Interactions Among Root-Knot Nematodes and Fusarium Wilt Fungi on Host Plants

Under natural field conditions, roots of crop plants are constantly exposed to many soil microorganisms. It has been estimated that one square meter of high-fertility field soil may contain as many as 3 x 1014 bacterial cells (300 g). 5 x 108 protozoa (39 g). I x 107 nematodes (12 g). and 400 g of fungi (95). Most soils. however. do not have such high microbial densities, and many microbes present are saprophytic or have weak pathogenic potential to plants. Also, only a small percentage of the plant pathogens present in any given soil is capable of parasitizing a particular plant species, generally under specific environmental parameters. However, the number of soil microorgan­ isms present in the root zone of any crop plant is sufficient for innumerable and diverse interactions. It is not surprising that many investigators have concluded that the majority of root diseases have a complex etiology (41. 119). As early as 1931, Fawcett (32) stated that nature does not work with pure cultures and that most plant diseases, partiCUlarly root diseases, are influenced by associated microorganisms. Root infections by one pathogen may modify the host response to subsequent infections by pathogens or saprophytes. The moist soil environment is favorable for activities of nematodes and also