Management of Southern Blight

Sclerotium rolfsii is a soilborne pathogen with a very wide host range of over 500 plant species. As with most fungal soilborne plant pathogens, disease management involves exclusionary practices, plant removal, soil removal or treatment, plant treatment, crop rotation, use of resistant varieties, or a combination of these practices. The specific management practices used depend upon the crop and cropping situation.

Excluding the Pathogen from an Area
Only pathogen-free plants, cuttings or seeds should be purchased from a reputable dealer. In a greenhouse, all pots and planting equipment must be clean and pathogen-free, and planting media also must be pathogen-free. If field planting is involved, areas that do not have a history of S. rolfsii should be selected. Even though southern blight is not common in areas with cold winters, the disease may be a problem during the growing season if infected transplants from greenhouses or warmer regions are planted.

Plant Removal
As with many plant diseases, removal of infected plants is an important aspect of disease control. Sclerotium rolfsii usually causes infection at the lower stem (crown) section of the plant, and once infection takes place, removal of the whole plant is necessary. Prompt removal of infected plants will help prevent the addition of abundant fungal inoculum to the soil. If infected plants are allowed to remain in an area, these plants will serve as a continuous source of inoculum. Plant removal is a practical measure in landscapes, gardens, nurseries, and greenhouses, but not in field situations.

Soil Removal
Sclerotia are reported to survive in the soil for 3-4 years. In gardens or landscapes, some localized soil removal and replacement may help to control S. rolfsii. The grower should take care not to increase the distribution of the fungal inoculum in an area when transporting soil. In nurseries and greenhouses, used potting media should not be saved for new plants. Plant debris and used media should be thoroughly cleaned out of production areas after each crop is removed. Recycled containers should be thoroughly cleaned so as to remove all old pot media.

Soil Treatment

Treating the soil with heat (including solarization), fungicides or fumigants, cultural manipulations, organic amendments, fertilizers, or biological treatments may help to control southern blight.

Heat. In some large nurseries or greenhouses, it may be possible to treat beds or bulk soil with aerated steam. All areas must be brought to a temperature of 160-180 degrees F for 30 minutes. Treated soil should be stored away from contaminated areas. Even after steam treatment, some sclerotia may survive and losses may occur.

Solarization. Solarization for southern blight control has been beneficial in select situations in the southern United States. Successful solarization requires that the soil is prepared properly as for planting. Adequate soil moisture must be present. Clear plastic sheeting, 0.025 - 0.4 mm thick, must be applied to the area for 4-8 weeks, depending upon the time of year (Figure 24). For example, plastic applied during the month of June in Alabama would require a shorter exposure time of 4 weeks due to the high temperatures. Treated areas should receive direct and full sunlight. Soil solarization will significantly reduce viable sclerotia. It will also help control other soilborne diseases, plant parasitic nematodes, and some weeds. To be effective in control of southern blight, solarization must be repeated every year. Soil solarization is expensive, and it is not practical for use in large field situations.

Figure 24

Chemical soil treatments. Soil fungicides or fumigants have been successfully used to control S. rolfsii. The soil fungicide pentachloronitrobenzene (PCNB) has been used on peanuts and some other crops since the 1940s. In a recent peanut trial, azoxystrobin applied as preplant and postplant furrow treatments significantly controlled southern blight. However, applying fungicides to soil may require large quantities of chemical which is not practical in many situations. Also, fungicide effectiveness is not always consistent from year to year. With some ornamentals in commercial production situations, the fungicides azoxystrobin, flutolanil, flutolanil + thiophanate-methyl, and/or tebuconazole are labeled for pre- and post plant drenches. Fumigants, such as metam sodium or dazomet (granular), are toxic to sclerotia and mycelium in the soil. But, even after treatment with fumigation, some sclerotia survive, and treatments must be repeated annually. These fumigant chemicals are restricted-use products and may only be applied by certified applicators (Figures 25, 26).The toxicity and cost of fumigants limit their usefulness in many situations. Methyl bromide was used for many years to help control southern blight but it is no longer available, due to its detrimental effects on the ozone layer of the atmosphere. In addition to the more conventional soil treatments of fungicides or fumigants, a soil insecticide (chlorpyrifos) is also labeled for use as a soil treatment to control S. rolfsii in certain peanut cropping situations.

