How to Manage Pests
UC Pest Management Guidelines
There are several different strains of Verticillium dahliae, and each strain varies in its ability to attack different host plants. Individual strains can infect surface cells of roots of nearly all plants but that doesn't mean it can thrive on all plants. Strains that only infect root surfaces on a particular host usually cannot reproduce or only reproduce sparingly. Strains better suited to infect an individual host grow internally in the plant and reproduce at higher levels.
Aggressive strains that infect mint and cause visual wilt symptoms (wilting and plant death) reproduce at very high levels within mint. Certain asymptomatic hosts of this pathogen (e.g., strawberry) may allow modest reproduction of the fungus. Strains more specialized on other crops (e.g., potatoes) usually do not attack mint, although exceptions may occur. The Verticillium that attacks alfalfa, V. albo-atrum, is a distinctly different species and does not infect mint. Native spearmint (Mentha spicata L.) found along waterways in northeastern California is considered resistant to V. dahliae.
Survival and Spread of the Fungus
Verticillium wilt is a serious disease of peppermint in the Pacific Northwest. Once the fungus is established in a field, oil yield may be reduced, stands may decline, and the ability of mint plants to compete with weeds may be reduced. Infected plants eventually die. Useful stand longevity may be reduced from 5 years or longer to as little as 2 to 4 years. Although the disease has been found in California, it is currently found at very low levels.
Activities that Increase Disease Damage
Keys to managing this disease include using only rootstock certified to be free of V. dahliae and eliminating field-to-field movement of microsclerotia and infected plant material. This includes cleaning all equipment and vehicles entering mint fields and cleaning shoes of people moving from field to field. Although fumigants are registered, their use is uncommon because the occurrence of this disease is so rare.
Prevention of Importation of the Fungus
Reduce spread of microsclerotia and infected plant material. Since little disease is present in California mint fields, the most important practice is to clean harvest equipment and vehicles that move between fields to prevent movement of contaminated plant material and microsclerotia in soil. Although the steam used in harvest containers (called "tubs") to extract oil is thought to kill the fungus, the outside surfaces of tubs and trucks can still transport infested plant material and soil. A pressure sprayer is satisfactory for cleaning vehicles and equipment moving between fields. All workers moving from field to field should clean their shoes to prevent movement of microsclerotia, spores, and infected plant material. Wash rubber or plastic boots with a 10% bleach solution. Cover leather boots with disposable plastic shoe covers. Minimize cultivation to avoid spreading microsclerotia. Do not use tillage for weed control in infested fields because tillage can move microsclerotia within fields, which can create new infestations. Plant perennial nonhost crops such as alfalfa and grass hay that are not cultivated to eliminate production of microsclerotia and their spread by tillage.
Reduce production of microsclerotia. Regularly inspect all fields for the presence of V. dahliae. Once V. dahliae is discovered in a field, it is prudent to remove the mint and plant a nonhost. Rotation to crops that do not encourage reproduction of V. dahliae for a minimum of five years can help reduce populations of V. dahliae. If the disease is found in a field and the field is not taken out of production, the numbers of microsclerotia can build to such high levels that a rotation of 10 or more years will be needed to allow the infestation to decline to tolerable management levels. Many rotation crops will not reduce microsclerotia populations of V. dahliae. Avoid growing red clover, as it seems to increase the disease. Do not rotate to mint, potatoes, or strawberry. Some crops that will not encourage reproduction of the fungus are grass hays (orchardgrass, fescue, or timothy), corn, sudangrass, alfalfa, cereals, onions, and garlic. The fumigants metam sodium or chloropicrin can reduce microsclerotia populations but are seldom used because of their high costs. Do not plant the Black Mitcham variety in fields known to be infested with V. dahliae; choose either the variety Murray or Todds, which are more tolerant of the disease.
Destroy microsclerotia by annual fall burning. If the disease is discovered, destroy microsclerotia by annual fall burning (flaming) that could otherwise overwinter and increase disease losses. Fall burning is commonly done shortly after harvest. After burning, do not stress plants with practices such as delaying water and nitrogen application because stress may aggravate the disease. Fall burning can cause injury to stands, so fields where V. dahliae is not present should not be fall-burned.
UC IPM Pest Management Guidelines: Peppermint