Effect of organic amendments on Verticillium wilt of cotton

Verticillium wilt is the most devastating disease of cotton in China and a challenge for producers to find effective means of control. Here, we report the effects of different organic amendments on the incidence of this disease and on the rhizosphere microflora of cotton plants. Seven organic amendments were evaluated for their suppressive effect on cotton Verticillium wilt caused by Verticillium dahliae Kleb. The results showed that organic amendments applied to soil reduced disease severity in both inoculated pots and naturally infested cotton field plots. The most effective control was achieved with crab shell (chitin), soybean stalk and alfalfa, and in pots the efficacy was 72%, 60% and 56% for vascular tissues, respectively. Rice chaff gave moderate control, while poultry manure, peanut cake and wheat straw showed a weak suppressive effect with efficacy of 21%, 28% and 11% for vascular tissues, respectively. Organic amendments increased the population size of rhizosphere microbes (including fungi, bacteria and actinomycetes), which varied at the different ages of the cotton plants. The organic materials with the best biocontrol capacity strongly stimulated the proliferation of antagonists to V. dahliae in the rhizosphere. However, poultry manure, peanut cake and wheat straw caused only small changes in the total numbers of microflora and the percentage of antagonists was lower. Extracts from organic amendments were highly inhibitory to V. dahliae. The changes undergone by rhizosphere microbes after the addition of organic amendments may contribute to suppression of cotton wilt and help to explain the protective effect of the amendments. The results indicate that application of organic amendments is an effective control measure against cotton Verticillium wilt.