. J. of Econ Entomol. 51:357-359.