Sclerotinia blight caused by Sclerotinia sclerotiorum was confirmed earlier this month in a commercial peanut field near Pocahontas. Sclerotinia blight symptoms were recognized by Gordon Drennan with the Clint William Co. and confirmed by extension plant pathologist on a runner peanut cv. Florun107. Symptomatic plants were observed in several large (2’ x 5’) foci with several smaller (1’ x 1’) spots clustered in a 1/10 acre section of a 30 acre field. Sclerotinia blight caused by S. minor was reported in 2013 on peanut; however, to my knowledge this is the first time that S. sclerotiorum has been detected on peanut in Arkansas. Sclerotinia sclerotiorum is not a new pathogen in the state as it has been reported on other crops like tomato and canola. Weather conditions forecasted for this week appear favorable for Sclerotinia blight so, it is very likely this disease will be detected in other peanut fields. Producers should be on the lookout for Sclerotinia blight to minimize spreading the fungus into Sclerotinia blight-free fields.
Sclerotinia blight is caused by the soilborne fungi S. minor and S. sclerotiorum. Typically, symptoms appear late in the peanut growing season when the high temperature remains in mid 80s (late September or October). The first symptom is “flagging” or wilting of infected branch tips. Parting the lower canopy is often needed to see light green water-soaked lesions on infected stems and pegs with white fluffy (cotton-like) hyphae (Fig. 1).
Older lesions appear bleached or straw colored to dark brown. Black irregular-shaped sclerotia can be found on or inside infected stems (Fig 2 and 3).
Sclerotia are the overwintering propagule for this disease that can remain viable for four years. Sclerotia produced by S. minor are small (0.5 to 3.0 mm), whereas sclerotia produced by S. sclerotiorum are large (4.0 to 10.0 mm), which can be helpful to differentiate between the two pathogens. Sclerotinia minor produces more sclerotia than S. sclerotiorum, which is why over several seasons it is typically more problematic on peanut than S. sclerotiorum. Yield loss occurs when peanut pods detach from infected stems and pegs during the digging and thrashing process, which are unable to be picked by commercial peanut harvesting equipment.
Sclerotia can hitch a ride on diggers or harvesters so take caution when moving equipment as sclerotia could be transferred to a new field. A field with Sclerotinia blight should be harvested last if possible, and harvesting equipment should be washed and cleaned to remove any peanut stems and as much soil as possible before moving to a Sclerotinia blight-free field. Once the disease is established there is no way to eradicate the disease thus, prevention is a key factor in disease management. In fields with Sclerotinia blight, there are a few management options. Some peanut cultivars are more susceptible than others, thus fields with a history of the disease should be planted in the least susceptible cultivar. Cultural practices consist of wide row spacing (> 30”) and be sure to avoid cultivars with rank growth to reduce conditions that are favorable for disease development. Although fungicides are available, (see MP 154) they are most effective when applied earlier in the season before disease development. Fungicides use to control Sclerotinia blight are not the same as those used to southern blight, another soilborne disease with similar symptoms. Sclerotia shape and size can be helpful to differentiate these two diseases. Southern blight has round tan to brown sclerotia, whereas Sclerotinia blight has black irregular-shaped sclerotia with a white core. Document fields with Sclerotinia blight for scouting purposes and management considerations for the next peanut crop.