Comments on the Pathogen
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
The fungus moves within fields and between fields passively—something must vector it to new fields. Because the fungus can survive and reproduce on a wide range of plants, any movement of infected plant material (particularly roots) can spread the fungus. The fungus survives between crops and through winter as hardened fungal bodies called microsclerotia, which are pinhead size and barely visible. Microsclerotia are produced internally in the plant and are released into the soil when the infected plant material decomposes. Microsclerotia are then easily moved by any activity that moves soil or water from location to location. For example, microsclerotia are moved in soil during tillage operations, on equipment as it moves from field to field, and on shoes. The fungus can produce another spore type in or on hosts, but these spores are not a major source of spread of V. dahliae. A major source of V. dahliae is infected mint rhizomes used as planting material. Pay special attention to prevent any movement of plant material and roots infected with V. dahliae as well as soil infested with microsclerotia.
Comments on the Disease
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
Wilt incidence increases with increasing soil populations of V. dahliae microsclerotia. Growing susceptible crops such as mint allows the greatest reproduction of the fungus. Other hosts remain asymptomatic but may allow modest reproduction of the fungus. Grower experience in the Midwest and Oregon has shown that in fields infested with mint V. dahliae strains, potato and red clover are poor crop rotation choices; grass hays (orchardgrass, fescue, or timothy), corn, sudangrass, alfalfa, cereals, onions, and garlic are desirable rotation crop choices.
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.
No peppermint variety is immune from this disease, and recent research suggests all current varieties are equally susceptible. However, the fungus appears to reproduce faster in the variety Black Mitcham, which is popular for its high oil yields. If infested fields are eventually replanted to mint, choose the varieties Murray or Todds instead of Black Mitcham because they have a higher tolerance to the disease.
Prevent Importation of the Fungus into California
Ordinances in Shasta, Lassen, Modoc, and Siskiyou counties established in 1993-1997 require that all mint rootstock be certified free from V. dahliae. From the very beginning of the modern era of mint production in northeastern California in 1991, growers avoided aggressive strains of the disease by planting clean greenhouse-grown rootstock and increasing acreage locally by digging from these first planted acres. Nevertheless, in the summer of 1997, V. dahliae was first detected in mint in California in the Fall River Valley portion of Shasta County. The history of that field did not indicate that V. dahliae was imported with the first mint; rather, it appeared that the infestation came from a previous crop other than mint. Because the fungus has been identified in mint in California, growers must minimize disease loss and spread to other fields. There are three basic approaches to managing V. dahliae in mint.
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. 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.