By Jaime Cummings of NYS Integrated Pest Management, and Janice Degni of Cornell Cooperative Extension
We had an interesting report from Janice Degni (CCE, SCNY dairy and field crops team leader) this week. While out scouting and measuring alfalfa stands in Onondaga County, Janice noticed some wilting plants. Upon closer inspection, she found sclerotia, the tell-tale sign of white mold on the alfalfa stems (Fig. 1). Though this disease can be found in alfalfa and clover stands, you are likely more familiar with white mold on soybeans. Two different species of this pathogen can be found on alfalfa. Sclerotinia trifolium is the one most commonly identified on alfalfa, but the same species found on soybean, Sclerotinia sclerotiorum, can also infect alfalfa.
Remember that these pathogens overwinter and survive many years in the soil as sclerotia. Sclerotia are the black structures you’ve likely seen on infected soybean stems. These two different fungal pathogen species are difficult to differentiate, but the trend is that S. sclerotiorum tends to infect in the spring and summer, while S. trifolium typically infects in the fall. Regardless of the species, the symptoms, epidemiology and management are similar.
The symptoms of Sclerotinia crown and stem rot in alfalfa include rotting crowns, cottony growth on stems and crowns, and wilting and rotting stems. Infected crowns tend to die-off, and may be confused with winterkill, if not for the tell-tale sign of sclerotia in the dead tissues. Infection in first-year stands is most problematic. Just like in soybeans, this disease can spread quickly through a field, either by spores or via fungal mycelium spreading among plants during a cool, wet spring. This can potentially thin a stand out rather quickly, and can leave plenty of those sclerotia behind as inoculum for future years.
This prolonged cool, wet spring we’ve been experiencing has provided ideal conditions for this disease that we infrequently encounter in our alfalfa fields. An integrated management approach is the best solution. Since this disease is most prominent in first year, fall-seeded fields, you may consider future spring plantings in fields where you find it. Or, get your fall seedings in as early as possible so that seedlings have a chance to establish before the sclerotia germinate and produce spores in the fall. Tillage buries the sclerotia, which can reduce the number of spores released and may decrease infection in the field. But, as in soybeans, this disease is difficult to manage. Dense stands and weedy fields create perfect conditions for this pathogen to thrive. However, a few alfalfa varieties exist with moderate resistance, and may be considered in fields with a history of this disease. Research in other states has shown that some fungicides are efficacious against this disease when applied in the fall (Table 1). We have limited options for fungicides labeled for white mold on alfalfa in NY, including Pristine and Endura. However, fungicide applications for this disease may not be cost-effective (always follow label instructions, restrictions, and pay attention to post-harvest intervals). Herbicides, such as Paraquat, have also been used to reduce weeds and open up the canopy to increase air-flow lessen disease development.
Once the weather warms up and fields dry out, this disease will likely halt, and some infected stands may recover and produce sufficient yields in subsequent years. But, keep in mind that those sclerotia will remain in the soil for many years waiting for perfect conditions to start the disease cycle again.
Interested in keeping up to date with pest and disease issues identified statewide by CCE and IPM staff? Subscribe to the NYS IPM Weekly Pest Report.