Corn Diseases Update

Common rust can be found in almost every corn field in South Dakota. The disease develops from spores blown up from southern states since the pathogen does not survive the winter in South Dakota. Typical symptoms of common rust are small, dark, reddish-brown raised pustules on the corn leaf. The pustules vary from oval to elongated in shape (Figure 1). Common rust rarely develops to reach yield limiting levels unless a very susceptible hybrid is planted, in which case a foliar fungicide at tasseling would be recommended.

 

 

Northern corn leaf blight (NCLB) is a fungal disease that is also common in South Dakota. The NCLB pathogen, Exserohilum turcicum, survives on corn residue and is spread through rain splash and wind. The wind-dispersed spores can travel to fields many miles away. The typical NCLB symptom is cigar-shaped lesions (Figure 2). They are typically 1 to 6 inches long, gray to tan colored and often observed on the lower and middle leaves. Under favorable weather (more than 6 hours of leaf wetness and 80°F), NCLB can continue to develop reaching yield limiting levels on susceptible hybrids. There are no set thresholds for applying a fungicide to control NCLB; however, if the planted corn hybrid has a poor rating for NCLB and one or two lesions can be seen on 50% of the plants, a fungicide is recommended.

For a list of fungicides and their efficacy in corn, see the list developed by the Corn Diseases Working Group.