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Cotton

Soil Sampling for Verticillium Wilt

If percent stem discoloration is high, or if you would like to determine how long to rotate out of cotton, consider sampling the Verticillium inoculum (microsclerotia) levels using soil sampling. To monitor inoculum levels from one season to the next, sample soil at the same time each year.

Take samples in the summer to help you decide whether to plant an alternate crop or a resistant variety the following season.

To sample Verticillium inoculum levels:

  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 that has 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 (30 cm) in at least three randomly chosen places per area.
  4. Place the soil in a bucket or bag.
  5. Mix all the soil collected from one area thoroughly and transfer about 50 to 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 (for example, 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 sclerotia 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.

Since microsclerotia are produced gradually as infected plant debris decays, 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.

Related information

Statewide IPM Program, Agriculture and Natural Resources, University of California
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