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Disease Management

Management of Sclerotinia occurs at several stages of crop development. Techniques include: making the crop microclimate less conducive for infection, utilization of effective fungicides to protect susceptible plants, elimination of sources of inoculum, and selection of resistant plants. Successful disease control commonly requires implementation and integration of multiple techniques.

Cultural practices that influence crop microclimate
Canopy management. Cropping practices that reduce the intensity and duration of a disease-favorable microclimate within the canopy can lessen white mold severity. Factors that may influence the microclimate include row spacing and orientation, nitrogen fertilizers, and cultivar selection. Studies on row spacing in legume crops consistently show that white mold incidence is lower in crops with wide row widths than those planted in narrow rows. Consequently, the management goal is to space rows at the distance that will maintain plant densities for maximum yield while providing for adequate room to facilitate air movement to reduce high moisture microclimates within the canopy. Precision seeding helps to optimize plant spacing and avoids clumping of plants. Because infection by ascospores of S. sclerotiorum and S. trifoliorum requires an extended period of free moisture, orienting rows parallel to the direction of the prevailing winds also may be of some value in quickly drying the canopy after a rain or irrigation event. In addition, to avoid dense crop canopies, applied nitrogen should not exceed the optimal rate for a particular crop. Lastly, when choices are available, cultivars that mature early and have a more upright, as opposed to a vining (prostrate), growth habit will generally have less disease.

A primary cropping practice for prevention of alfalfa crown and stem rot (caused by S. trifoliorum) is to plant seed in the spring or late summer rather than the fall. Warmer temperatures at these times inhibit germination of S. trifoliorum sclerotia; however, planting must be coordinated to ensure adequate soil moisture for the crop.

Sprinkler irrigation. When a crop is irrigated, the goal is to manage irrigation events to reduce the frequencies of 12 to 24 hour periods of leaf wetness, especially during the bloom period, when flower petals can become colonized by the ascospores of S. sclerotiorum. Water deprivation at bloom, however, may compromise the quality of the harvested product. Therefore, a strategy known as ‘irrigation cut-off’ can be used to manipulate the duration of time in a day that foliage remains wet after irrigation. This technique simply requires initiating daily irrigation events in the very early morning and stopping all irrigation near noon. The early afternoon ‘cutoff’ allows time for the sun to dry the foliage before nightfall, thereby helping to avoid prolonged periods of plant wetness.

To reduce disease due to S. minor, hyphal germination of sclerotia can be reduced by allowing the soil surface to dry thoroughly between irrigation events. Each irrigation event must therefore provide sufficient water to allow for a prolonged dry period.

Chemical protection of susceptible plants
Fungicides, applied as protectants before infection, especially during the bloom period, are effective in inhibiting infection by ascospores in fields with a history of white mold, and several registered fungicides are available for this purpose. The number of fungicide applications required for disease control depends on the length of the crop season and the period of time that ‘weak’ tissues (flower petals) are available for colonization by ascospores. A single fungicide application carefully timed during the bloom period may be sufficient in some crops such as snap beans and potato, whereas 2 to 3 applications may be necessary in crops with a longer bloom period, such as lima bean. In order to be effective, it is necessary that fungicides penetrate deep into the canopy to adequately cover the flowers and the places on the plant where the senescing petals might adhere or become lodged.

In lettuce production, fungicides can be used to prevent infection by S. minor, provided that the chemical penetrates and persists on the soil surface to provide a barrier protecting the bottom leaves and stems of the plants. In peanut, a combination of plant pruning and fungicide application was shown to decrease infection by S. minor, possibly because a reduced canopy allowed the chemical to more adequately reach the soil surface.

Management of overseasoning inoculum
Crop rotation with nonhost crops reduces the number of sclerotia in the soil by loss of viability over time. In addition, sclerotia may germinate in the absence of a host crop, but without subsequent host infection, new sclerotia are not returned to the soil and numbers are gradually reduced. Crop rotation is most effective when initiated before white mold becomes a serious disease problem in a field.

Even in the absence of a susceptible crop, sclerotia can remain viable in the plow layer of field soil for up to 5 years. If numbers of sclerotia in a field are low, rotations of 3 to 5 years with a nonhost crop may be sufficient. Once the pathogen is well established in a field, and soil is highly infested with sclerotia, crop rotation may be of less value because of the long survival time of these propagules. Consequently, growers may be faced with the difficult problem of finding and growing an economically viable, non-susceptible rotation crop for more than 5 years.

Crop rotation is further limited by the wide host ranges of Sclerotinia spp. Corn, wheat, and sorghum are nonhost crops of S. sclerotiorum that can be planted in rotation with susceptible crops such as soybean and sunflower. For lettuce infected with S. minor, broccoli grown as a nonhost crop has been shown to reduce the number of sclerotia in the field. Host crops of S. trifoliorum are limited to forage legumes, so grains and annual forage grasses may be used in crop rotation. A number of annual legumes have been shown to be hosts to Sclerotinia spp., so they should not be used as cover crops.

Biological control. Several fungi have been shown to be parasites of sclerotia of S. sclerotiorum. One of these organisms, Coniothyrium minitans, has been released as a commercial product for suppression of white mold due to S. sclerotiorum. In practice, dried spores of this parasite are sprayed onto pathogen-infested crop debris either at the end of a season, or onto the soil surface before planting. Parasitization of sclerotia has been shown to reduce the number of apothecia formed by S. sclerotiorum. In several studies, soil treatment with C. minitans has lessened white mold severity in the crop that follows the treatment. This treatment is most useful when combined with other components of an integrated disease management program.

Tillage effects on white mold are complex. Although sclerotia can survive in the plow layer for several years, only the sclerotia near the soil surface germinate to produce apothecia and ascospores. Therefore, burying infested residues with a moldboard plow can prevent the apothecial germination of sclerotia. A subsequent plowing in another season, however, can bring these sclerotia back to the surface, and any tillage operation can also contribute to the dispersal of sclerotia. An alternative recommendation to control S. sclerotiorum in a soybean–corn (host-nonhost) rotation has been to follow a diseased soybean crop with the use of no-till in the corn crop. This type of tillage leaves the sclerotia near the surface and promotes their germination under the corn. Consequently, the number of viable sclerotia that survive to the next soybean season is greatly reduced.

Weed control. The host range of S. sclerotiorum is very broad and includes many of the most important broadleaf weeds of cultivated crops: Canada thistle, Jerusalem artichoke, lambsquarters, mustard, nightshade, pigweed, ragweed, shepherd’s purse, sow thistle, velvet leaf and vetch. If these weeds occur in a field, they may provide inoculum for a host crop. When rotating to a nonhost crop, the effect of tillage (or lack of it) on weed control is an additional consideration, as poor suppression of broadleaf weeds may lessen the benefit of crop rotation.

Host resistance
Cultivars able to resist infection by S. sclerotiorum would be a predictable, convenient, and inexpensive option for controlling white mold. Unfortunately, identification and selection of highly effective physiological resistance to this pathogen has proven to be a difficult challenge. As a consequence, commercial cultivars that show high levels of resistance to white mold are relatively uncommon. When available, these cultivars, at best, generally have only a partial impact on the overall amount of disease.

Biotechnological techniques are being used to create novel types of resistance to diseases caused by Sclerotinia spp. For example, the transfer into peanut of a gene from barley that encodes for an enzyme to degrade oxalic acid (i.e., oxalate oxidase) makes a peanut plant much more resistant to infection by S. minor. Such genetically modified cultivars may become a major means of Sclerotinia disease management in the future.

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