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 nematode" 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.
Killing nematodes by means of a hot solution of 0.5% acetic acid in water keeps them in a better condition than heating them in a drop of water. A pipette for heating and transferring the acetic acid solution and a small dish for the evaporation of alcohol from an alcohol-glycerin mixture are described. Killing Killing of nematodes by heating them gently in a drop of water on a slide followed by fixation in cold f.a. 4 : 10 or f.a. 4 : 1 did not always give satisfactory results especially with tylenchs. Clotty appearance of the oesophageal region, distortions again especially of the oesophagus, splitting of the tissue just beneath the lateral field and tendency of the nematodes to shrink slightly when transferred to glycerin by the alcohol method occurred fairly commonly. Spiral nematodes and Pratylenchu.r exhibited these fixation artefacts more than T'ylenchorhynchu.r for example.
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