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| The ARS-led National Sclerotinia Initiative supports 20 different projects aimed at neutralizing white mold's economic threat to seven different crops: sunflower, soybean, canola, edible dry beans, chickpeas, lentils and dry peas. This consortium of federal and state university scientists includes 10 land grant universities and five crop commodity groups, and is led by the Agricultural Research Service, the USDA's chief scientific research agency. |
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Sclerotinia sclerotiorumis a plant pathogenic fungus that causes important diseases known as white mold, Sclerotinia stem rot, wilt or stalk rot, or Sclerotinia head rot on a wide variety of broadleaf crops. It is commonly found damaging dry beans, sunflowers, soybeans, canola, peanut, and lentils. |
Germination of White Mold spores
| Sclerotinia sclerotiorum is a plant pathogenic fungus that causes important diseases known as white mold, Sclerotinia stem rot, wilt or stalk rot, or Sclerotinia head rot on a wide variety of broadleaf crops. It is commonly found damaging dry beans, sunflowers, soybeans, canola, peanut, and lentils. There are many other crops that are susceptible such as alfalfa, field pea, potato, mustard, safflower, flax, borage, crambe, buckwheat, chickpea, lupine, faba bean and numerous vegetables such as lettuce and carrots. Some of these crops are rarely damaged by S. sclerotiorum, while other are quite susceptible. This pathogen is known to infect about 408 species of plants. Numerous weeds such as marsh elder, lambsquarters, pigweed, Canada thistle, sow thistle, and wild mustard are also hosts and can play a role in disease cycles.
These tufts of "white mold" develop into hard black bodies, the sclerotia, some of which survive many years in the soil. Leaves of infected plants turn yellow and wilt. In dry weather affected stems have a bleached or whitish appearance. Infected seed is discolored, chalky and lightweight. White mold is favored by rainy weather before and at flowering, moderate temperatures and long periods of high humidity, and keeping the lower canopy of plants wet more or less continuously for up to two days. |
| Sclerotinia stem rot has been the most serious disease of canola in North Dakota and Minnesota in recent years, with average incidence (percent infected plants) as high as 19 percent in North Dakota in 1993 and 19 percent in Minnesota in 1997. Estimated state-wide losses from Sclerotinia were as high as 13 percent in North Dakota (1993) and 13 percent in Minnesota (1997). In severely infected fields losses were estimated as high as 50 percent. |
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White mold is a sporadic disease which is most serious when wet weather occurs at flowering. When the surface soil is wet, the hard black survival structures, called sclerotia, germinate to form tiny mushroom-like bodies that liberate millions of wind-borne spores. These spores colonize dead bean tissue, particularly the dead blossoms, then the fungus invades green tissues, causing a watery soft rot. In wet weather infected tissues are tan colored and soft, with tufts of fluffy white fungal growth. These tufts of "white mold" develop into hard black bodies, the sclerotia, some of which survive many years in the soil. Leaves of infected plants turn yellow and wilt. In dry weather affected stems have a bleached or whitish appearance. Infected seed is discolored, chalky and lightweight. White mold is favored by rainy weather before and at flowering, moderate temperatures and long periods of high humidity, and keeping the lower canopy of plants wet more or less continuously for up to two days. |
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White mold of pea occurs worldwide under wet conditions on foliage and stems with excessive vine growth. White mold is caused by Sclerotinia sclerotiorum, a fungal pathogen that has a wide range of host plants including many cultivated crops such as beans, Brassica spp., vegetables, potato, sunflower, soybean, and pea. The pathogen infects stems, leaves and flowers that are in contact with the ground due to lodging. It produces prominent black sclerotia within mycelium and in cavities of infected stems. Sclerotia can germinate and produce mycelium for infection, but they most often develop apothecia, a sexual fruiting body produced after a period of dormancy. Within the apothecia, asci develop which eventually release ascospores into the air. The disease is most severe at the late growth stage when plants are flowering and the canopy is closed. |
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The Sclerotinia diseases are some of the most important diseases of sunflower in the Northern Great Plains. Three diseases are recognized in the field: Sclerotinia wilt, middle stalk rot, and head rot. Wilt is distinct because it begins as a root rot, whereas Sclerotinia head rot and middle stalk rot are above-ground diseases caused by airborne spores. All three diseases are caused by the same organism, Sclerotinia sclerotiorum, a very destructive fungus which is often called the "white mold" fungus. Sclerotinia sclerotiorum is a serious problem not only in areas where sunflower and dry bean have been cultivated for many years, but also in areas where irrigated susceptible crops are grown. As production of sunflower in North Dakota moved west, there was an increase in the incidence and severity of Sclerotinia diseases in areas with no previous history of the pathogen. This fungus can be a serious problem in the drier sunflower production areas. Sclerotinia sclerotiorum attacks approximately 374 species of broadleaf plants. The row crops commonly damaged in the Northern Great Plains are sunflower and dry bean; however, many other crops are susceptible, such as soybean, buckwheat, flax, lentils, pea, potato, mustard, crambe, rapeseed or canola, Jerusalem artichoke and safflower. Not all of these other crops are as susceptible to S. sclerotiorum as sunflower and some are rarely damaged. Many broadleaf weeds such as marsh elder, common lambsquarters, redroot pigweed, Canada thistle and wild mustard also are hosts. |
