The Effect of Various Factors on the Expression of Genetic Resistance to Root-knot Nematode in Snap Bean. Tomato and Lima Bean

Abstract

Cultivars of vegetables resistant and tolerant to root-knot nematode (Meloidogyne incognita Kofoid and White, Chitwood) bred in Hawaii were tested to determine the factors which lower the effectiveness of the resistance. Emphasis was placed on possible formation of pathogenic races of M. incognita which could result from continuous cropping of resistant cultivars, and the effect of high soil temperature on expression of resistance. Cultivars tested were 'Manoa Wonder' snap-bean (Phaseolus vulgaris L.). 'Healani' tomato (Lycopersicon esculentum Mill.), 'Kailua' soybean (Glycine max Merr.), and 'White Ventura N' lima bean (Phaseolus lunatus L.). Field tests were conducted in 3 root-knot nematode infested fields (fields P-1, P-2, and Q-1) at the Poamoho Experimental Farm where galling of the resistant cultivars had occurred. Tests were conducted throughout the year to obtain seasonal effects on the performance of the cultivars. Root-knot nematode populations established in the greenhouse on a susceptible tomato cultivar from galled roots of both resistant and susceptible cultivars from fields P-2 and Q-1. These were used in tests to investigate the formation of a more virulent race of root-knot nematode, the effect of soil temperature and level of inoculum on the rate of galling, and the development of a root-knot nematode population which can parasitize resistant cultivars of more than one crop. Data used to measure the resistance were the gall formation and egg-mass production of the root-knot nematode on the plant roots. 'Manoa Wonder' snap-bean showed considerably increased galling when exposed to the field P-2 M. incognita population. This population showed an increase in pathogenicity on 'Manoa Wonder' under continuous planting. No increase of pathogenicity was observed in the field Q-1 population. Effectiveness of the genetic resistance was reduced when the soil temperature was kept at 29° + 1°C, but was effective under fluctuating soil temperatures (21° - 33° C), although scattered galls were sometimes found. The resistant tomato cultivar 'Healani' was galled heavily by a more virulent race of M. incognita established in field Q-1. It was not galled by the field P-2 M. incognita population. The level of resistance was significantly lowered at a soil temperature of 29° + 1°C, and, at the higher inoculum level it was completely ineffective. However, the resistance was effective under fluctuating soil temperature (21° - 33° C). It was not possible to induce the formation of a pathogenic race of the root-knot nematode population from field P-2 five times the basic rate, which indicates that the genetic resistance of 'Healani' is strong. Soybean cultivar 'Kailua' was observed to have a stable, strong genetic resistance. Repeated inoculation with root-knot nematodes did not result in any increased pathogenicity. Under fluctuating soil temperature (21° - 33°C) and increased, continuous soil temperatures up to 34° + 1°C, with the inoculum levels tested, the resistance proved to be excellent. The tolerant lima bean cultivar 'White Ventura N' was partially resistant with its degree of galling dependent upon the level of inoculum available. Increased soil temperature also increased the ability of M. incognita to cause galling and produce egg-masses on the roots. The attempt to induce a root-knot nematode population which is able to parasitize more than one resistant cultivar was not successful.