Heritability of Resistance to Rhizoctonia Fruit Rot in a Wide Base Cucumber Population

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Cucurbit Genetics Cooperative Report 7:25-26 (article 12) 1984

J.T. Sloane and T.C. Wehner
Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609

Rhizoctonia fruit rot is a major disease problem in the southern U.S., reducing yields by an average of 7 to 9% annually (3). No single-gene resistance to the disease has been found, however. Recently, screening methods have been developed and lines identified that have quantitatively-inherited resistance to the disease (2). In order to plan a breeding program to incorporate that resistance into commercially acceptable cultivars, a measurement of the heritability of the trait is needed. The objective of this study was to measure the heritability of Rhizoctonia fruit rot resistance in the wide-base cucumber population, NCWBP.

Methods. A wide-base population was developed by intercrossing all available cucumber Plant Introduction lines (from the Regional Plant Introduction Station at Ames, Iowa), breeding lines and cultivars, totaling 1063 lines. The lines were intercrossed in 1981 and the pickling cucumber types harvested and intercrossed in 1982 to form the North Carolina Wide Base Pickle (NCWBP) population. After intercrossing, 112 plants were selected at random and tested for Rhizoctonia fruit rot resistance. The plants were flagged and seed harvested from each plant at maturity for the progeny test. The progeny (half-sib families) were planted in 1983 and 3 plants in each family tested for fruit rot resistance. The test involved harvest of 1 fruit, approximately 50 mm in diameter, from each plant. Fruit were taken to the laboratory and placed on soil that had been inoculated with oat grains (3200/m2) infested with a mixture of 12 Rhizoctonia isolates obtained from cucumber production areas in North Carolina. The soil was held in flats on a mist shelf system that applied 3 ml of water for 1 min every 3 days. After 10 days, the fruit were scored for percent of the surface infected with Rhizoctonia. Analysis of the data was by parent-progeny regression, which has been shown to estimate 1/2 the narrow-sense heritability, h2N (1).

Results. The population had a fairly high level of resistance and the detached fruit test was not as severe a test as has been developed more recently. However, there was a wide range of disease reaction, and the population mean of 2.1% fruit damage was considered to be an intermediate level between resistant and susceptible (Table 1). The heritability of resistance was estimated to be 0.24, considered low to moderate. Progress for incorporating resistance into cultivars should be steady. The formula for calculation of gain from selection was used to estimate that half-sib family selection (where the best 10% of 300 families tested were intercrossed in isolation each year) would result in a population mean of 0% fruit damage after 1 cycle of selection. However, more recent research has resulted in the development of a more severe screening procedure, so progress toward a higher level of resistance might not be so rapid. It appears that resistance can be transferred to commercially acceptable cultivars with slightly more effort than that being spent on transfer of high levels of anthracnose resistance.

Table 1. Rhizoctonia fruit rot resistance in the NCWBP population, and estimates of genetic variance and heritability from parent-progeny regression analysis of a sample of 112 plants.z

 

Statistic

Value

Mean

 2.1%

Range

– Highest

 12.0%

– Lowest

 0.0%

b(parent-offspring)

 0.12

h2N

 0.24
zData were based on a single fruit per plant evaluated for percent of surface infected after 10 days of exposure to soil infested with Rhizoctonia solani.

 

Literature Cited

  1. Falconer, D.S. 1979. Introduction to Quantitative Genetics. Longman Inc., New York. 340 p.
  2. 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.
  3. 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.