Three major diseases drive fungicide decisions in Virginia-type peanuts: 1) late leaf spot; 2) southern stem rot; and 3) Sclerotinia blight.  Although there are many fungicide options growers can use, few of those options control all three diseases at once and the most effective time to spray is different for each disease.  Peanut leaf spot typically occurs during the mid-latter part of the growing season under favorable environmental conditions.  Most fungicide sprays applied to peanuts target this disease which is caused by Cercospora aracidicola (early leaf spot) and Nothopassalora personata (late leaf spot).  Typically, Virginia growers experience much more late leaf spot than early leaf spot. Sclerotium rolfsii is a soilborne pathogen that causes southern stem rot or “white mold”.  Infection by this fungus is favored by high temperatures and wet conditions.  Sclerotinia blight is caused by the fungus Sclerotinia minor which, unlike S. rolfsii, is favored by cool, wet conditions.  Virginia growers use a days after planting (DAP) schedule for all diseases, or online advisories for leaf spot and Sclerotinia blight at https://webipm.ento.vt.edu/cgi-bin/infonet1.cgi.  Sclerotinia blight and southern stem rot sprays may also be timed by scouting for the first presence of disease.  Since both Sclerotinia blight and southern stem rot are soilborne diseases, field disease history plays a major role in making fungicide application decisions as well.  To further complicate the issue, few fungicides are available to control all three diseases at once.  Figure 1 shows peanuts infected with both late leaf spot and Sclerotinia blight occurring simultaneously, which was observed in many fields in 2020.

Figure 1. Peanut leaves showing late leaf spot symptoms on leaves and peanut stems showing symptoms and signs of Sclerotinia blight
Figure 1. Peanut leaves showing late leaf spot symptoms on leaves and peanut stems showing symptoms and signs of Sclerotinia blight.

In 2020, the trials set up to evaluate fungicides and fungicide programs for control of peanut leaf spot demonstrated that some fungicide program treatments significantly controlled late leaf spot while simultaneously controlling Sclerotinia blight.  Figure 2 shows that some of the 2- row plots have more green peanut tissue than others (especially treatments 2, 3, 5 and 7) which resulted from some peanut leaf spot treatments conferring control of Sclerotinia blight, which was the main cause of dead (brown) plant tissue across most of the experiment.  Typically, growers make separate fungicide applications of different fungicides that control soilborne diseases such as southern stem rot and Sclerotinia blight.

Figure 2. Aerial photograph of a peanut leaf spot trial showing differences in live (green) tissue and dead (brown) tissue resulting from Sclerotinia blight (credit: Sarkar and Cazenave).
Figure 2. Aerial photograph of a peanut leaf spot trial showing differences in live (green) tissue and dead (brown) tissue resulting from Sclerotinia blight (credit: Sarkar and Cazenave).

Leaf spot ratings demonstrated that all fungicide program treatments significantly controlled late leaf spot across three trial locations (Figure 3).  Sclerotinia blight was severe in two of the three trials (Hare Rd 34 and TAREC) with significant control of Sclerotinia blight observed in all treatments containing Miravis compared to the non-treated check at the Hare Rd 34 location (Figure 4).  The TAREC location demonstrated a similar trend but with no statistical differences among treatments.  These data demonstrate that control of late leaf spot and Sclerotinia blight can be achieved simultaneously with applications of Miravis alone and in combination with Elatus.  The added benefit of Elatus is that it can provide control of southern stem rot.  

Figure 3. Area under the disease progress curve (AUDPC) for late leaf spot across fungicide programs over three locations in 2020. Means followed by the same letter(s) are not significantly different at P≥0.05 using Fisher’s Protected LSD test.
Figure 3. Area under the disease progress curve (AUDPC) for late leaf spot across fungicide programs over three locations in 2020. Means followed by the same letter(s) are not significantly different at P≥0.05 using Fisher’s Protected LSD test.
Figure 4. Area under the disease progress curve (AUDPC) for late leaf spot across fungicide programs over three locations in 2020. Means followed by the same letter(s) are not significantly different at P≥0.05 using Fisher’s Protected LSD test.
Figure 4. Area under the disease progress curve (AUDPC) for late leaf spot across fungicide programs over three locations in 2020. Means followed by the same letter(s) are not significantly different at P≥0.05 using Fisher’s Protected LSD test.

Dr. David Langston, the current leader of the Plant Pathology program at the Tidewater AREC, plans to build on these results to further identify fungicides and spray programs that control both peanut leaf spot and Sclerotinia blight, allowing growers make fewer fungicide applications and reduce fungicide costs.