Microbial biodiversity as related to crop succession and potato intercropping for management of brown rot disease

Potato brown rot, caused by Ralstonia solanacearum, ranked globally as the second most important bacterial plant pathogen. In the present study, the influence of different cropping programs in potato brown rot management was investigated in four infected fields in Egypt. Two districts were selected as sandy soils in Giza (Wardan) and Behera (Ganuob El-Tahrir) governorates. The other two were selected as silty clay in Minufyia (Talia) and Beni-Suef (Sids) governorates. The followed crop succession included corn, potato intercropped with cabbage, onion, cowpea, wheat, corn again, and ended by potato. The pathogen was undetectable after corn, onion, and wheat. It decreased in cowpea and cabbage rhizospheres in the clay soils. The pathogen was undetectable at all districts, except at Sids, where the pathogen was significantly decreased but was not eradicated. This was possibly attributed to the high ratio of NO3- and Na+ at this district. Decreased R. solanacearum density after corn coincided with the high ratio of fluorescent pseudomonads, endospores, and actinomycetes, being most clear in the poor soils (Wardan) and less clear under iron excess at Ganoub El-Tahrir as well as the clay soils. Corn rhizosphere supported an array of antagonistic actinomycetes such as strains similar to Streptomyces intermedius, Streptomyces albidoflavus group, Streptomyces argenteolus group, and Streptomyces erythrogriseus. Intercropping potato with cabbage decreased the density of the pathogen in rhizosphere, which is associated with greater antagonistic fluorescent pseudomonads, Bacillus spp. and Serratia spp. Onion soil and rhizosphere associated with abundance of antagonists and fluorescent pseudomonads, followed by S. maltophilia and Bacillus spp. Wheat soil and rhizosphere supported fluorescent pseudomonads and antagonistic Streptomyces spp., especially in sandy soils. The pathogen was undetectable after planting the ending potato in the three districts, Wardan, Ganoub El-Tahrir, and Talia. This was accompanied by a general oligotrophism and increased ratio of fluorescent pseudomonads, endospores bacteria, and actinomycetes along with a diversity of R. solanacearum antagonists such as S. maltophilia, Citrobacter freundii, Acinetobacter sp., Delftia sp., and Serratia marcescens.