Looming threats, leading players

GROWERS SHOULD BE ON THE LOOKOUT for two emerging corn diseases in MFA territory—tar spot and bacterial leaf streak—according to Dr. Kaitlyn Bissonnette, University of Missouri plant pathologist.

“Tar spot been found in four northeastern Missouri counties: Lewis, Clark, Scotland and Marion,” she said. “With tar spot, you can see about 30 to 40% yield loss in hard-hit areas. This is an emerging disease, so we haven’t seen those levels, but we need to be monitoring it because as it moves, that potential for yield loss increases.”

Tar spot can be found anywhere on the corn plant, but often is found in the lower canopy from mid-to-late August through harvest. It can be identified by its stromata, which are fungal structures that appear as small, raised, black, circular spots.

Bacterial leaf streak (BLS) has been found in Ray and Chari­ton counties in Missouri and often resembles gray leaf spot, Bissonnette said. BLS often infects leaves after strong winds and rains have caused leaves to rub together. To identify this particular disease, growers should look for water soaking and long, necrotic lesions with irregular margins, she said.

Growers are asked to contact their local MU Extension office or Bissonnette at This email address is being protected from spambots. You need JavaScript enabled to view it. if they detect either of these diseases in their fields.

Though not considered emerging diseases, gray leaf spot, southern rust and northern corn leaf blight should also be top-of-mind for producers, Bissonnette said.

“Those are usually the main players we usually think about in corn,” she said.

In soybeans, she encourages growers and scouts to watch for frogeye leaf spot, cercospora leaf blight, sudden death syn­drome and soybean cyst nematode. Target spot is also a concern in soybeans and cotton for some parts of the state.

In wheat, growers should look for fusarium head blight and stripe rust, though the latter depends on the season.

“Stripe rust requires a living host to survive because it’s an obligate parasite,” Bissonnette said. “It doesn’t overwinter in residues, so that means that it has to stay alive in places like Louisiana or Arkansas, and then it’ll blow into our area.”