Cucurbit Genetics Cooperative Report 7:92-93 (article 41) 1984

The Reliability of a Seedling Test for Resistance to Root-Knot Nematodes in Cucurbits

Boukema, I.W., G.T.M. Reuling and K. Hofman

Institute for Horticultural Plant Breeding (IVT), P.O.B. 16, 6700 AA Wageningen, The Netherlands

A seedling test is used to screen for resistance to root-knot nematodes (Meloidogyne incognita Chitw.) in cucurbits (1). The level of resistance is measured by determining the mean number of root-knot galls per plant. To verify whether this seedling test gives a reliable prediction of the resistance level, the reaction of four Cucumis species (Table 1), which showed different levels of resistance in former tests, was compared in a) the seedling test, and b) a test which approaches more the glasshouse situation (pot test).

The seedling test (1) was carried out in a growth cabinet at 24 C in five replications of eight plants per plot. Each seedling was inoculated with 50 larvae. After five weeks the number of galls per plant was counted and the pooled larvae production per plot was determined by hatching larvae from egg masses on the roots. The roots were therefore cut up and placed on nematode filters in tap water at room temperature.

The pot test was carried out in a glasshouse at a minimum temperature of 20 C, rising to 25-28 C on sunny days. Cucumis seedlings were transplanted to pots containing five liters of sieved glasshouse soil and inoculated one week after transplanting with 92 larvae per plant. The test was set up in six replications with six plants per plot. At 7, 10 and 17 weeks after inoculation two plants per plot were assessed. A gall index was assigned to the rootballs (scale 1-10, 1 = 0-10%, 10 = 90-100% of the roots covered with galls). Larvae production was determined by hatching larvae from egg masses on the roots per plot, as described for the seedling test.

Of the pot test, only results of the observation at 17 weeks are given, because they will approach most closely the value of the resistance of the studied genotypes in practice. At that date the gall indexes of the four Cucumis species differed significantly from each other (Table 1). The roots of the susceptible C. sativus were almost completely disintegrated at that time, while those of the moderately susceptible C. anguria var. longipes were partly disintegrated. The rootballs of the moderately resistant C. metuliferus and the resistant C. zeyheri 2x were still intact, but the former showed many more galls than the latter. Rather large numbers of larvae were produced on C. metuliferus, only very few on C. zeyheri. It should be noted that the C. zeyheri plants were initially growing very slowly, with almost no side roots. This may have influenced their high resistance level in this test.

Table 1. Means of the number of galls, of the number of larvae and of the gall index per plant in the seedling test and in the pot test, respectively 5 and 17 weeks after inoculation.
Cucumis species	Cv. or Accession No.	Seedling test		Pot test
		galls	larvaez	gall-index	larvaez
C. sativus	G6	39.6 ay	5821 a	10.0 a	-x
C. anguria var. longipes	1751	28.2 b	4126 b	8.4	-x
C. metuliferus	1822	10.3 c	202 c	3.9 c	27.755 a
C. zeyheri 2x	0181	7.4 c	6 d	1.1 d	10 b
zFor the analyses of variance a square root transformation was made.
yMeans showing a common letter are not different at p=0.05.
xNumber of larvae could not be determined because of disintegration of the roots.

At the first and the second observation date of the pot test, the ranking order of the gall index and the number of larvae agreed in most cases with that of the gall index after 17 weeks. However, the two resistant genotypes could not be distinguished.

In the seedling test the number of galls gave a good distinction between the susceptible, the moderately susceptible and the two resistant genotypes (Table 1). Both tests revealed that none of the genotypes are completely resistant. For the four genotypes studied, the level of resistance can be more precisely predicted if, besides the number of galls, the larvae production is measured in the seedling test.

Literature Cited

1. Nijs, A.P.M. den and K. Hofman. 1983. An efficient procedure to screen for resistance to root-knot nematodes in cucurbits. Cucurbit Genetics Coop. Rpt. 6:96-98.