Cucurbit Genetics Cooperative Report 10:31-32 (article 19) 1987

A Seedling Test for Resistance of Cucumber Lines to Fruit Rot Caused by Rhizoctonia solani

Todd C. Wehner

Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609

Mary Palmer

U.S. Department of Agriculture, A.R.S., University of Wisconsin, Madison, WI 53706

This research was supported by a grant from Vlasic Foods, Inc.

Fruit rot caused by Rhizoctonia solani Kuhn. is one of the 3 most important cucumber diseases in North Carolina. Significant differences among lines were found for resistance to the disease in both field and detached-fruit (lab) tests (2). Differences were significant and heritable, and there are commercially acceptable lines that are at least moderately resistant, such as M21 and Marketmore 76.

Plant breeding programs would be able to incorporate fruit rot resistance into new cultivars more efficiently if an easy test could be used for preliminary selection work. The field test is useful for final selection trials, but involves much work to inoculate and evaluate plants. The detached-fruit test is useful for isolating particular factors for evaluation, but we are able to handle more lines with the field test.

A seedling test for Rhizoctonia resistance like the one now used for scab resistance would be ideal if it were correlated with the field test. Previous efforts to develop a correlated test using a damping-off test were not successful, since it was not correlated with fruit rot resistance (1).

The objective of this study was to develop a seedling test that would be easy to run and significantly correlated with the field test for fruit rot resistance.

Methods. Plants were grown in the greenhouse at the University of Wisconsin, Madison. A randomized complete block experimental design was used with 2 stages (cotyledon and 2nd leaf), 5 lines (M21, Marketmore 76, Poinsett 76, Sumter and Supergreen), 4 inoculation methods (water spray control, soil drench, spray, and agar blocks), and 4 replications of 10 plants each. The lines were chosen to represent a range of resistance. Inoculation methods involved spraying the plants with water, drenching the base of each plant with 1 ml of inoculum, spraying inoculum on the leaves until droplets formed, or placing 4 agar blocks on one leaf per plant.

Inoculum was prepared by transferring inoculum (Rhizoctonia-infested soil, isolate R5-H-2) to petri plates containing potato dextrose agar. After 2 days at room temperature, 10 ml of distilled water was added to each plate and the culture rubbed with a rubber policeman. The resulting liquid was ground in a blender for 1 min., and a concentration of 8.5 x 10⁴ fragments/ml was produced for the spray or drench treatments. Agar blocks were produced by placing a 2 x 2 mm piece of inoculated potato dextrose agar onto petri plates containing water agar. After 6 days at room temperature, 5 mm diameter disks were punched out of the water agar using a cork bore. The disks were then placed, inoculum side down, on the leaves.

Results. The best treatment was the agar block method at the 2nd leaf stage (Table 1). The spray and drench methods did not work, possibly because of the grinding of the mycelium which apparently reduces the virulence of the fungus (Dr. E. Echandi, personal communication, 1986).

The cotyledon stage treatments did not work, but the plants were left in the chamber an extra day, so that may have been the cause (data not shown). The agar block test of plants at the 2nd leaf was correlated with previous field and detached-fruit (lab) tests, but not with the damping-off test (Table 2). Additional work is needed to refine the method, but it will undoubtedly be useful in preliminary screening work to incorporate moderate resistance into new cultivars.

Literature Cited

1. Booy, I., T.C. Wehner, and S.F. Jenkins, Jr. 1986. Evaluation of resistance of cucumber lines to damping-off caused by Rhizoctonia solani. Cucurbit Genet. Coop. Rpt. 9:5-9.
2. Wehner, T.C. and S.F. Jenkins, Jr. 1986. Field and detached-fruit tests for resistance of cucumber lines to fruit rot caused by Rhizoctonia solani. Cucurbit Genet. Coop. Rpt. 9:41-43.