| The primary survival (overwintering) structure of S. sclerotiorum is the sclerotium. A sclerotium is a hard resting structure consisting of a light colored interior portion called a medulla and an exterior black protective covering called the rind. The rind contains melanin pigments which are highly resistant to degradation, while the medulla consists of fungal cells rich in ?-glucans and proteins. The shape and size of sclerotia depend on the host and where they are produced in or on infected plants. |
| What is the origin of sclerotia in a field? There are four primary methods that fields are infested with sclerotia. The most common is by susceptible crops or weeds being infected by ascospores coming from adjacent infested fields. The fungus then produces sclerotia on those plants and some are returned to the soil when the field is harvested. Wind transported soil or crop debris infested with sclerotia are also known to contaminate adjacent fields. Contaminated machinery can introduce sclerotia into a field. Surface irrigation water or rain water moving naturally between fields can also move sclerotia to previously clean fields. Seed contaminated with sclerotia is another method of introducing the fungus into clean fields. |
| The basic disease cycle of Sclerotinia diseases begins with the overwintering of sclerotia in the soil. Sclerotia are conditioned to germinate by the overwintering process. In the growing season, overwintered sclerotia can germinate in one of two methods. Probably the most common is carpogenic germination which results in the production of a small mushroom called an apothecium. Carpogenic germination usually requires the sclerotia to be in wet soil for one to two weeks prior to germination. The apothecium forms spores called ascospores which are ejected into the environment. Most will fall on susceptible plants in the immediate area of the apothecia, but some can travel long distances by wind. Ascospores require free moisture plus a food base such as senescent flower petals or damaged tissue to produce a small colony that can then infect the plant. The pathogen produces oxalic acid and numerous enzymes that break down and degrade plant tissue. The requirement of moisture for carpogenic germination and growth of the pathogen are reasons why rainy periods or irrigation are associated with outbreaks of disease on certain crops. Disease development is favored by moderate temperatures of 15 - 25 C. |
| The other method of germination is myceliogenic, where the sclerotium produces mycelium. A primary crop where myceliogenic germination plays a major role in the disease cycle is in Sclerotinia wilt of sunflower. Sclerotinia wilt is caused by sclerotia germinating and infecting the sunflower roots. Most other Sclerotinia or white mold diseases, such as on dry beans, soybean, canola and sunflower head rot are initiated by carpogenic germination and infection of above ground plants parts by ascospores. |
| How long do sclerotia survive in the soil? Few studies have quantified sclerotia survival in the field. There are many factors affecting survival such as soil type, previous crops, initial population of sclerotia and environmental conditions, but how and to what degree they affect survival is not well understood. High temperature and high soil moisture combined are probably the two most deleterious environmental factors. Microbial degradation, however, is the principal reason for a decline in the populations of sclerotia. There are many fungi, bacteria and other soil organisms that parasitize or utilize sclerotia as carbon sources. One reason that crop rotation is recommended for Sclerotinia is to allow the natural microbial population to degrade sclerotia. Two important fungal parasites involved in the natural degradation of sclerotia are Coniothyrium minitans and Sporidesmium sclerotivorum. Both these fungi have been touted as possible biocontrol agents for sclerotia, and some commercial products are now available. |
| The effect of tillage on survival of sclerotia is poorly studied and no generalizations can be made to aid in management of the pathogen. There is evidence that leaving the sclerotia on the soil surface enhances degradation whereas burying the sclerotia enhances survival. It is thought that the more dramatic changes in temperature and moisture on the soil surface are deleterious to sclerotia. |
| Because of the numerous crops infected by this pathogen, there are many strategies for control. Fungicides have been use with some success such as with dry bean and canola. Crop rotation continues to be used for certain crops such as sunflower where inoculum densities in the soil play a major role in disease development. Host resistance has been an elusive goal of many control programs. Most Sclerotinia diseases are not controlled principally by host resistance. However, some moderate levels of host resistance such as in dry beans and soybean have been found and can aid in integrated control programs. Disease escape mechanisms via plant architecture also have a role in reducing disease. Cultural controls such as wider row spacing or lower plant populations that reduce the microclimate favorable for disease development are used with some crops. Sanitation practices such as with vegetable production, and clean seed programs to keep sclerotia out of seed lots are also useful practices in some crop production systems. Biological control has only recently been tried on a commercial scale, but the results of farmer's acceptance of this method remains to be determined. Sclerotinia continues to be a very difficult pathogen to control. |
