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1. IntroductionSclerotium Root Rot is a serious disease of sugarbeets. During the last few seasons it has caused a large amount of crop loss in the Sacramento Valley of California. The pathogen, Sclerotium rolfsii, is a soilborne fungus widely distributed in the southern U.S., as well as the warmer parts of the world. The disease is also known by the name "southern blight" because of its prevalence in the southern regions of the country. The fungus has a very broad host range, causing disease in over a thousand species of dicotyledonous plants, and millions of dollars of loss, on a variety of crops, each year.
2. Disease SymptomsSymptoms of this disease can be severe. The initial symptom of infection is wilting of the leaves (see image #1). Observation of the root reveals the presence of a white cottony growth on its surface (see image #2). Click image for full page view.
This white growth is called mycelium which is the vegetative growth form of the fungus. As the infection proceeds, the mycelium will eventually cover the entire surface of the root. The tissues of the root become soft and have a water-soaked appearance that is evident when cut open (see Image #3). Many spherical, dark brown, structures may be observed on or in the infected tissue, or in the surrounding soil (see image #4). Click image for full page view.
These structures resemble mustard seeds and are calledsclerotia, the survival form of the fungus. These structures will remain in the soil over the winter and be available to cause infection during following years. In the fall of the year symptoms are confusing. Beets may have symptoms of Sclerotium root rot, but without signs of the fungus. This is due to changing environmental parameters. As temperatures cool, other soil microorganisms become more competitive and attack the pathogen, the mycelium may be destroyed and the sclerotia may not develop. Sclerotia may persist for several years depending on the soil conditions. Many factors affect the survival of the sclerotia, such as temperature and moisture. The sclerotia will survive for 5 years or more in dry conditions, but survival under cool and moist conditions is much shorter. Sclerotia will persist for a long time in soils low in organic matter and available nitrogen.
3. Disease InfectionInitial infection occurs by the germination of the sclerotia. The optimal soil temperatures for germination are from 70° to 86° F. After the sclerotia germinate, the fungus grows to the sugarbeet root. Chemicals produced by the fungus attack the root tissue in advance of the fungus. These chemicals cause severe damage and even death of the tissue, making the root more easily invaded by the fungus. Once the fungus has infected a root, it is then able to grow through the soil and infect adjacent roots in the row. Because of this ability to spread down the row, a few initial infections can result in a large amount of disease. Operations that move soil have the potential to infest new fields with the disease. Soil adhering to implements, vehicles, or shoes has the potential to infest fields.
4. Disease SeverityCertain conditions may increase the amount of disease caused by Sclerotium rolfsii. Disease severity may increase during periods of temperature and soil moisture fluctuations, which apparently stimulate the sclerotia to germinate. The disease usually appears when the canopy is fully developed and soil temperatures are high. Free soil water is not required for infection, as it is for some other soilborne diseases, but moist soils favor disease development. Plant to plant spread of the fungus is more likely in moist soil. Cultivation practices may throw soil into the crowns of the sugarbeets which may increase infection.
5. Disease Control
Control of this disease is difficult. Certain crop rotations may reduce the inoculum levels of the fungus in the soil. Growing nonsusceptible crops such as corn, wheat or barley or growing winter crops such as peas, in infested fields, results in rapid decline in the population of sclerotia. Rotations with susceptible crops such as beans or sunflowers are likely to maintain high levels of sclerotia and result in severe disease during the next sugarbeet crop.
Other cultural practices may also be effective control measures. Deep plowing will bury the sclerotia deep in the soil so they will not be able to infect the crop. Sclerotia more than 5" deep in the soil will not germinate and will be destroyed by other soil microorganisms.
Incorporation of organic matter into soil has been shown to reduce the disease severity due to Sclerotium rolfsii. The addition of compost or oat straw or corn stalks has been effective. The disease control observed with these amendments may be due to the release of ammonia which is toxic to the sclerotia. The amendments also increase the populations of soil microorganisms. Some of these microorganisms will attack and kill the sclerotia.
Reduction of Sclerotium root rot has been accomplished with certain nitrogen fertilizers. Anhydrous ammonia, urea, ammonia sulfate and other ammonium forms of fertilizers reduce the population of viable sclerotia in soil. This reduction is apparently due to the stimulation of antagonistic soil microorganisms that are attacking and killing sclerotia. The use of large amounts of nitrogen on sugarbeets would result in low sugar concentrations in the root, so nitrogen application alone is probably not a viable control measure. The timing of the nitrogen application may be more important that the quantity. Side dressing the nitrogen may stimulate the beneficial microorganisms to become established on the root surface and protect the sugarbeet from infection. Additional studies are being planned to determine the optimum rates and application methods for these fertilizers to achieve the maximum disease control and sugar yield.
Although no sugarbeet varieties are resistant to this pathogen, preliminary work conducted by the USDA indicates that resistance may be possible. The development of disease resistance screening techniques is underway to identify resistant sugarbeet germplasm.
If adequate land is available, growers should avoid fields with high populations of the pathogens, and choose non-infested fields to grow sugarbeets. The Holly Sugar Company's plant pathology laboratory is able to determine the population of this pathogen in soil samples from grower's fields. Contact your Holly agriculturist for more information. Although several fungicides are active against the fungus, none are available that will economically control the disease in sugarbeets. Large amounts of these chemicals are required, and they are difficult to apply in amounts large enough to be effective.
Soil fumigants, such as metham-sodium, may be possible control tools. Since the sclerotia are usually found near the soil surface, they should be readily accessible to attack by these types of chemicals. Researchers in India have shown the sclerotia to be killed by metham-sodium. More work is needed to determine rates and application methods for this fumigant. Other chemicals, not developed as soil fungicides, are reported to be active against Sclerotium rolfsii. In a study conducted in India, aldicard and phorate were shown to reduce disease incidence in greenhouse and field trials. Additional experiments are needed to determine the usefulness of these chemicals.
Biological agents have shown promise for the control of this disease. Gliocladium virens is a soil fungus that is pathogenic to Sclerotium rolfsii. It has been tested for disease control in tomato with encouraging results. The biological control agent is added to the soil in alginat pellets containing wheat bran. The fungus is able to become established and parasitize the sclerotia of Sclerotium rolfsii, killing them.
Management of this disease will be due to an integrated approach on the part of the growers. Proper rotations with nonsusceptible crops are a major aspect of control. Use of some chemicals will aid in limiting the disease pressure. Optimal use of nitrogen fertilizers will help reduce inoculum. Proper sanitation will keep the fungus out of non-infested fields. The development of biological agents and resistant germplasm will contribute to disease control. A total management scheme to maintain a healthy crop and a healthy soil is needed to overcome this malady.
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