Influence of soil pH and calcium nutrition on resistance of alfalfa to bacterial and verticillium wilt.

Alfalfa plants of a resistant, a susceptible and a highly susceptible strains were grown in unlimed soil at pH 5.8 and in limed one at pH 6.9 and inoculated by the pathogens of vascular wilt, Corynebacterium insidiosum and Verticillium albo-atrum. Two types of liming were performed: 1) before inoculation and 2) after inoculation. Liming of the soil led to an increase in number of resistant plants. In susceptible plants the external symptoms of disease on the plant tops were delayed or alleviated. This phenomen was more conspicuous with Verticillium wilt than with bacterial wilt. The favourable effect of liming was less distinct in resistant strains than in susceptible ones. For an increase in resistance, post-infection liming of the soil was more effective in the case of bacterial wilt, while pre-infection liming provided the best results in the Verticillium wilt. The nitrogen content in the dry matter of roots from plants grown in limed soil was higher by more than a quarter as compared to roots from plants growing in unlimed soil.