The objective of this study is to analyze the effect of different temperatures on the occurrence of cotton Verticillium wilt and the mechanism of regulating host defense response, reveal the dual effects of temperature on pathogen and host, and to provide a theoretical basis for the green control and temperature regulation of this disease.

Cotton Verticillium wilt resistant variety Zhongzhimian 2 (ZZM2) and susceptible variety Jimian 11 (JM11) were employed as experimental materials, indoor experiments and disease nursery experiments were jointly used to set constant temperatures (22, 25, 28, and 32 ℃) and natural temperature variation to determine the effect of temperature on the growth, infection and colonization of Verticillium dahliae, and the occurrence of cotton Verticillium wilt. The mechanism of temperature regulation of cotton host defense response was analyzed by utilizing indicators such as reactive oxygen species (ROS) outbreak, H₂O₂ content, callose accumulation, and the expression of defense-related genes.

On the culture media, 25 ℃ was the optimal temperature for the mycelial growth of V. dahliae, and the range of 22-28 ℃ was suitable for spore yield. Compared with the culture media, the leaf extracts of resistant variety ZZM2 and susceptible variety JM11 had a promoting effect on the growth of V. dahliae, and JM11 promoted stronger. When the temperatures were between 25 and 28 ℃, both ZZM2 and JM11 suffered from severe Verticillium wilt, and low temperature 22 ℃ and high temperature 32 ℃ were not conducive to the occurrence of Verticillium wilt. Meanwhile, under 25 ℃ treatment, the colonization ability of V. dahliae in cotton was strong, JM11 was more susceptible to V. dahliae infection than ZZM2, which was basically consistent with the results of the cotton Verticillium wilt disease index. Furthermore, temperature significantly affected the host defense response of cotton: compared with 22-28 ℃, whether inoculated with V. dahliae or not, the ROS outbreak produced by ZZM2 and JM11 under 32 ℃ treatment was stronger; under 25 ℃ treatment, the H₂O₂ content in ZZM2 and JM11 leaves was the lowest; under 32 ℃ treatment, the accumulation of callose in ZZM2 and JM11 leaves was higher, which was 2.04 and 1.80 times higher than that of the untreated control, respectively; the expression level of PAL, POD, and PPO defense-related genes in cotton leaves decreased under treatments at 25-28 ℃, which was lower than that under treatments at 22 and 32 ℃.

Temperature has a dual effect on the growth of V. dahliae and host defense response, which in turn affects the occurrence of cotton Verticillium wilt. Regardless of constant temperature or natural temperature variation, 25-28 ℃ are beneficial for the colonization of V. dahliae in cotton, significantly reduce the host defense response, and lead to severe cotton Verticillium wilt.