Seedling Tests for Belly Rot Resistance in Cucumber

Cucurbit Genetics Cooperative Report 20:13-15 (article 7) 1997

Michael S. Uchneat and Todd C. Wehner
Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609

Introduction. Cucumber (Cucumis sativus L.) is an important vegetable crop of the Southeastern United States, supplying cucumbers to much of the country during the early summer and fall. Belly rot, caused by the soilborne fungus Rhizoctonia solani Kuhn AG-4, is an important cucumber disease in that area, particularly in the fall crop. In North Carolina, the average crop loss to belly rot was 5 to 10% (2). Losses in individual fields were reported as high as 80% (3). Nationwide, belly rot resulted in $4 to 5 million of losses per year (3).

Field and detached-fruit screening tests have been developed for belly rot resistance (6). Those tests require that plants be grown to the fruiting stage in the field. Therefore, the tests are time-consuming and must be run during the growing season. A seedling test method, if developed, could have three advantages over field tests of the fruits for resistance. Seedling tests are faster, permitting resistant plant to be identified before the time of pollination. They can be run off-season, permitting more than one generation per year using greenhouses or winter nurseries. And finally, seedling tests provide an additional measure of resistance, making it possible to distinguish between mechanical barriers such as netted fruits, or thick fruit skin.

Seedling tests have been developed for scab, a leaf-spot/fruit rot disease of cucumber caused by Cladosporium cucumerinum Ell. & Arth. (5). Wehner and Palmer (7) ran seedling tests using R. solani mycelium grown on agar and applied to cotyledons in a spray, drench, or agar disk treatment. Only the agar disk treatment gave differences among cultigens, and they were correlated with field and detached-fruit tests only at the 10% level.

The objective of this study was to evaluate several screening methods to identify a seedling test for belly rot resistance that was easy to run and correlated with field tests.

Methods. All seedling tests were conducted in a green-house in Raleigh, North Carolina, during the spring and summer of 1992. The experiment was a randomized complete block design in a split-plot treatment arrangement with five test methods as whole plots, and five cultigens as subplots. There were three runs of two replications and a four-plant subsample. Five cultigens differing in belly rot resistance were chosen for the experiment: ‘Supergreen’ (subsceptible middle- eastern slicer), ‘Marketmore 76’ (resistant American slicer), M 21 (resistant American pickle), PI 165509 (resistant wild type, netted skin), and PI 432855 (susceptible green-house slicer).

Plants were grown in 100 mm (370 ml volume) pots using Metro Mix 220 as a substrate. Two seeds were planted in each pot on three planting date (11 April, 25 May, 17 June), and each pot was thinned to one plant after one week. No fertilizer was used during the test. Day temperatures ranged from 27 to 35 C and night temperatures were not allowed to fall below 21 C. All plants were inoculated when the second true leaf had fully expanded. Inoculations for the three runs were on 28 April, 10 June, 1 July.

Five methods were evaluated in the seedling tests. Those included a potato dextrose agar disk placed on the second true leaf (referred to as the leaf disk method) and against the hypocotyl (referred to as the hypocotyl disk method), a leaf dip and a soil drench using a mycelial suspension, and a syringe inoculation, also using the mycelial suspension. Agar disks were produced by growing R. solani for 7 days at room temperature on 100 mm diameter petri plates containing 20 ml potato dextrose agar. A cork borer was used to punch 7-mm-diameter disks from the plates. The mycelial suspension was produced by blending the contents of a 100- mm-diameter petri plate with 100 ml of sterile water for 30 seconds in a blender (Hamilton Beach model 632-1, lowest speed). The leaf dip method was applied by dipping 60% of the second true leaf into a 100 ml beaker containing the mycelial suspension and a drop of Tween 20. The soil drench method consisted of applying 1 ml of the suspension to the substrate at the hypocotyl. The syringe injection technique involved injecting the plant stem 10 mm above the soil surface until the suspension began to ooze from the wound. After inoculation, the plants were placed in a growth camber for two days at 27 C with a fogger to maintain high humidity. The isolate used in this study (Rs-143-N) was of North Carolina origin.

