How to Manage Pests
UC Pest Management Guidelines
Pathogen: Verticillium dahliae
(Reviewed 5/13, updated 5/13)
In this Guideline:
Symptoms and Signs
Leaves wilt and show interveinal yellowing before becoming necrotic. Light to dark brown vascular discoloration is prominent in the stem and branches. Defoliation and death of plants may occur. Symptoms caused by Verticillium generally appear after first flower. Symptoms on younger plants may be caused by Fusarium.
Comments on the Disease
Verticillium survives in the soil as microsclerotia that germinate in the vicinity of roots. Disease severity depends on the number of root penetrations. Verticillium wilt is favored by cool air and soil temperatures.
The concentration or density of inoculum in soil is a major factor in choosing management strategies for Verticillium wilt. Where the density is low, you can generally prevent increases by following a regular rotation with nonsusceptible crops, particularly grass family crops such as corn and small grains. Tolerant cotton varieties reduce losses, but they do not prevent inoculum from increasing. Once the inoculum reaches a high level, it may be necessary to rotate out of cotton for several years or employ special techniques such as soil solarization to reduce it.
Monitoring and Treatment Decisions
If there is evidence of Verticillium in the field or a new variety of cotton with unknown resistance to Verticillium has been planted, sample stems from crop maturity through harvest. If the percent of stems with discoloration is high or if you want to determine the length of time to rotate out of cotton consider taking soil samples to determine the level of inoculum.
Take samples in summer and follow with these steps in each area with a different cropping history:
Laboratories that analyze soil samples for Verticillium report results in number of microsclerotia per gram of soil. Where a single (susceptible) cotton variety is planted without rotation a level of 10 or more microsclerotia per gram usually results in significant yield loss.
Microsclerotia are produced gradually as infected plant debris decays, consequently the number found in the soil increases after plowdown and peaks in midsummer the following year. Therefore, take soil samples at the same time each year in order to monitor changes in inoculum level to determine if it is safe to plant Verticillium-susceptible cotton.
UC IPM Pest Management Guidelines: