Sclerotinia sclerotiorum is a plant pathogenic fungus. It is responsible for the development of white mold, cottony rot, stem rot, crown rot, and blossom blight. While the exact cause is unknown, there are a variety of possible treatments, including fungicides and crop rotation. If you are interested in getting rid of white mold, read on to learn more about this fungal problem.
Symptoms
When white mold sclerotinia invades plants, it produces acetic acid. The white mold sclerotia form in stem pith cavities, which are surrounded by hard black sclerotia. The diseased stems and leaves eventually die. Plants with a white mold infection are recognizable by their dry lesions and pale, bleached-white stems.
In the first stage of the disease, the organism will be in the form of sclerotia, which overwinters in the soil. When these ascospores re-grow in the plant’s tissue, they develop into small tan mushrooms called apothecia. These apothecia release airborne ascospores. The disease will also develop on plant parts that are above ground.
Sclerotia form in plants and decay them. Its outer rind is black, while the interior is light-beige. They are irregular in shape, typically 2 to 5 mm in diameter and up to 25 mm long. The sclerotia of S. sclerotiorum and S. trifoliorum measure between 0.25 and 3 mm. Infected plants can survive for years in the soil.
When the white mold sclerotiorum fungus attacks a plant, the infection can be asymptomatic or very noticeable. Plants can develop crown cankers, stem rots, wilts, and seedlings may suffer damping-off. Brown spots may also appear on the petals of flowers. Cottony masses of fungal threads may also develop on stems and soil. Ultimately, the infected plant will have a white mold covering its stems and leaves.
White mold is a fungus that attacks over 300 species of plants. It is a serious disease that can cause significant yield losses. While it is easy to identify, the symptoms of a white mold infection are usually accompanied by a whitish, fluffy growth on the stem. Symptoms of the disease usually appear between R3 and R6 of the growth cycle. When the lesions begin to form, they progress up the stem, above and below nodes, and girdle the entire plant. The white mycelium covers the area with the infection during periods of high relative humidity. Black sclerotia are embedded in the mycelium, and these are visible on the lesions.
Life cycle
The life cycle of a fungus is complicated, and white mold sclerotia have the potential to persist for up to five years in the soil. However, low populations can be controlled by a three to five-year crop rotation. At the other extreme, a 10-year rotation is necessary for high populations. Fortunately, a number of cultural practices can minimize sclerotia rot. For example, weed eradication and crop rotation with non-susceptible hosts will reduce inoculum levels. The sclerotia will germinate on non-susceptible plants and will not complete the life cycle. In fields with a history of white mold, non-susceptible crops should be grown in order to minimize inoculum levels.
The sclerotia develop on the surface of the infected plant tissue. Later, the fungus forms hard, black sclerotia. These spores are most commonly found on the outer surface of diseased plant tissue, but they can also be found inside soft host tissues such as leaves, stems, or developing fruits. During this time, the disease-causing fungi are most active during the spring and summer seasons.
While variety resistance is not a widely used management strategy for sclerotinia, some foliar-applied fungicides have shown some efficacy against the white mold fungus. The fungicides inhibit sclerotia growth, but their effectiveness varies according to their chemistry. Three fungicides of three chemistry classes have been registered for control of white mold in soybean. These fungicides are most effective when applied in a canopy management program and are limited in their upward movement.
The life cycle of sclerotinia sclerotiorum involves the production of the survival structure known as sclerotium. The sclerotia are capable of remaining viable for three years. In addition to sclerotium, sclerotia also produce ascospores. Those spores can travel a considerable distance if they are not destroyed.
Fungicides
The fungus Sclerotinia spp. has been found to attack various parts of plants. Its ascospores infect the upper portions of plants, causing symptoms such as flower blight, stem rot, and fruit rot. If not treated, the fungus may continue to grow and cause further damage to plants. Fortunately, there are fungicides available for control of this problem.
