Relationship between occlusion of xylem elements and disease symptoms in leaves of cotton plants infected with Verticillium dahliae

The relationship between plugging of xylem elements and subsequent wilting, chlorosis, and necrosis of limited leaf areas was studied in cotton plants (Gossypium hirsutum L.) infected by Verticillium dahliae. In contrast with xylem elements which extended into nonchlorotic areas of infected leaves, the lumens of xylem elements extending into chlorotic areas of infected leaves were occluded with a dark brown substance, and the vessel walls were noticeably thickened. There was a positive correlation between the number of occluded xylem elements and the degree of chlorosis: among 72 infected leaves tested, the average percentage of occluded xylem vessels in veins extending into chlorotic areas and nonchlorotic areas of leaf blades were 28 and 6, respectively. These results suggest that regional chlorosis and necrosis of leaves of cotton plants infected by V. dahliae may be due to water stress created by localized plugging of xylem elements. When sections from midribs or petioles of leaves of healthy plants were placed in solutions of several enzymes or other substances, only methyl alcohol (2.5 × 10−5 M to 2.5 × 10−2 M) or pectin methyl esterase at 100 μg/ml caused browning and plugging of xylem vessels typically associated with wilt leaf symptoms of Verticillium wilt.