Ratings were made 5 to 12 days after inoculation, depending on the rate of disease development for the three tests (10 May, 15 June, 7 July). Plants were rated on a 1 to 9 scale with a rating of 1 indicating no disease, and a rating of 9 indicating the plant was dead. Data were analyzed using the GLM procedure of SAS (4).

Table 1. Five seedling test methods compared with field performance.^z

Cultigen	Fruit skin	Field (%)	Leaf disk	Leaf dip	Syringe	Hypocotyl disk	Soil drench
Marketmore 76	Thick	3	3	2	2	3	3
M 21	Thin	5	2	2	2	3	3
PI 165509	Netted	5	3	2	3	4	5
PI 432855	Thin	5	3	2	3	4	3
Supergreen	Thin	8	3	2	3	5	5
Mean		5	3	2	3	4	4
LSD (5%)		3	1	1	1	1	1
Correlation with field test		1.00	0.32	-0.34	0.77	0.75	0.65

^z Fruit skin was thin, thick, or netted. Field test was the mean % of the fruit surface showing symptoms of belly rot. Seedlings were rated 1 to 9 (1=no disease, 9=plant dead). Methods evaluated were a 7-mm-diameter potato dextrose agar disk damaged with R. solani and placed on the second true leaf (leaf disk) and against the hypocotyls (hypocotyls disk), and, a leaf dip, a soil drench, and syringe injection using a mycelial suspension.

Results. Different levels of disease were observed in each of the three runs (data not shown). Variation in symptom expression was most evident in the leaf disk, leaf dip, and syringe methods, suggesting those methods were not reliable. The hypocotyls disk and soil drench methods provided more consistent symptom expression, and both methods appeared to be measuring the same reaction.

The methods producing consistent results over runs were the soil drench and hypocotyls disk (Table 1). Although they had the greatest range of the five methods, the differences were not correlated with field performance. Symptom expression resembled damping-off with lesions and girdling occurring at the soil line. The soil drench method was easier of the two to apply, and future studies might concentrate on refining the method to provide improved results.

In the soil drench method, the most resistant cultigen was ‘Marketmore 76’, ‘Supergreen’ was one of the most susceptible, and M 21 was intermediate. Those three cultigens were tested for damping-off by Booy et al. (1). In that experiment, ‘Marketmore 76’ and M 21 were among the most susceptible, and ‘Supergreen’ was among the most resistant of the cultigens tested. That suggests the soil drench and damping-off methods may be testing different resistance mechanisms.

Nonce of the seedling test methods we evaluated was correlated with field resistance for the cultigens tested. The soil drench method was one of the better methods for distinguishing resistance, and was easy to apply. In addition, it seemed to test the type of resistance not related to the mechanical barrier provided by the fruit skin. It would be beneficial to screen a larger number of cultigens using that method to determine the effectiveness of the method.

Literature Cited

1. 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.
2. 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.
3. Lewis, J.A. and G.C. Papavizas. 1980. Integrated control of Rhizoctonia fruit rot of cucumber. Phytopathology 70:85-89.
4. SAS Institute Inc. 1988. SAS/STAT user’s guide, release 6.03 edition. Cary , NC 27512-8000 , USA .
5. Vakalounakis, D.J. and P.H. Williams. 1989. A cotyledon screen for resistance to scab (Cladosporium cucumerinum) in cucumber (Cucumis sativus) seedlings. Ann. Appl. Biol. 115:443-450.
6. 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.
7. Wehner, T.C. and M. Palmer. 1987. A seedling test for resistance of cucmber lines to fruit rot caused by Rhizoctonia solani. Cucurbit Genet. Coop. Rpt. 10:31-32.