By Mandy Bish and Peng Tian
Tar spot is a corn disease that was first confirmed in the United States in 2015 and detected in Missouri in 2019. The disease is caused by a fungus, Phyllachora maydis, which can survive Missouri winters, allowing it to persist from season to season. The impact of tar spot varies each year and is dependent on weather conditions, hybrid susceptibility, and timing of disease onset. Tar spot can be observed without subsequent yield losses. However, severe outbreaks can occur when conditions favor disease spread and result in significant yield losses of 20 to 60 bushel/acre in the north central United States1.
Tar spot identification
Infected plants produce small black raised spots that are visible on the leaves, sheaths or husks of corn (Figure 1A). These spots, called stromata, are fungal structures that cannot be scraped off the leaf surface. Stromata may be surrounded by tan or brown halos called “fisheye” lesions (Figure 1B). Stromata can be present on both sides of the leaves and on healthy green and senescing (dying) brown tissues. These lesions can appear throughout the season from corn in vegetative growth stage through corn senescence.
Source : missouri.edu