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15
Oct
2013
Peanut disease alert: Sclerotinia blight of peanut detected in Arkansas
Author: Travis Faske, Extension Plant Pathologist

Sclerotinia blight was detected earlier this month in a commercial peanut field near Pocahontas, AR.  This is a common soilborne disease of peanut in Texas, Oklahoma, Virginia, and North Carolina; however, until now it has not been reported in Arkansas.  In fields with a history of sclerotinia blight it is common to have pod losses of 10% of expected yield.  This disease is caused by a fungus that has a wide host range of more than 200+ plant species in 21 plant families.  Therefore, it is possible the fungus has gone undetected in Arkansas until observed on a susceptible host like peanut, growing in cooler fall conditions.  Producers and consultants should be able to recognize this disease in order to minimize the spread of the fungus to uninfected fields.

Sclerotinia blight is caused by a soilborne fungus, Sclerotinia minor.  Typically symptoms appear late in the peanut growing season when the high temperature remains near 80°F (late September or October).  The first symptom is the “flagging” or wilting of infected branch tips and petioles (Fig. 1).  Initial symptoms are small, light green water-soaked lesions near the soil line and lower canopy often with white fluffy hyphae surrounding the infected stem (Fig. 2).  Hyphae can be seen when relative humidity is high after a heavy morning dew or light rain.  Older lesions appear bleached or straw colored with a distinct separation between diseased and healthy tissue.  Two additional diagnostic characteristic of sclerotinia blight include shredding of infected stem tissue and production of small (0.5 to 3.0 mm diameter) black sclerotia on and inside infected stems (Fig. 3).  The sclerotia size and color are key signs of this disease when compared to the more common southern blight.  Southern blight is characterized by small (size and shape of mustard seed) white sclerotia that turn tan and brown as they mature.  Sclerotinia blight was observed in several 3 to 4 ft. diam. foci near the low end of the furrow irrigated field near Pocahontas.  Yield loss occurs when peanut pods detach from infected stems and pegs during the digging and thrashing process and are unable to be picked by commercial peanut harvesting equipment.

Figure 1.  Flagging of peanut stems caused by sclerotinia blight

Figure 1. Flagging of peanut stems caused by sclerotinia blight

Figure 2.  Water-soaked, straw colored lesions with white hyphae caused by sclerotinia blight.  Small black sclerotia (right side of pic.) on straw colored lesion are produced by Sclerotinia minor.

Figure 2. Water-soaked, straw colored lesions with white hyphae caused by sclerotinia blight. Small black sclerotia (right side of pic.) on straw colored lesion are produced by Sclerotinia minor.

Since sclerotinia blight is a soilborne disease, it does not readily produce windblown spores but can spread by hitching a ride on harvesting equipment.  A field with sclerotinia blight should be harvested last, if possible, and harvesting equipment should be washed and cleaned to remove any fungal propagule (e.g. sclerotia) before moving to a sclerotinia blight-free field.  Once the disease is established, there is no way to eradicate the disease in the field; thus, prevention is a key factor to minimize the spread of this pathogen.  Sclerotia can remain viable for several years (4+) under favorable conditions.  Though fungicides can be used to manage this disease, this year it is too late in the growing season to recommend a fungicide application.  It is important to note that fungicides used to manage sclerotinia blight are not the same as those used to manage southern blight, the most common peanut disease in the state.  Therefore, field disease history will be important for fungicide selection when rotating to a field with a history of sclerotinia blight.  Some peanut cultivars have a moderate level of resistance, but they are often susceptible to tomato spotted wilt virus, a more widespread peanut disease in the state.  Other management options are forthcoming at future peanut meetings and on this blog site.  Hopefully the majority of Arkansas peanut producers will likely not have to manage this disease for several years.

Figure 3.  Black sclerotia of Sclerotinia minor inside infected stems and shredding of plant stem tissue caused by S. minor.

Figure 3. Black sclerotia of Sclerotinia minor inside infected stems and shredding of plant stem tissue caused by S. minor.

 


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