IV AGARKOVA

Institute of Genetics and Plant Experimental Biology, AS, Tashkent PO Box 97, Uzbekistan

Background and objectives
Nematodes, typical inhabitants of the cotton rhizoshere, enhance the severity of Verticillium wilt infection. The entire ecological and faunal nematode complex, which is characteristic for cotton fields, takes part in intensifying and spreading the disease. It has been discovered that the number of plants infected by wilt in susceptible cotton varieties increases by 23-37%, and the yield decreases by 24-41%, when the density of nematodes in soil goes up to 2-3 times.

Results and conclusions
At present one of the most prospective measures for wilt control is field management. In order to develop protective anti-wilt measures, taking into account the nematodes' role, the influence of 22 cultural control methods and rotational crops has been studied. The control variant was a cotton monoculture. The structural change of qualitative composition of soil nematode fauna was noted after the application of manure-mineral composts, bio- and poultry manure. These measures increased the number of eusaprobionts up to 28%; their quantity did not exceed 14% in control. Eusaprobionts, being typical inhabitants of rotting environments, promote wilt spreading to a lesser extent than the nematodes of other ecological groups.

Various agricultural methods to influence the density of nematode populations in soil have been defined. Of 22 agrotechnical actions studied, nine decreased nematode numbers (from 25 to 67%), eight did not have any marked influence (within 15%); and five increased the density of nematode populations (from 40 to 177%). Thus various anti-wilt measures change the field nematode complex. If protective anti-wilt crop rotations are carried out, taking into account their influence on nematode populations, their anti-wilt effect can be enhanced. So the anti-wilt steps reducing nematode numbers in soil must follow those which promote nematode numbers to support and prolong their useful protective action. It is best to use every practice separately in a system of defensive steps. In the case of combining two anti-wilt agricultural methods or rotational crops, it is necessary for one of them to promote reduction of nematode density in the rhizosphere of plants. Two or more agrotechnical actions should not be used one after another to enhance the anti-wilt effect if both promote a rise in nematode numbers. It may cause a sufficient accumulation of nematodes and then the total anti-wilt action will be less prolonged and more weakened.