Stay Ahead of Peanut Fungal Pathogens This Summer

After weeks of extreme July heat and dry conditions across much of the Panhandle, August has been a washout so far.  The widespread rainy weather of the last few days has provided a brief reprieve from the heat and has been a welcome sight for those of us that have been a little on the dry side.  However, late summer rainy weather also keeps growers out of fields, delaying spray and scouting schedules, and might be an ominous sign of fungal disease occurrence to come.

Controlling peanut fungal diseases is costly, in terms of both purchasing and applying crop protection chemicals to prevent devastating crop losses.  Most extension crop budgets project ~$100 per acre for six to eight fungicide applications per season.  In practice, higher input growers may incur fungicide and application costs in excess of $150 per acre. These expenses are necessary insurance though, as crop losses associated with fungal diseases could be upwards of 75%, in the absence of control (Shokes and Culbreath, 1997).  Let’s look at the major fungal diseases often battled locally and discuss some strategies to stay ahead of them this season.

White mold attacking peanut plants. Credit: Daniel Leonard, UF/IFAS

White Mold, caused by the pathogen Sclerotium rolfsii, has already been a problem in Panhandle fields and will continue to be a major concern that, if untreated, can lead to severe yield loss.  Over the past month, growers should have been scouting for the telltale feathery white mycelium growth and wilting or “flagging” stems that indicate white mold infection.  Fortunately, white mold prefers hot, dry conditions, so the temperature moderation prolonged rain brings may help slow down disease occurrence.  Though white mold disease progress may slow this week, growers still need to be on top of scouting and preventative spraying once things heat back up.  It is important to note that fungicides to control white mold must reach the soil to be effective.  Rainfall within a few days of fungicide application will move the fungicide to the soil.  Alternatively, spraying while the leaves are folded during nighttime hours will allow the spray to reach the soil surface where it can more actively control the fungus.  Remember to rotate through products with varying active ingredients and strong efficacy on a regular schedule as things dry out and field conditions permit.  Peanut Rx is a great tool to help growers plan fungicide programs to reduce white mold and other fungal disease risk and maximize crop protection.

Chemical burn leaf spotting on peanut. Credit: Dr. Barry Tillman, UF/IFAS

The other major fungal diseases in Panhandle peanuts are the leaf spots:  Late Leaf Spot (Nothopassalora personata) and occasionally Early Leaf Spot (Passalora arachidicola) both thrive in rainy high humidity weather, not unlike the conditions of this last week.  Late Leaf Spot, as its name suggests, typically occurs later in the growing season and is usually the more problematic leaf spot disease for Panhandle peanut producers.  In several leaf spot trials at the NFREC in Marianna last year (2024), we did not observe Late Leaf Spot in any plots until mid-September.  However, each year is different, and if rainy patterns continue, we could easily see earlier outbreaks this season; it’s important for growers to begin scouting for the lesser seen Early Leaf Spot and our archnemesis Late Leaf Spot sooner rather than later.  All these pathogens are more easily managed before they get out of hand and cause defoliation, which ultimately leads to yield loss at harvest.

One final note – in looking around area peanut fields over the last several weeks, I have seen lots of suspicious leaf spots that, at first glance, appear to be disease, but in each case so far have simply been pesticide burn – relatively easy to distinguish by the light-colored centers of the dark spots.  While they can initially cause concern, these benign spots are actually a sign your fungicide program is working!

The takeaway from this article is to be vigilant in scouting for fungal diseases and consistent with your spray program as summer continues and rainy periods set in.  For more information on fungicide trials for white mold and late leaf spot and breeding efforts to introduce cultivars resistant to these diseases, be sure to attend the NFREC Peanut Field Day on August 14!