We haven’t applied fungicides yet. I only aim to apply if we see tar spot — it has not shown up yet. How much should I find before spraying? What product should we use? When? Is there a point when it is too late to spray if it shows up?

 

The Indiana certified crop adviser panel answering this question includes Danny Greene, Greene Crop Consulting, Franklin; Abby Horlacher, Nickel Plate Consulting, Frankfort; Brian Mitchem, Farmer 1st Agronomy Consulting Services, Decatur; and Dan Quinn, Extension corn specialist, Purdue University.

 

Greene: I assume that if you only plan to spray for tar spot, then you have very good resistance to other diseases. Scout and pay attention to reports for when tar spot establishes or is noted in your county. A smartphone app called Tarspotter uses weather and predictive models to help those in the field anticipate issues.

 

Scout your fields and note the corn growth stage when tar spots’ fungal fruiting structures called “stromata” can be found on corn tissue. If you have spot numbers increasing prior to or at R3, spray as soon as is practical. R4 applications may not be cost-effective, depending on upcoming weather conditions. At R5 or later, there is little chance of a cost-effective outcome for this year’s yield.

 

Horlacher: If your field has had a history of tar spot, then you need to pay attention to weather patterns on when to spray. If you see tar spot, it can be too late to reduce its full impact on yield.

 

Humid, warm and rainy conditions increase the chances of tar spot. If it is hot and dry, you will have a lower infection rate of tar spot. If it is August, you haven’t sprayed fungicide, and rain and heat are coming, spray a full-spectrum fungicide with multiple modes of actions. In our area, we use a lot of Delaro Complete.

 

Mitchem: We have not seen a consistent benefit from spraying fungicides past dough stage for tar spot. Missing the R1 application window leaves your crop vulnerable to tar spot. Tar spot is the one disease in susceptible areas best treated as a preventative on acres vulnerable to infection.

 

If you identify tar spot between V12 and R1 and find consistent lesions on multiple plants, plan for a fungicide application. One challenge is how fast it moves from initial identification to significant yield loss. The recommendation from Extension plant pathologists’ work shows Veltyma, Delaro Complete and Aproach Prima have shown the most consistent performance on tar spot.

 

Quinn: If you are finding tar spot in the lower to mid canopy before tassel emergence, during pollination or a few weeks after pollination, consider a fungicide application. Tar spot can move quickly in the canopy, especially in cooler or humid conditions.

 

If the corn is in the late R4 growth stage or R5 growth stage and tar spot is observed, it is highly unlikely a fungicide application will provide a benefit. The best application timing for fungicide application is from R1 to R3. Protection during the reproductive and early kernel formation stages is critical.

 

There are many products available to help control tar spot, and the Fungicide Efficacy Table from the Crop Protection Network is a very useful tool to understand the performance of those products.