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