An Efficient Procedure to Screen for Resistance to Root Knot Nematodes in Cucurbits

Cucurbit Genetics Cooperative Report 6:96-98 (article 49) 1983

A. P. M. den Nijs and K. Hofman
Institute for Horticultural Plant Breeding, P. O. Box 16, Wageningen, The Netherlands

Screening for resistance to root knot nematodes is normally carried out in nematode-infested soil. Seeds, seedlings or young plants are placed in the infested soil and, after several weeks to months, root knot development and/or number of egg masses are observed on the rootball or on roots washed free of soil. We have used this method for testing accessions of wild species of Cucumis, melons and cucumbers for resistance to Meloidogyne incognita Chitw. However, we found it time-consuming and, due to loss of roots during washing, not very accurate. Besides it required much glasshouse space, the more so since intervening crops of tomato appeared necessary to keep the infestation level of the soil up to standard. We therefore welcomed the opportunity to adapt for use with root knot nematodes an efficient mass screening method for resistance to sugar beet nematodes developed by Lubbers and Toxopeus (1).

Pregerminated seeds are sown in 36 ml PVC tubes open at both ends and filled with silver sand moistened with nutrient solution. The tubes are arranged in asbestos containers, which are kept in a growth cabinet at 24°C, 70% RH and 10 hr days ca. 30 W/m² light intensity. In this environment the plants form a miniature vine with very small leaves with a reasonably normal root system. In this way we can test up to 2700 plants on 1 m² in about 5 weeks. The inoculum is prepared by shortly grinding galled cucumber or tomato roots containing mature egg masses in a blender and depositing the suspension on a nematode filter with tap water. Very large numbers of active larvae can easily be gathered in a few days. The prehatched larvae can be stored for some time in a refrigerator. Before inoculation the suspension is diluted to the desired concentration. Preliminary tests pointed to 50 larvae per plant as the best concentration in our system. Thus far we have mainly used one well-defined Dutch population of M. incognita for inoculum.

Seedlings are inoculated, as soon as the cotyledons have expanded, by injecting 1 ml of suspension into the top 2 cm of each tube with a veterinary syringe. Delaying the inoculation yielded root systems with only root knots in the lower part, illustrating the insensitivity of grown roots to infection. After 4–5 weeks, plants are carefully removed from the tubes, put on a screen, and the sand is simply rinsed off the roots with tap water. The plants are best examined on the screen, immersed in water, against a black background. Well-developed root knots are frequent on susceptible checks, containing egg masses which are visible to the naked eye. No secondary infection is evident at this time.

The repeatability of the results obtained with this procedure is illustrated by the mean number of rootgalls per plant of several representative accessions which were tested 3 times (Table 1). Each population was usually tested in 3 randomized replicates of 8 plants. Accessions behaved fairly consistently, but a large variation in number of galls occurred within many accessions. Resistant species not only possessed few galls, but these were small and only rarely contained egg masses.

Table 1. Mean number of root galls per plant of several species of Cucumis.

		Root galls per plant in 3 screening tests
Species	Accession	81-1	82-1	82-3
C. anguria L.	1761	19	17	17
C. ficifolius A. Rich	1729	13	7	9
C. metuliferus Naud.	1768	8	4	8
	1822	2	2	2
	1994	9	5	2
C. prophetarum L.	1752	13	6	5
	2016	6	4	2
C. zeyheri Sond. 2x	0181	5	4	4
C. zeyheri Sond. 4x	1807	13	8	11
C. sativus L.	1745	20	17	28
(primitive)	1759	14	23	16

None of over 70 cucumber cvs. tested so far exhibited appreciable resistance. Cucumber cv. Sakata Kohaifushinari from Japan was reported as having 24% (2) or 4% (3) resistant plants. In our tests the numbers of galls of 140 plants of this cv. averaged 22, and that of the susceptible check cv. G6, 24. The percentage of plants with less than 5 galls were 1 and 3, and with less than 10 galls were 7 and 14, respectively, for the two cultivars. Our nematode population may differ from that used by Udalova.

In conclusion, the described method appears to be efficient in space and time (ca. 10 work days per test of 2700 plants). The reliability of the results derives in part from the uniform inoculum and the minimal damage to the roots for observation.

Literature Cited

1. Lubbers, J.H. and H. Toxopeus. 1982. Het ontwikkelen van bietecystenaaltjes-resistente bladramenas en gele mosterd. Zaadbelangen 36:66–69.
2. Udalova, V.B. 1978. (Evaluation of varieties from the Cucurbitaceae for resistance to Meloidogyne incognita). Referativnyi Zhurnal (1979) 5.55.408.
3. Udalova, V.B. 1979. (The search for initial varietal forms of cucumber resistant to M. incognita). Referativnyi Zhurnal (1980) 1.65.278.
4. Stettler, R.T. and R.P. Curies. 1976. The mentor pollen phenomenon in black cottonwood. Can. J. Bot. 54:820–830.
5. Varekamp, H.Q., D.L. Visser and A.P.M. den Nijs. 1982. Rectification of the names of certain accessions of the IVT-Cucumis collection. Cucurbit Genetics Coop. Rpt. 5:59–60.