Cucurbit Genetics Cooperative Report 7:23-24 (article 11) 1984

Evaluation of Screening Methods and Sources of Resistance for Rhizoctonia Fruit Rot in Cucumber

Sloane, J.T., T.C. Wehner and S.F. Jenkins, Jr.

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

B. Graves

Southern Mississippi Experiment Station, Beaumont, MS 39423

Cucumber yield losses in the southern U.S. averaged 7 to 9% and reached as high as 40% in some areas (2). Cultural practices for control have not been economical to apply, and single gene resistance has not been found to date. Previous research (1) has identified sources of quantitative resistance, however. This study involved the evaluation of several methods for screening for Rhizoctonia fruit rot resistance, and the identification of resistant and susceptible lines for use in breeding programs.

Methods. The most resistant and most susceptible lines (totaling 149) from previous tests were evaluated using 2 screening methods. The lab test involved placing 50 mm diameter fruit harvested from field plots onto soil inoculated with Rhizoctonia. The soil was inoculated with oat grains (4800/m²) that had been infested with 12 isolates of the fungus. The 12 isolates had been collected from North Carolina production areas. The fruit were scored after 10 days in a mist chamber where the soil was kept moist by applying 3 ml of water every 3 days for 1 min. Fruit were scored for percent of surface infected with Rhizoctonia.

The field test was run in a similar manner to the lab test, except that the fruit were not removed from the plants. Inoculation rates and procedures were similar except that the field plots were inoculated at vine tip-over stage. The first and second most infected fruit were flagged and scored twice weekly in each plot during the period 25 to 46 days after inoculation. Any plots having scores of 1% fruit infection or less were checked to determine if there were fruit in the plot that were more infected. This helped prevent us from confusing resistant plots with escapes. A second field test was run in Mississippi on naturally-infested soil using the same disease evaluation procedures as for the inoculated-field test.

Results. Data from the 1983 tests were combined with previous data to determine which lines of the initial 1063 screened were most resistant. The 4 most resistant lines were PI 163216, PI 197088, PI 280896 and PI 357852 (Table 1). Plants in some of the lines were segregating for plant type and fruit type, so selections were made within lines to stabilize resistance and other horticultural characteristics. The correlation between the lab and field tests was significant, but perhaps not high enough to use as a screening method. The field test was about as easy to run and was more closely correlated with results from a test run in Mississippi (data not shown). A more severe test is needed to distinguish among levels of resistance found in the best lines and in selections made from them.

Table 1. Belly rot resistance (percent of fruit surface infected) for the most resistant and most susceptible lines in 3 laboratory and 3 field tests.
Line-Selection	Origin	Laboratory test			Field test
		1981	1982	1983	1982	1983	Miss.
PI 163216-4	India	0	0	0	0	0	0
PI 163216-6	India	0	0	0	0	0	0
PI 197088-5	India	0	0	0	0	0	0
PI 357852-1	Yugoslavia	0	0	-	0	-	0
PI 280096-2	USSR	0	0	-	0	-	0
PI 181752-3	Syria	22	3	-	18	0	6
PI 177360-5	Turkey	0	0	10	9	6	8
PI 419108-5	China	0	5	6	7	4	8
PI 419108-6	China	0	5	6	7	4	8
PI 344433-1	Iran	23	-	20	16	-	14
PI 267741-4	Japan	0	1	20	13	5	15
LSD (5%)		-	7	7	7	6	5
Mean		-	3	3	3	4	6
CV (%)		-	126	115	76	63	80
No. replications		1	2	3	2	2	1

Table 2. Correlation of Rhizoctonia fruit rot scores (percent of surface infected) among laboratory and field tests run in 1983.
Laboratory test Raleigh	Raleigh vs. Clinton	Field test Clinton	Clinton vs. Mississippi	Field test Mississippi
	0.50**		0.40*

Literature Cited

1. Sloane, J.T., T.C. Wehner and S.F. Jenkins, Jr. 1983. Screening cucumber for resistance to belly rot caused by Rhizoctonia solani. Cucurbit Gen. Coop. Rpt. 6:29-31.
2. Sumner, D.R. and D.A. Smittle. 1976. Etiology and control of fruit rot of cucumber in single harvesting for pickles. Plant Dis. Reptr. 60:304-307.