Chemical fungicides are not available for the control of sclerotinia on all crops. A variety of microbial and biochemical products are available to delay resistance to S. sclerotiorum. The fungicides used in a biofungicide program include Bacillus amyloliquefaciens and Coniothyrium minitans.
When choosing a fungicide for this disease, you should consider how often it has been present in your field. Are there weather conditions that favor white mold development? What types of crops are susceptible to it? Are there many susceptible varieties in your field? Several factors can affect how invasive it is. Fungicides for white mold sclerotinia are effective when applied early enough to suppress its growth in the canopy.
Some management practices for sclerotia include planting non-host crops. These include soybeans, small grains, forage legumes, sunflowers, and cole crops. Planting non-host crops will also keep the fungus’ numbers low. While this won’t stop the disease from causing significant yield losses, it does help minimize the damage caused by the disease.
Several studies have been conducted to evaluate the efficacy of fungicides for white mold sclerotia. To determine which fungicides are most effective, researchers have used replicated non-sprayed check strips. These strips were primarily planted in low areas where the molds are likely to grow. There are several fungicides available, but they do not all provide similar efficacy. Nonetheless, several products have proven to be effective in the control of this disease, including a few commercially available fungicides.
Crop rotation
While there are no known preventive methods for white mold, crop rotation can limit the growth of sclerotia in a field. Sclerotia is capable of living in soil for as many as 10 years. It attacks a wide variety of broadleaf crops, including corn and wheat. Although peanuts and wheat are not considered a good rotation crop, flax is resistant to white mold and is a viable alternative for the disease.
To minimize the spread of the disease, grow susceptible crops in alternate fields. This will help reduce the overall disease pressure. In addition to crop rotation, cropping practices should include weed control. The Petoskey News-Review reports that sclerotia can survive for up to 10 years. In addition to corn and soybean, the disease affects alfalfa and sunflower.
There are numerous varieties of soybeans, ranging from moderately resistant to susceptible, which are classified according to their susceptibility to the disease. In addition, some cultivars of soybeans are slower to die, which can help farmers maintain a high yield even in the presence of sclerotia. A nonhost crop is a good choice for rotation when soybeans are being grown in a field with a history of the disease. Other good nonhost crops are broccoli, Jerusalem artichoke, lambsquarters, nightshade, pigweed, and ragweed.
Sclerotia germinates under wet and cool conditions, so it is essential to rotate crops in order to minimize their impact. In addition to weed control, crop rotation is also essential for white mold sclerotinia. Soybeans should be grown under shaded conditions, as the crop canopy closes during rainy seasons. During the harvest, the spores and survival structures will be dispersed back into the soil.
Resistance to sclerotia
In a recent study, scientists found that soybean cultivars were resistant to the fungus Sclerotinia sclerotiorum. The fungus is a common cause of stem rot in oilseed rape grown in the Anhui Province of China. Fungicides such as carbendazim are effective against this mold. In this study, 74 isolates of S. sclerotiorum were collected from 13 regions of Anhui and tested for resistance to carbendazim.
The resistance-testing of plants for white mold has been a problem. Although there are several methods to measure plant health, greenhouse tests are often inadequate for determining plant disease severity. Early tests involved applying a small inoculum to the stem of seedlings and supplementing them with nutrients. More recent tests have focused on partial resistance to white mold developed by Miklas and Grafton and Dickson et al. (1982).
The development of a screening procedure for physiological resistance to white mold requires the development of more reliable methods to identify resistant bean germplasm. Physiological resistance to S. sclerotiorum can be detected indirectly through oxalate treatment. To measure oxalate resistance, cut seedlings were placed in a 20 mM oxalate solution. The genotypes were then rated according to how rapidly they wilted.
Phylogenetic analyses of these isolates revealed differences among S. sclerotiorum isolates from five Brazilian states. The resistance sources studied in this study included G122, A195, and Cornell 605. The A195 strain exhibited the highest resistance levels. In addition, one isolate from Parana, Goias, and A195 strains were resistant to S. sclerotiorum in field tests.