Connie Strunk
Plant Pathology Field Specialist
Stalk rots in corn can develop when the crop is nearing maturity and the sugars are being mobilized for grain fill. This leaves the corn stalk vulnerable to infection by stalk rot pathogens. Corn stalk rots can cause yield losses through premature plant death which leads to reduced grain fill, plant lodging which results in harvest problems, and lodged plants which may develop ear molds if the ears contact the soil. Scouting for corn stalk rots may help to determine the type and extent of stalk rots and this information can be used in future disease management planning. Nothing can be done about stalk rots affecting corn this season, but changes can be made for the next growing season. Scouting can help determine which fields need to be harvested early before heavy lodging occurs. Scouting for stalk rots can be done by using a push method on 10 corn stalks at five or more random stops in a field. The push method involves gently pushing the corn stalk to about 45 degrees. Stalks with rots will crumple at the first or second node above the soil or may fail to regain the erect position. Fields with 10% or more incidence of stalk rot should be harvested early. Several pathogens cause stalk rots in corn but the most common stalk rots are caused by Gibberela zea, Fusarium spp, and Colletotricum graminocola (Anthracnose). Under wet spring weather, Physoderma brown spot can develop and this often results in stalk rot. Severe Physoderma stalk rot causes heavy lodging in low spots in a field (Figure 1).
Figure 1. (Above) Heavy corn lodging caused by Physoderma stalk rot. Photo credit: Emmanuel Byamukama
Gibberella stalk rot is characterized by reddish discoloration which can be noticed when the infected stalk is split near the soil surface (Figure 2). Stalks with advanced symptoms may develop small black perithecia around the internode. Anthracnose stalk rot is recognized by a shiny black color on the outer surface of the stalk when the leaf sheath is peeled off (Figure 3). Splitting stalks with anthracnose stalk rot reveals discolored pith. Fusarium stalk rot is not easy to diagnose but corn stalks with disintegrated pith (Figure 4) and without obvious discoloration on the stalk may indicate Fusarium stalk rot.
Figure 2. (Above) Corn stalk infected with Gibberella zea. Notice the pink discoloration of the pith, the diagnostic sign of the pathogen. Photo credit: Emmanuel Byamukama