Issue No. 18, Article 8/July 24, 2009
Conditions Favorable for Sclerotinia Stem Rot (White Mold) on Soybean
White mold (aka Sclerotinia stem rot), caused by the fungus Sclerotinia sclerotiorum, is a disease of soybean and other broadleaf crops. Historically, white mold on soybean has been observed sporadically in central Illinois, more frequently in northern Illinois, and rarely in southern Illinois. Cool (temperatures below 85°F) and wet conditions, especially when soybean plants are blooming, are favorable for the development of white mold. These cooler-than-normal and wet conditions have been occurring in central and northern Illinois the last couple of weeks, increasing the risk of white mold in these areas.
White mold gets its name from the fuzzy white growth that can be observed on affected soybean plants. This growth is the mycelia of the fungus that causes the disease. Symptoms include wilting leaves, stems that appear to be "bleached," and shredding of the stem tissue. Small black structures known as sclerotia can be found on and inside plants that have been affected by white mold.
Soybean stem affected by white mold (aka Sclerotinia stem rot).
The disease cycle of white mold is complicated, and favorable environmental conditions and soybean growth stages must intersect for the disease to occur. The fungus overwinters in the soil as sclerotia. These sclerotia can survive in the soil for many years. Under wet soil conditions, the sclerotia germinate and form small mushroom-like structures known as apothecia. Airborne spores (ascospores) are discharged from the apothecia and land on soybean plants. Ascospores that land on senescing petals of soybean flowers are the most likely to cause infection. As the soybean flower petals senesce, the ascospores begin to germinate, grow, and infect the stems. If wet and cool conditions continue, the disease continues to develop throughout the plant. Eventually, sclerotia will form on and inside the affected plants. Many of the sclerotia will be blown out of the back of the combine during harvest, adding more "inoculum" back into the field.
Apothecia (mushroom-like structures) growing from a sclerotium. (Image courtesy of J. Venette, North Dakota State University.)
Management of white mold in soybean is difficult, and multiple practices must be integrated to achieve the best control. These management practices include the following:
- Growing partially resistant varieties. No soybean varieties are completely resistant to white mold, but some are less susceptible than others. These partially resistant varieties will be less affected by white mold than susceptible varieties. Many seed companies have ratings available for the susceptibility of their varieties to white mold. Information about susceptibility of some varieties can be found at the VIPS website (www.vipsoybeans.org).
- Row spacing and seeding population rate. In areas where white mold is a severe problem year in and year out, wider (30-inch) row spacings may reduce the disease's impact. Because wider spacing could impact the yield potential of soybean, it is recommended for white mold control only in areas where severe white mold is observed frequently. High plant populations can decrease the airflow through the canopy, which can increase the spread of white mold. For effective management, it is important to follow recommended seeding rates and to avoid high seeding rates.
- Foliar fungicides. Currently, two foliar fungicides are registered for control of white mold in soybean. Topsin M (and other thiophanate-methyl products) and Domark are registered for use on soybean and include white mold on their labels. Topsin M has been evaluated in University of Illinois soybean white mold trials in the past; however, we have not tested Domark on soybean under white mold disease pressure. In the University of Illinois trials, Topsin M was effective at controlling white mold, but timing was critical (Table 2). Also, the yield benefit of controlling white mold with fungicides is only fully realized when disease incidence is moderately high. A research trial ongoing this year at the U of I Northern Illinois Agronomy Research Center near DeKalb is evaluating several foliar fungicide products, including Domark and Topsin M, for control of white mold.
- Avoiding bin-run seed. The white mold fungus can be seedborne, so to avoid bringing the pathogen into your fields it is important not to plant bin-run seed.
- Biological control. A biological control product marketed as Contans WG is available for control of white mold. This product contains the fungus Coniothyrium minitans, a parasite of the white mold fungus's sclerotia. Contans WG has not been evaluated in University of Illinois research trials, though research at North Dakota State University indicated it is effective at colonizing and killing sclerotia. But depending on the level of sclerotia present, disease incidence may not be affected: in fields with a high load of sclerotia in the soil, enough sclerotia may survive to still cause a substantial level of disease. It is important to note that Contans WG should not be applied to flowering soybean plants. Rather, it should be applied to the soil in the fall after harvest or in the spring prior to planting.--Carl A. Bradley
Carl A. Bradley