An interaction between Meloidogyne incognita (Kofoid and White) (Nematoda: Heteroderidae) with the fungus Sclerotium rolfsii (Saccardo) or Cylindrocladium crotalariae (Bell and Sobers) on three soybean varieties

The interaction on soybeans between the nematode Meloidogyne incognita and the fungus Sclerotium rolfsii, and between the nematode Meloidogyne incognita and the fungus Cylindrocladium crotalariae was studied under greenhouse conditions. Three soybean varieties chosen for their resistant and susceptible qualities to M. incognita were used: 'Lee,' 'Hill,' and 'Mack.' Treatments involved pathogens inoculated alone or in various combinations. Predisposition by nematodes was studied by nematode inoculation 14 days prior to fungus inoculation and by simultaneous nematode-fungus inoculation. Significant nematode-fungus interactions were found to occur as indicated by measurements of plant height, top weight and root weight and by ratings of nodule formation, nematode gall and egg mass formation and root necrosis. Using root necrosis as a measurement of disease development, the greatest necrosis for treatments using C. crotalariae occurred where M. incognita was inoculated at planting and C. crotalariae was added after 14 days. Not only does this indicate the occurrence of an interaction between the pathogens, but it also indicates that predisposition by nematodes had occurred. Evidence that formation of nematode galls is associated with root nodules was also found. Greatest nodule formation was found on treatments where nematodes were inoculated alone; on these plants extensive gall formation was evident. Disease development in plants was also measured by percentage plant mortality. Highest mortality for plants treated with either fungus occurred where the nematode and fungus were combined simultaneously.