Description and Symptoms
Root rots (Pythium graminicola, Fusarium spp., Exserohilum pedicellatum) of corn are very common and are caused by several soilborne pathogens. Root rots occur to some extent in every field, causing economic losses under wet conditions. Above ground symptoms include stunting, uneven growth, chlorosis, small or poorly filled ears, or wilting. In saturated conditions, effects of root rot are difficult to distinguish from the effects of oxygen deficiency and poor root development. Root symptoms are more reliable for diagnosis. Initially, small yellow-brown lesions develop on roots; roots later turn dark brown and are obviously decayed. Fine root decay may not be apparent without careful examination. Fine roots slough off after rotting; their absence is another root rot symptom. Root rot is generally reduced by standard seed fungicide treatments on corn. Planting corn when soil temperature is above 50°F and soil moisture is not excessive may also reduce risk.
The best time to scout for root rots is VE through R2. Corn rootworm feeding can lead to subsequent root rot development. Use a trowel to dig up entire system when searching for symptoms.
• Target fields that have had significant foliar disease.
• Target hybrids with low stalk rot and/or standability scores.
• Evaluate at least 100 plants per field (20 plants in 5 locations).
• Use the "push test" or the "pinch test" to determine standability. If 10 to 15 percent of plants lodge or are rotted, schedule an early harvest.
Photo by Gary Munkvold