Update On Indiana Corn Foliar Disease Risk

 

Frequent rains, mild temperatures, and high humidity across Indiana have increased the risk for foliar disease development in corn. So far, disease observations have been limited, but with more rain and mild temperatures forecasted over the next week, more disease may begin to appear in Indiana corn. The two diseases that I have seen most frequently in Indiana this week are Holcus leaf spot and northern corn leaf blight.

 

Holcus leaf spot is very common in Indiana fields this year. I have received many emails and calls asking if these are early symptoms of northern corn leaf blight or gray leaf spot, and it is important not to confuse Holcus leaf spot with other foliar diseases. Holcus leaf spot is a disease caused by the bacterium Pseudomonas syringae pv. syringae, and is characterized by round, discrete lesions that are initially pale yellow to white and then enlarge and turn gray or brown. Lesions have a water-soaked halo and on certain hybrids, the margin of the lesion may appear brown or purple (Figure 1). Holcus leaf spot is seen sporadically in Indiana, and is not known to limit yield. Although the disease may cause concern based on symptom appearance, no in-season treatment is available or necessary.  Fungicide applications will not have efficacy against this bacterial disease.

 

 

 

 

Northern corn leaf blight (NCLB) is the more concerning disease present in corn.  NCLB is caused by the fungus Exserohilum turcicum, and is characterized by long, tan, cigar-shaped lesions on leaves (Figure 2). This disease caused yield loss in some areas in 2014, and it has already been observed in the lower canopy of a few Indiana fields this year. Currently we do not have thresholds available for determining when fungicide applications are needed for NCLB, but we follow similar guidelines as gray leaf spot, in that fungicides will typically be more beneficial when applied to susceptible hybrids in high-risk fields for disease development, such as those that are in minimum or no-till. Scouting fields around V14, or just prior to tassel emergence, can help determine the level of disease pressure in a field. When scouting, if 50 percent of the plants in a field have disease lesions present AND additional factors or conditions that favor disease development are in play (residue present, favorable weather conditions) a fungicide application may be warranted. Research in Indiana indicates that fungicides are most effective at preventing yield loss due to NCLB when applied at the tasseling to early silking (VT-R1) growth stage. 

 

 

 

 

Before deciding on any in-season disease management strategy this year, keep this in mind: The condition of corn in Indiana varies widely, and farmers should think carefully before making decisions to add inputs into their crop. Consider expected crop price, crop condition, planting date, predicted weather conditions and what impact these factors may have on your economic return this year.

 

It is important to remember that a V4-V6 application of fungicide to corn will not protect the ear leaf or above from disease that develops around tasseling. Farmers may be concerned about foliar diseases like anthracnose leaf blight (caused by the fungus Colletotrichum graminicola), which can be present in young corn. Typically, symptoms of this disease are confined to lower leaves throughout the growing season, and do not typically require management.

 

Farmers considering applying foliar fungicides at early growth stages of corn to improve the health of stressed or damaged corn should consider that past research indicates that the most consistent yield advantage from a fungicide application occurs when fungicides are applied in response to a high risk of disease development at VT-R1.