Figure 25	Figure 26

Cultural practices. Cultural modifications for management of S. rolfsii in the landscape include deep plowing, lime additions, aerification, and thatch removal. In some situations, deep plowing will provide disease control. At depths below 20-30 cm [8-12 in.]), sclerotia do not survive longer than 45 days. Also, deep plowing removes sclerotia from contact with root tissues. Although deep plowing is effective the first time, it may not work in subsequent years because sclerotia are returned to the upper soil layer. Keeping the soil pH at 6.5 by the addition of lime will help to prevent rapid fungal growth. Aerification of the soil and removal of thatch or other plant debris will also aid in suppressing S. rolfsii growth. In greenhouses, areas should be kept open with good plant spacing to help keep relative humidity low. Also, keeping the temperature below 25ºC / 77ºF and maintaining well-aerified plant media will discourage fungal growth. Nurseries must be designed to carry drainage water away from container areas.

Soil amendments, fertilization, biological agents and plant-produced chemicals. Soil can be treated with organic amendments, fertilizers, or biological agents to help control S rolfsii. The addition of organic amendments such as compost, oat or corn straw, or cotton gin trash to soil sometimes reduces southern blight incidence and development. This effect may be due to the increase of toxic ammonia and/or the increase of certain soil microorganisms in the soil. Furfuraldeyde, an organic (sugar derivative) amendment, has been shown to change the soil microflora, and this change has been related to a decrease of S. rolfsii in the soil in lab and greenhouse studies. Also, neem oil and pine bark extracts or pine bark powder have resulted in reduced growth of S. rolfsii. To date, these amendments have not been widely used for disease control, but future refinements may allow their use in the field. Fertilizer studies have shown that treatments with ammonium, calcium nitrate or calcium sulfate may help control southern blight. Increased nitrogen may inhibit sclerotia germination, alter host susceptibility, or alter the soil microorganisms. Biological control of S. rolfsii has been achieved to some degree with bacteria (Bacillus subtilis), actinomycetes, a mycorrhizal fungus, or certain Trichoderma species. There have been several studies to attempt to explain the mechanism for Trichoderma inhibition of S. rolfsii. Many studies have shown disease control by biological agents in laboratory and greenhouse tests, but disease control is less effective in the field. When control is seen in field studies, the required quantity of the biological product may be very high and not practical in most agronomic situations. In addition to the above, certain compounds in some aster roots and mustards are being studied for their inhibitory activity against this pathogen. At the present time, soil amendments, fertilization products, biological agents and plant-produced chemicals have not been widely adopted for southern blight control.

Crop Rotation
When soilborne plant pathogens are involved, crop rotation is a common and preferred method of disease control. This practice is not used much with southern blight because of the unusually wide host range of S. rolfsii. Corn is reported to be a non-host of this pathogen. When infested fields are rotated to corn, inoculum levels in the soil slowly decrease, and lower disease incidence will be seen in subsequent years. Some field research has shown that peanut rotations with cotton, ‘Pensacola’ bahia grass (Paspalum notatum), or switch grass (Panicum virgatum) resulted in a decreased incidence of white mold (stem rot) in peanut fields.

Plant Treatments
Plant treatments with fungicides for control of southern blight are generally protective in nature. Most fungicides are labeled for use on select ornamentals, vegetables, or some field crops. On peanuts, good disease control has been achieved by foliar applications of azoxystrobin products, flutolanil, or tebuconazole. PCNB is a fungicide that was used for many years to control S. rolfsii on some ornamentals, peanuts, some vegetables, and as a seed treatment. The product is available as a granular or liquid formulation. In 1998, PCNB no longer provided control of S. rolfsii in some Texas peanut fields. Fungal isolates tolerant to PCNB had developed. Southern blight control in peanut usually involves protective treatments with fungicides. However, good disease control has also been achieved with the insecticide chlorpyrifos, which is hydrolyzed into a fungicidal product, or with the nematicide ethoprop (mode of action unknown). For many years the only fungicide labeled to control southern blight on aucuba and several other ornamentals was PCNB. We now have the fungicides azoxystrobin, flutolanil, flutolanil + thiophanate-methyl, and tebuconazole in addition to PCNB available for use on select ornamentals for control of southern blight. Many of these products are labeled for pre- and post plant drench or broadcast application.

Resistance and Transgenic Plant Resistance
The use of resistant varieties or cultivars is always a much preferred method of disease management or control. Unfortunately, many common host plant species of S. rolfsii do not contain cultivars or varieties that exhibit high levels of resistance to this fungus. Nevertheless, research in this area continues. Some recent cultivar studies were done on Hosta, peanut, and cowpea. In 2003, 18 Hosta cultivars in a greenhouse study were evaluated for their resistance to S. rolfsii infection and disease development. Variable symptom severity resulted, but complete resistance to S. rolfsii was not reported. Recent inoculation studies with peanuts and cowpeas showed there were differences in disease susceptibility between some cultivars. And, preliminary results from a recent laboratory study with transgenic carrots indicated a reduced susceptibility to S. rolfsii.

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by The American Phytopathological Society