By C. M. Ocamb and D.A. Johnson
Cause A fungus, Verticillium dahliae, which lives in soil and in diseased plants. It is soilborne and once established in soil is almost impossible to eradicate due to microsclerotia, which germinate and infect roots. The fungus grows throughout the vascular system and up into mint stems. After diseased plant parts die, microsclerotia form and remain several years in soil. The pathogen is spread with infected rhizomes used for planting and in contaminated surface irrigation water. Co-infection of V. dahliae and the root lesion nematode, Pratylenchus penetrans, increase disease incidence and severity.
The VCG 2B strain of V. dahliae is most aggressive and prevalent in mint. This strain infects potato but symptoms and damage on potato are very mild or do not occur. The VCG 4A and VCG 4B strains, which are frequently obtained from potato, infect mint but symptoms and damage on mint are mild or not evident.
Native spearmint is relatively resistant, but Scotch spearmint and Black Mitcham peppermint are susceptible. Redefined Murray Mitcham is moderately resistant, but this variety is not as vigorous as Black Mitcham in the Columbia Basin. Grasses including corn and related crops do not sustain the population, but fallow or grass rotations alone may take many years to effectively reduce soil populations below economic levels.
Symptoms First, upper leaves twist and curl. Leaves are bunched at the top of the plant. Infected plants are stunted and foliage is yellowish to red or bronze. Lower leaves die first, then the aboveground part of the plant. With flowering or other stresses, stems or plants may die too rapidly for these symptoms to be observed.
Sampling Soil sampling is not recommended. Laboratories can determine the number of Verticillium propagules per unit of soil. However, there are no clear guidelines on what constitutes an economic threshold nor do we know whether laboratories' estimates are comparable. Also, soils may contain strains of the fungus that are not pathogenic to mint, which soil-only assays are not able to determine.
- Use certified Verticillium-free stock for new plantings.
- If possible, plant in fields that have not had a history of growing mint. Rotate infested fields with corn and alfalfa.
- Take severely infested areas of a field out of mint production to prevent spread to new areas or fields. Do not plant mint there for 6 to 10 years unless soil is fumigated before planting.
- Remove badly infested fields from mint production before wilt becomes so severe that mint will no longer grow.
- Begin a program of field flaming when wilt first appears in a planting. Fields severely infested with wilt will not respond to flaming.
- Western Oregon-if possible, flame within 1 week after harvest to allow regrowth before frost. This will strengthen the rootstock. If necessary, irrigate after harvest to keep soil moisture adequate for growth. Flame at 1.75 to 2 mph with 40 psi gas pressure.
- Central Oregon and Malheur County, OR-Flame in fall in fields after wilt has appeared, and cultivate only when necessary to control soil insects and soil compaction and for stand regeneration.
- Plant varieties Todd's Mitcham, Redefined Murray, Robert's Mitcham or other improved varieties, which have some resistance to Verticillium wilt and may be cropped successfully on infested soils. In addition to the above cultural controls, observe the following guidelines in using these varieties.
- Never immediately replant these varieties in a field where peppermint has failed due to Verticillium wilt.
- On heavily infested soil, rotate out of mint at least 5 years before planting unless soil fumigation precedes planting; otherwise the new planting is likely to fail. Even 5 years may not be sufficient if growing something other than cereals or grass seed.
Chemical control Soil fumigation, along with cultural control methods, may help to control wilt. Additional benefits include controlling nematodes that affect mint production. Soil must be properly fumigated for satisfactory results. Factors to consider in soil fumigation include soil preparation, temperature, moisture, and soil surface seal.
Consider the field's history in deciding whether to use fumigants and, if so, at what rate. Wilt potential, or the probability of disease, is related directly to the amount of wilt in the last peppermint crop and the number of years since the last peppermint crop that the field has been in crops other than mint.
Verticillium wilt potential is high in fields where peppermint has failed due to Verticillium wilt and the fields have been rotated to other crops 4 years or less. Wilt potential is low in fields that had a small amount of wilt in peppermint, in fields never in peppermint but that have received Verticillium inoculum from windblown dust, water, etc., and in fields moderately to lightly infested but rotated out of mint 5 years or more to crops other than cereals or grass seed.
- Telone II at 25 to 30 gal/A as a broadcast treatment in the spring or fall. 5-day reentry. Restricted-use pesticide.
- Telone C-17 at 25 gal/A for a low-wilt-potential field, at 27.5 gal/A for a high-wilt-potential field. 5-day reentry. Restricted-use pesticide.
- Vapam HL at 37 gal/A for a low-wilt-potential field, at 75 gal/A for a high-wilt-potential field. Note: Low rate may not adequately control nematodes. 48-hr reentry and/or while tarps are being removed. Restricted-use pesticide.
Reference Johnson, D.A., and Santo, G.S. 2001. Development of wilt in mint in response to infection by two pathotypes of Verticillium dahliae and co-infection by Pratylenchus penetrans. Plant Disease 85:1189-1192.