Experiments were carried out to determine the role of Pratylenchus penetrans (Cobb, 1917) Filip. & Stek. 1941 in the peach replant problem. It is shown that this nematode is a true plant pathogen since it invades and causes necrosis of peach roots in the absence of bacteria and fungi. Necrosis of the root tissue occurs rapidly and appears to take place in advance of the invading nematode. The main mechanism involved in the formation of the lesion is the production of phytotoxic substances through hydrolysis of the cyanophoric β-glucoside, amygdalin. P. penetrans is capable of hydrolyzing this substance in vitro. It is concluded that, although P. penetrans is the most important primary parasite and a true plant pathogen, its main role in peach replant failure appears to be its ability to incite root degeneration by providing extensive infection courts for soil microorganisms.

The fungus Rhizoctonia solani Kühn and the root-lesion nematode Pratylenchus minyus Sher & Allen were closely and consistently associated in naturally occurring infections of winter wheat. The resultant root rot was characterized by markedly yellowed and stunted patches of plants in the wheat fields during spring growth. In greenhouse and field experiments the combined effects of the fungus and the nematode upon growth of the wheat was almost twice as great as the effect produced when either pathogen was controlled by a selective soil treatment. By pure culture techniques it was not possible to show that in the penetration of wheat roots the fungus was dependent upon the nematode or that the converse was true.

Studies conducted in steam-sterilized soil inoculated with Verticillium alboatrum and Pratylenchus penetrans demonstrated a synergistic relationship between fungus and nematode. At low and intermediate levels of Verticillium inoculum, the nematode increased wilt and the increase may be a function of the number of nematodes present. Except at the high inoculum levels of Verticillium, significantly larger numbers of P. penetrans occurred in eggplant roots in the presence than in the absence of the fungus. The nematode alone had no adverse effect on root condition or growth of eggplant.

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Further evidence was accumulated indicating that the peach replant disease decreases in severity as the interval increases in time between removal of the old orchard and the planting of new trees. The ameliorating effect of the interval may be related to the decline of Pratylenchus penetrans (Cobb, 1917) Sher & Allen, 1953. In experiments, increased growth of peach was associated with the control of P. penetrans by nematocides and P. penelrans was found to be the first nematode to attack newly developing peach roots. The populations of P. penetrans within such roots increased rapidly, then gradually declined. Generally, ectoparasites appeared later, increased relatively slowly, and occurred infrequently until late in the second growing season. The soils of all commercial peach nurseries in Ontario contain P. penetrans and the nematode may be transported in the roots of nursery stock.