Cucurbit Genetics Cooperative Report 20:16-17 (article 8) 1997
Michael S. Uchneat and Todd C. Wehner
Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609
Eddie Echandi
Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695-7616
Cucumber (Cucumis sativus L.) is an important vegetable crop in the United States, with a large production area in the southeast. North Carolina is the second leading state in the production of pickling cucumber (9), and fourth leading state in the production of slicing cucumber (8).
Belly rot is a severe problem in many cucumber-producing areas. In the southeast, the disease is often severe because of the warm and humid growing season, which favors disease development. Belly rot is caused by the soil- inhabiting fungus Rhizoctonia solani Kuhn, anastomosis group 4 (AG-4). In North Carolina, belly rot is present in 72% of all cucumber fields (6). Symptom development on apparently healthy fruits can occur in as little as 24 h. Symptoms appear as sunken, brown necrotic lesions, which often develop a hard, corky layer (2).
Traditional fungicides have been used in an attempt to control belly rot, but those methods do not provide complete control, and often are not economical (3,4). A better means of control would be genetic resistance. Resistant cultivars, along with proper cultural practices, should provide an economical means of controlling belly rot. Cucumber screening tests have been developed using field and detached-fruit studies (7) to identify resistant cultigens (breeding lines, cultivars, and plant introduction accessions). However, those tests required much time and resources, and the data are not obtained until after the plants flower. A seedling test that was correlated with field and detached-fruit tests would use less time and resources, and would permit identification of resistance in time to pollinate selected plants.
The objective of this study was to determine the value of R. solani damping-off tests of seedlings for prediction of resistance to belly rot in field and detached-fruit tests.
Methods. All damping-off tests were conducted in a greenhouse in Raleigh, North Carolina during the spring and summer of 1992. The experiment was a randomized complete block design with two runs, three replications, and six cultigens. Four cultigens were chosen that ranged in resistance for belly rot: ‘Supergreen’ (susceptible Middle
Eastern slicer), M 21 (resistant American pickle), PI 432855 (susceptible greenhouse slicer), and PI 165509 (resistant wild-type cucumber with netted skin). Two cultigens were chosen that ranged in resistance to damping off in a previous test (1): ‘National Pickling’ (resistant pickling type) and ‘Addis’ (susceptible pickling type).
Inoculum was produced by autoclaving flasks containing a 2:1 ratio (by volume) of oat grains and tap water two consecutive days for 90 min at 121 C. Disks were then punched out of R. solani cultures growing on potato dextrose agar in petri plates. The disks were transferred into the flasks which were kept at room temperature to allow the fungus to colonize the oat grains. Finally, the oat grains were air dried under a hood and refrigerated at 5 C until needed.
All damping-off tests were conducted in flats (505 x 365 x 50 mm) containing a 1:2:1 mix of sand, soil, and Metro Mix 220. The mix was watered prior to filling the flats. Flats were partitioned into two equal units (251 mm long). At the center of each half-flat a small depression was made, 25 oat grains colonized with R. solani were placed in the depression, and the oats covered with soil. Twenty-five uninfested oat grains were used in the control treatments. A template was then used to make 20 evenly-spaced holes in each half flat. A seed was then placed into each hole and covered with soil. The two runs were planted on 28 April or 11 July.
Ratings for the two runs were made 14 days after planting, on 12 May or 25 July. Emergence was recorded and compared with the controls. Each plant was rated for damping-off on a 1 to 9 (1=healthy, 9=dead) scale, and treatment means were computed. Data were analyzed using the GLM procedure of SAS (5).
Results. There were no significant differences among cultigens for resistance to damping-off regardless of whether the analysis was done by run or pooled over runs. We were unable to distinguish ‘Addis’ and ‘National Pickling’, which were the most resistant and most susceptible, respectively, in previous damping-off tests run using a different method (1). In most cases, all plants near the centers of the flats (near the oat grains) died, while those around the edges were relatively free of disease. Emergence was not significantly reduced by the fungus, although there were significant differences among cultigens for emergence. Those differences in emergence were likely due to differences in seed quality.
It was interesting that, while the damping-off tests did not show significant differences among cultigens, there were significant correlations with field tests and detached fruit tests (Table 1). Booy et al. (1) used different methods to measure damping-off, and found no correlation with belly rot.
Future research might be done allowing more time for the oats to infest the soil prior to planting to allow better establishment of the fungus.
Table 1. Comparison of damping-off evaluations to belly rot resistance observed in the field.
Damping-off |
|||||
|---|---|---|---|---|---|
Cultigen |
Field testz |
Detached testy |
Run 1 |
Run 2 |
Mean |
| National Pickling | 4.1 | – | 4.4 | 3.4 | 3.9 |
| Addis | 4.4 | 3.7 | 3.9 | 3.5 | 3.7 |
| M 21 | 6.7 | 3.4 | 4.2 | 4.0 | 4.1 |
| PI 165509 | 7.2 | 6.4 | 4.1 | 4.5 | 4.3 |
| Supergreen | 8.0 | 24.7 | 5.0 | 4.0 | 4.5 |
| PI 432855 | 9.9 | 27.8 | 4.7 | 5.1 | 4.9 |
| Mean | 6.7 | 13.2 | 4.4 | 4.1 | 4.2 |
| LSD (5%) | 4.5 | 6.2 | 2.2 | 1.9 | 2.0 |
| Correlation with field test | 1.00 | 0.83 | 0.61 | 0.92** | 0.97** |
| Correlation with detached test | 0.83 | 1.00 | 0.91* | 0.60 | 0.87* |
z Ratings were based on the percentage of the fruit surface damaged averaged over 1991 and 1992 field tests.
y Detached fruit ratings are based on the percentage of the fruit surface damaged during 1991 experiments. Each value is based on the mean of nine fruits.
*,** indicates significance at 5 or 1 % level, respectively.
Literature Cited
- Booy, G., T.C. Wehner and S.F. Jenkins, Jr. 1987. Resistance of cucumber lines to Rhizoctonia solani damping- off not related to fruit rot resistance. HortScience 22:105-108.
- Jenkins, S.F. and C.W. Averre. 1981. Fruit rots of pickling and slicing cucumbers. Plant Pathology Info. Note #219. Dept. of Plant Pathology, North Carolina State University, Raleigh, NC.
- Halterlein, A.J., G.L. Sciumbato, and W.L. Barrentine. 1981. Use of plant desiccant to control cucumber fruit rot. HortScience 16:189-190.
- Jones, J.P. 1961. Comparative effects of soil fungicide treatments on soil rot and damping-off of cucumber. Plant Disease Rptr. 45:376-379.
- SAS Institute, Inc. 1988. SAT/STAT user’s guide: Release 6.03 ed. SAS Institute, Cary, N.C.
- St. Amand, P.C. and T.C. Wehner. 1991. Crop loss to 14 diseases in cucumber in North Carolina for 1983 to 1988. Cucurbit Genet. Coop. Rpt. 14:15-17.
- Wehner, T.C. and S.F. Jenkins. 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.
- U.S. Department of Agriculture. 1981. Agricultural statistics. U.S. Government Printing Office, Washington D.C. p. 171.
- U.S. Department of Agriculture. 1990. Agricultural statistics. U.S. Government Printing Office, Washington D.C. p. 154.