Andrew Kness, Agriculture Extension Agent
University of Maryland Extension, Harford County
There have been multiple reports of white mold on soybean in the region, which is no surprise considering how wet and humid we have been over the past few weeks. This is one disease in soybean that can cause significant yield loss if it gets out of hand.
White mold is caused by the fungus, Sclerotinia sclerotiorum, which infects a wide range of crops, including alfalfa, snap bean, lima bean, sunflower, lettuce, tobacco, canola, many weed species, and the list goes on (over 100 species). Once you have white mold in a field, you’re pretty much stuck with it for the foreseeable future. Tiny pebble-like dark structures, called sclerotia (Figure 2), which are tightly wound balls of melanized fungal hyphae, enable the pathogen to survive in your soil for 7-10 years. The reason white mold doesn’t cause a severe epidemic every year is because it requires very specific conditions to form fruiting bodies (apothecia, Figure 1) and produce spores that will infect a host. In most years in Maryland, temperatures are too hot for this pathogen to cause a lot of damage in our soybean crop; however, this year has certainly been wet enough, so if temperatures cool off in August, this could be a potential issue.
In order for sclerotia to form apothecia and produce spores, soils need to reach near-saturated conditions and soil temperatures need to be between 54-75 ºF. In soybean, hyphae from nearby germinated sclerotia can infect vegetative tissue near the base of the plant, or spores from the apothecia can be splashed or wind-blown onto cenesing flower petals where they may germinate and enter the plant through the flower. Once inside the plant, the fungus grows and disrupts the flow of water and nutrients, causing non-descript symptoms, which include general yellowing, drying, and wilting of the plant (Figure 2). These symptoms alone are not enough to diagnose white mold; however, the presence of sclerotia on or inside stem is diagnostic (Figure 3). Sclerotia are produced in and on the stems to serve as overwintering structures and inoculum for subsequent years.
In-season management of white mold in soybean can be achieved with a fungicide application prior to symptom development, which is around R1. Fungicides that are effective against white mold include picoxystrobin (Aproach), fluazinam (Omega), and boscalid (Endura). Always read and follow the pesticide label before application. To help you determine if a fungicide application is warranted, you can use the Sporecaster app; a white mold forecasting tool. You enter information about your field and Sporecaster uses the field location and weather data to assess your risk for developing white mold.
If you already have white mold and you’re past R2, a fungicide isn’t going to help you. What you will want to do at this point is assess white mold disease severity at maturity (R7). Do this by randomly sampling multiple plants in multiple areas of a field and counting the number of plants with white mold and divide that number by the total number of plants sampled and multiply by 100. This will give you a percentage. As a general rule of thumb, for every 10% disease incidence of white mold, you can expect about a 2-5 bushel per acre yield hit. If you sample a field and you have greater than 20% incidence of white mold, then you should reconsider your soybean variety for that field. There are no soybean varieties with complete resistance to white mold, but there are significant differences in partial resistance and tolerance between varieties. Consult with your seed dealer and select a variety with a higher white mold disease rating for that field.
Additional management options for white mold include increasing air movement between the rows and decreasing canopy humidity (remember, this pathogen needs very wet conditions to germinate). You can achieve this by planting on wider rows (15” rows are less susceptible than 7.5” rows) and/or backing off on your population. Tillage has a minimal effect on white mold since the sclerotia can survive for so long. Extended rotations to non-hosts can help reduce disease pressure, but will not eradicate the pathogen.
After such a wet year like we’ve had, it would be a good idea to scout your fields for white mold and at least make note of them and harvest those fields last. Sclerotia will end up contaminating your combine, and you do not want to spread them to clean fields. When moving from a white mold infected field, you should thoroughly clean the combine and equipment (including tillage and sidedress equipment or anything that digs and can move soil) to avoid spreading sclerotia to new fields.
