Author : Mulyadi

Summary : Forty-eight soybean cultivars grown in pots were screened for resistance to M. incognita. Of these cultivars, 7 were resistant; 24 moderately resistant; 7 moderately susceptible; and 9 susceptible. The mechanism of resistance is both pre- and post infectional. Pre-infectional resistance was indicated by reduced larval penetration, reduced eggs hatched, and increased larval mortality caused by root exudates and decreased attractiveness of roots to nematodes. Post infectional resistance was shown by the delayed development of larvae to adult, low egg production, high phytoalexin, and low tryptophane productions in root extracts. The degree of resistance and susceptibility was influenced by plant age at time of inoculation and by inoculum level of the nematodes. Young were more susceptible than old soybean plants, and susceptibility was increased with increasing inoculum level of M. incognita. The concentration of glyceollin, a phytoalexin, in roots of M. incognita-inoculated resistant plants was higher than those in susceptible plants indicating that it plays a role in disease resistance. Histopathological observations of galled roots of susceptible cultivars showed hypertrophy and hyperplasia of cells surrounding the nematode head. Giant cells are thick walled, multinucleate and with granular cytoplasm; they disrupted the vascular elements. The resistant cultivars had fewer and smaller giant cells than susceptible cultivars. Host suitability, together with abundant giant cells, tryptophane, and available nutrients are vital in nematode development and reproduction, and determines the resistance or susceptibility of soybean cultivars to M. incognita.