Tar spot, a fungal disease caused by Phyllacora maydis, has made its earliest appearance in Missouri, affecting corn crops in Holt and Marion Counties, along with six Iowa counties. While the incidence and severity remain low, farmers are advised to be vigilant and monitor their fields closely. This outbreak emphasizes the need to delay fungicide applications, with treatments between VT and R3 growth stages demonstrating the most effective results.
When scouting for tar spot, farmers should look for black lesions or stroma on corn leaves, occasionally surrounded by brown rings resembling "fish eyes." Considering several factors can help assess the risk of disease progression, such as prior confirmed cases in the field or neighboring areas, corn following corn rotations, and vulnerable corn varieties.
The recent study by Webster et al. highlights that cooler minimum air temperatures contribute to increased tar spot risks. The average minimum air temperatures in northern Missouri during June 2023 were notably cooler compared to previous years, potentially contributing to the early onset of the disease.
While air temperatures are expected to rise in the coming days, the current low disease incidence, ongoing research, and drought conditions suggest delaying fungicide applications. However, early spraying should consider potential implications on ear development and the likelihood of a second application if the disease continues to spread.
Farmers in affected regions can benefit from the Tar Spotter App, developed by the University of Wisconsin, for guidance on fungicide applications. Furthermore, subscribing to the MU Crop & Pest Text Alert System and attending the MU Pest Management Field Day on July 6th can provide timely updates on tar spot management strategies.
For further information and to aid in tracking the disease's progression, farmers are encouraged to submit suspect samples to the MU Plant Diagnostic Clinic.
Photo Credit: GettyImages-oticki
Categories: Missouri, Crops, Corn, Weather