Four relatively new diseases should be on your radar screen, say Iowa State University (ISU) Extension plant pathologists.
Northern leaf blight (NLB) and Goss’ wilt are previously known diseases now re-emerging in the Corn Belt, says ISU Plant Pathology Specialist Daren Mueller. Mueller also raised a red flag on two relatively new diseases: Physoderma brown spot and bacterial stalk rot.
Northern leaf blight produces large tan-to-brown cigar-shaped lesions that begin in the canopy. It’s a fairly aggressive fungus more common under overcast, wet, cool conditions. Once present, it can develop rapidly, and while some hybrids offer resistance, it’s not always complete. Fungicides can manage this disease, but proper timing is critical.
Most of the reported cases in 2010 were in Iowa, but instances of NLB occurred in other Corn Belt states like Ohio, where it’s a growing concern, according to Pierce Paul, Ohio State University cereal pathologist.
Goss’ wilt reappeared in 2007 in Illinois after a nearly 30-year absence and is now widespread west through Iowa, Minnesota and especially Nebraska. Like NLB, it’s favored by stormy conditions (specifically hail damage), produces lesions at the top of the plant and develops rapidly. It’s caused by bacteria, so fungicides provide no benefit. In large patches, it can reduce yields to almost nothing.
Identifying Goss’ can be a challenge, since it can look like NLB, Stewart’s wilt or even a nitrogen deficiency, says Carl Bradley, assistant professor of crop sciences at the University of Illinois.
Leaves infected with Goss’ wilt will have large brown, yellow or gray lesions with wavy margins. They may also display water-soaked tissue, a greasy appearance, a shiny ooze and freckles. In contrast, NLB produces a large gray-to-tan lesion with distinct margins on the leaves, which may look “dirty” under high-humidity conditions.
Physoderma brown spot usually appears in random plants scattered through a field. It shows up early in the season, producing bands of very small yellow to orange spots and, in some plants, dark brown blotches along the mid-leaf vein. It can be mistaken for Southern rust or eyespot, but it appears earlier in the season than rust and at warmer temperatures than eyespot. Fungicides are typically not an effective control.
Bacterial stalk rot benefits from warm, humid conditions and can kill entire plants prematurely. One diagnostic tool is the distinctively bad smell described as blend of bad silage or even dead fish. When it does not kill the entire plant, it can result in lodging with stems twisting or breaking at infected nodes. Like Goss’ wilt, this disease is not controlled by fungicides. In 2010, Mueller found at least 10-15 infected fields across a 120-mile stretch of central Iowa.