Crop residue return reduces cotton Verticillium wilt by altering potassium nutrition and root exudates

Crop residue return can improve soil fertility and crop yield, but it may also increase the risk of soil borne disease. Here, we investigated cotton biomass, Verticillium wilt incidence, root exudates, soil Verticillium dahliae abundance, soil available potassium (K) concentrations, and leaf K concentrations in two 8-year residue management experiments under wheat-cotton rotation in Nanjing and Dafeng of China. Six treatments were compared based on uniform rates of nitrogen and phosphate fertilizers: three residue return treatments without K fertilizer (wheat residue, WR; cotton residue, CR; wheat residue plus cotton residue, WR + CR), two K fertilizer treatments (150 kg K2O ha(-1), K150 and 300 kg K2O ha(-1), K300), and CK treatment without residue or K fertilizer. Our results showed that the incidence and disease index of Verticillium wilt were the lowest in K300, followed by K150, WR + CR, WR or CR, and the highest in CK at both sites. The responses of cotton biomass, leaf K concentrations and soil available K concentrations to different treatments were generally opposite to that of the disease severity. The disease index was significantly negatively correlated with the concentration of K in soil or cotton leaf, while it had significant correlations with root exudates but not soil microbe abundances. Structural equation modeling indicated that K nutrition affected cotton Verticillium wilt disease index by affecting alanine, glutamate and total phenol in cotton root exudates. These findings suggested that K nutrition status was the key factor affecting the occurrence of cotton Verticillium wilt, and residue retention could increase cotton resistance to Verticillium dahliae by improving soil K availability and subsequently regulating specific root exudates.