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Disease Management
Preplant management options
Site selection: Most of the land now cropped to potatoes and peppermint is infested with Verticillium. In landscapes where susceptible plants (Table 2) have been grown previously, the soil can be infested. Even in settings where non-susceptible hosts such as wheat, barley, and legumes, have been grown, Verticillium has been isolated from the roots, indicating that these nonhosts may serve as a reservoir of the pathogen in some environments. Therefore, consideration must be given to the cropping history and Verticillium wilt history of the site. Laboratory tests can estimate the population size of Verticillium in the soil.
Table 2. Plants resistant or susceptible to Verticillium wilt
Resistant or Immune |
Susceptible |
Trees |
|
| beech, birch, conifers (all), crabapple, dogwood, eucalyptus, hackberry, hawthorn, honey locust, hornbeam, katsura tree, mountain ash, mulberry, plane tree, poplar, sweetgum, sycamore, willow | ash, black locust, box elder, camphor tree, carob, carrotwood, elm, catalpa, cork tree, elder, elm, flannel bush, fringe tree, golden-rain, tree, horse chestnut, Japanese, pagoda tree, Kentucky, coffee tree, linden, magnolia, maple, oak, peppertree, persimmon, Prunus sp., redbud, Russian olive, sassafras, serviceberry, smoke tree, tree of heaven, tupelo, tulip tree, walnut, yellowwood |
Shrubs |
|
| arborvitaejuniper | fuchsia, heather, lilac, privet, rose, sumac, Viburnum sp. |
Vegetables |
|
| asparagus, beanpea | cabbage, celery, cucumber, eggplant, lettuce, melons, pepper, potato, pumpkin, radish, rhubarb, tomato |
Flowers |
|
| African violet, ageratum, baby’s breath, hollyhock, periwinkle, primrose, zinnia, | aster, begonia, carnation, chrysanthemum, dahlia, geranium, impatiens, peony, petunia, snapdragon, sunflower |
Fruit |
|
| apple, pear | barberry, blackberry, black raspberry, grape, Prunus sp., raspberry, Ribes sp., strawberry, watermelon |
Weeds |
|
| grass, monocots (all) | dandelion, groundsel, lambsquarters, nightshade, pigweed, sagebrush,shepardspurse, velvetleaf |
Field crops |
|
| corn, monocots (all), | alfalfa, clover, cotton, hops, mint, rapeseed, safflower |
Miscellaneous |
|
| cacti, ferns | |
Reference: McCain, A.H., R.D. Raabe, and S. Wilhelm. 1981. Plants Resistant to or Susceptible to Verticillium Wilt. University of California Leaflet 2703. |
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Crop rotation: Crop rotation should be viewed as a preventative, not a curative disease management option for two reasons. First, the host range of Verticillium is very wide. Second, the survival structure, the microsclerotium, persists in soil for a long time, even in the absence of a susceptible host. Therefore, rotation programs should be initiated early, before inoculum builds up to high levels in the soil. Populations of the pathogen decline very slowly through mortality over time. Fallowing a field (leaving it uncropped and weed-free during the growing seasons) or planting non-susceptible crops for several consecutive seasons do not lead to significant reductions in the populations of microsclerotia. Rotation in general is a less effective disease management tool compared to other methods, but is generally a good management practice for multiple reasons. Recent studies, however, have demonstrated that specific rotations such as those with broccoli lead to significant, long-term reductions in the populations of microsclerotia. Hence, attempts to identify hosts that are not only non-hosts of Verticillium spp., but also bring about significant reductions in microsclerotia in soil, that are compatible with production practices in a given area are urgently needed.
Chemical fumigation: Application of soil fumigants is an effective, but expensive control tactic. Fumigants are applied through either shank injectors (Figure 14) or irrigation systems in either the fall or the spring. When injected, the soil is sometimes covered with plastic to prevent volatilization of the fumigant (Figure 15). Depending upon the chemical, rate, and environmental conditions at time of application, reduction in soil populations have ranged from 85-95%. Fumigation rates need to be high when soil populations of Verticillium are large or when populations need to be reduced for multiyears, as with perennial crops such as peppermint and maple. Some soil fumigants can be detrimental to the environment, and some of these fumigants will no longer be registered for use in the next few years.
 Figure 14 |
 Figure 15 |
Cultivars: Resistant or partially resistant cultivars of some susceptible plant species, including potato, peppermint, and maple, are available (Table 3). In the presence of very high Verticillium soil populations, however, even resistant cultivars may exhibit symptoms of Verticillium wilt.
Table 3. A selection of Verticillium resistant potato, peppermint, and Norway maple cultivars
Potato |
Peppermint |
Norway maple |
|
Resistant Abnaki, Century Russet, Desiree, Elba, Gemchip, Ranger Russet, Reddale, Rideau, Russette, Targhee |
Moderately resistant Allegany, Atlantic, Centennial, Russet, Frontier Russet, Hampton, Katahdin, Maine Chip, Monona, Norchip, Norking Russet, Portage, Prestile, Russet Burbank, Russet Nugget |
Partially resistant Todd’s Mitcham, Murray Mitcham, Roberts Mitcham |
Tolerant Jade Glen, Parkway |
Pathogen-free planting stock: Species which are planted from bare-root stock or seedlings should be obtained from Verticillium wilt-free nurseries. In Oregon, a certification program is in place for ensuring Verticillium-free peppermint stock. Potatoes are not certified for this disease.
Management during the season
Fertilization: Applying optimal rates of nitrogen and phosphorus reduces the severity of Verticillium wilt symptoms in some plant species. Optimal soil fertility helps to produce plants which may be more resistant to infection by Verticillium.
Irrigation: Limiting the amount of water applied to the field can reduce severity of the disease in some plant species, such as potato. Overwatering, especially early in the growing season, results in increased infection and, thus, increased disease severity.
Management after harvest
Propane flaming: High temperatures are lethal to Verticillium propagules. Because microsclerotia reside within the crop residue, flaming crop residue at the end of a growing season reduces the amount of inoculum returned to the soil. Following peppermint harvest in the fall, propane torches are pulled behind a tractor to flame the crop refuse (Figure 16). The effect on the disease is to slow down the rate of inoculum increase in these fields.
 Figure 16 |
Pruning: In woody host plants, pruning of symptomatic limbs to remove all discolored sapwood, followed by burning, has prolonged the life expectancy of some trees.
Copyright © 2000 by The American Phytopathological Society