Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, Roger Schmidt, Nutrient and Pest Management Program, University of Wisconsin-Madison
Rain, and the return of more humid weather, has meant that risk of tar spot of corn and white mold of soybean has increased over the past week. Now is the time to think about your in-season management plan for both of these diseases. Let’s dig in a bit on what the risk looks like for each disease.
Tar spot in Corn
This week we added Fond du Lac County to the tar spot map (Fig. 1). We also continue to see tar spot slowly increasing in plots and production fields on the Arlington research station. Looking back at our records from last season, we are tracking almost identically with what happened last year. I know folks think it is dry, but the tar spot fungus doesn’t care. It might move slow in these conditions, but the elevated humidity provides adequate leaf wetness for the disease to slowly progress. Should it start raining more regularly I expect the disease to pick up speed.
Corn is rapidly approaching (if not already at) the optimal window of opportunity (VT-R3) for spraying fungicide to control tar spot. Given the high risk for tar spot across much of the state (Fig. 2), now is the time to call in that fungicide application if you are planning on it. Given the possible constraints on locating a custom applicator, getting the order in earlier than later may ensure application of fungicide by the R3 corn growth stage. Get out and scout, scout, scout!
White Mold of Soybean
White mold risk has increased from reasonably low last week, to mostly moderate across the state, this week (Fig. 3). Risk trends are also increasing, indicating that weather is continuing to become more favorable for white mold development. As we approach the R3 soybean growth stage, it will be important to make a decision on fungicide application, especially if you haven’t already applied a fungicide. If rain moves in over the next 7-10 days, expect risk to continue to increase. In irrigated fields we have been able to find apothecia (the mushroom-like structure that produces spores that infect soybean). This corroborates the increased risk we are seeing even in non-irrigated fields.
Tarspotter, the Tar Spot Risk of Corn Forecaster
The purpose of Tarspotter is to assist farmers in making management decisions for tar spot of corn. Farmers can easily input site-specific information about their corn field into this app, which combines this information with research-based models to predict the best timing for tar spot treatment.
iPhone and iPad: https://apps.apple.com/us/app/tarspotter/id1454584176
Sporecaster, White Mold Risk in Soybean app
The purpose of Sporecaster is to assist farmers in making management decisions for white mold in soybean. Farmers can easily input site-specific information about their soybean field into this app, which combines this information with research-based models to predict the best timing for white mold treatment.
iPhone and iPad: https://itunes.apple.com/us/app/sporecaster/id1379793823
The Field Prophet app combines both Tarspotter and Sporecaster for iPhones and iPads
For those who like all of their disease prediction tools in one place you might check out the Field Prophet version of the Tarspotter and Sporecaster apps. The app, which runs both corn and soybean disease risk models, allows for true 7-day forecasting and will display 7-day trends to better inform your management decisions. You can use the Field Prophet app for free for the next 6 months. You might find this tool as a handy alternative to Tarspotter and Sporecaster. Field Prophet is a UW-Madison spin-off company with a mission to ensure growers will have these tools to use into the future.