The Relation Between Nematode Density and Damage To Plants

An equation for the relation between the density of stem nematodes or virus transmitting nematodes in the soil and the proportion of attacked plants can be derived if two suppositions are made: 1. the average is the same at all densities. 2. the nematodes act independently of each other. If now y = the proportion of the plants that is not attacked and P = the density of the nematode then In this equation z is a constant < 1 and equal to the proportion of the plants not attacked at a nematode density P = 1 (competition curve of Nicholson). The results of a pot experiment by Sayre & Mountain and field observations by Kaai are in close agreement with this equation. To describe the relation between the density of populations of root infesting nematodes and the yield of attacked plants an equation must express two phenomena: 1) Up to a certain density (the tolerance limit) the yield is not affected (damage only to tissue that is not essential to the growth of the plant, power of recovery of the plant), 2) a certain minimum yield (Omin) remains unaffected by the nematodes even at the highest densities (because of temporary or continuous inaccessability of part of the host tissue). The equation in which Op = yield at nematode density P, 0max = yield in the absence of nematodes and T = tolerance limit, fulfills these requirements. The results of many experiments reported in the literature are in good agreement with this theoretical relation between nematode density and yield. In most cases Omin is between 0.1 Omax and 0.5 Omax. The tolerance limit depends on the nematode species, the plant species and external conditions.