Cucurbit Genetics Cooperative Report 7:25-26 (article
12) 1984
Heritability of Resistance to Rhizoctonia Fruit Rot in a
Wide Base Cucumber Population
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
- Falconer, D.S. 1979. Introduction to Quantitative Genetics.
Longman Inc., New York. 340 p.
- 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.
- 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.
