Verticillium Wilt

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.

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.

• Use tolerant varieties. These may include most modern Acala and Pima varieties; however, the Verticillium resistance evaluation programs of the University of California and most seed companies are less rigorous than in the past. In areas with a history of Verticillium wilt, be cautious when planting large areas with varieties where resistance to Verticillium is unknown. Many of the non-Acala Upland varieties are more susceptible.
• Rotate cotton with corn, wheat, barley, sorghum, safflower, or rice.
• Delay first irrigation if disease pressure is high (more than 10 microsclerotia per gram of soil) and air temperatures are cool.

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:

1. Before taking soil samples for Verticillium assay, contact the lab you will use and follow any special instructions
2. Take a sample from each area with a different cropping history; keep samples separate according to crop history
3. Use a shovel or soil tube to collect soil to a depth of 1 foot in at least three randomly chosen places per representative area
4. Place the soil in a bucket or bag
5. Mix thoroughly all the soil collected from one area and transfer about 50-100 g (2-4 oz) to a plastic bag or moisture proof container
6. Label each sample with a field number and other appropriate identification (i.e. cropping history)
7. Keep samples cool and deliver as soon as possible to the lab

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.