Although white mold has been sporadic and inconsequential in most Illinois soybean field over the past few years, this disease is now appearing in soybean fields in parts of Illinois. Jim Morrison, Extension educator at the Rockford Center, has reported white mold near Freeport, and other reports have come in from the northern third of the state. White mold, also called Sclerotinia stem rot, is caused by the fungal pathogen Sclerotinia sclerotiorum.
This disease can be very destructive, especially during cool and moist weather and when growing conditions favor high yields, and it can be one of the easiest soybean diseases to diagnose. The names of the disease are descriptive in that infection and disease development by the pathogen Sclerotinia sclerotiorum result in soft rot and darkening of stem, leaf, and pod tissues and can usually be recognized by the white moldy growth on the infected plant parts. These symptoms are often preceded by gray to brown discoloration and wilting of the upper leaves. In addition, as the fungus grows and infects soybean tissue, it produces sclerotia, which are gray to black spherical to elongated structures about 1/8 inch to 1/4 inch around or in length.
White Mold on soybean showing white fungal growth and sclerotia.
Even though white mold has only recently started appearing in Illinois soybean fields this season, much of the disease was initiated earlier in the summer when the nights were cool and moist in July. In simple terms, the life and disease cycles of S. sclerotiorum are as follows. The sclerotia are the resting and overwintering structure of this fungal pathogen and are a key to the life cycle. Sclerotia are produced on or in infected tissues and are released at harvest into field soil where they can survive for many years. The sclerotia near the surface of the soil germinate and produce small mushroom-shaped structures (apothecia), and the apothecia produce numerous spores when conditions are favorable. The spores land on senescing flowers and initiate infections, which spread on the stem or other plant parts. Infection typically occurs during cool and moist weather conditions when plants are flowering and after the canopy has partially closed.
Management of white mold in fields with a history of this disease is based primarily on decisions made prior to planting. A first step is to choose soybean cultivars with the highest levels of partial resistance; many cultivars are rated for their reaction to white mold, and information on resistance to white mold and other diseases can be found at the Varietal Information Program for Soybeans (VIPS) Web site.
In addition, several cultural practices can help to reduce white mold incidence and severity. These include late planting, wide row spacing, low plant populations, moderate to low soil fertility, good control of broadleaf weeds, and avoiding rotation with sunflowers and other bean crops. Fungicides have been shown to be ineffective or inconsistent for management of white mold of soybeans. In a recently published study with results from Illinois, Indiana, Ohio, and Wisconsin (Mueller et al., Plant Disease. 2002), the fungicide Topsin M applied at the R1-R3 growth stages did reduce disease severity when disease pressure was low but provided inconsistent control when disease pressure was high. Thus, white mold cannot be easily managed when environmental conditions are favorable for disease development and fields contain large numbers of sclerotia in the soil, but these steps can be made to reduce disease damage under less severe circumstances.
At this point in the season, soybean fields should be scouted to determine whether and where different diseases, including white mold, may be damaging the crop. Remember, once soybean plants are dead and decomposing, it can be very difficult to diagnose the problem and determine what may have contributed to suboptimal yields. --Dean Malvick