Conditions for Corn Diseases are Favorable

With all of the rain that has fallen over McLean County this month, conditions have become favorable for some foliar corn diseases.  The UK Plant Pathology Lab in Princeton has received samples from Western Kentucky of corn with northern leaf blight, gray leaf spot, common rust, and/or Diplodia leaf streak.  As of July 12, southern rust has not been detected in Kentucky however UK Plant Pathology Dr. Carl Bradley warns corn producers to be on the lookout for the following diseases:

Northern Leaf Blight
Northern leaf blight can be a major yield robber in susceptible corn hybrids.  Symptoms of this disease are observed on leaves as elliptical (“cigar-shaped”) lesions that have a light to tan-colored center.  Some hybrids may have excellent resistance to northern leaf blight, while others may be extremely susceptible.  It is important to know how susceptible a hybrid is to this disease to be able to determine the risk of disease occurrence and likelihood of a profitable foliar fungicide application.

This disease may be easily confused with Diplodia leaf streak, which also may appear as elliptical-shaped lesions on leaves.  Lesions of Diplodia leaf streak will contain black pin-sized structures known as “pycnidia” inside lesions, which can help distinguish it from northern leaf blight.  Diplodia leaf streak appears to still be a minor disease in Kentucky, and management of this disease is likely not needed at this time.

Gray Leaf Spot
Gray leaf spot is likely the most commonly-observed foliar disease of corn in Kentucky.  It is observed as rectangular lesions that occur on leaves.  Hybrids can vary greatly in their level of susceptibility to this disease, but no hybrid is completely resistant.  Conditions have been very favorable for gray leaf spot over the last several days.  On susceptible hybrids, expect that gray leaf spot will be very apparent within the next few days.  Disease progress can be “sluggish” with this disease at first, since it can take several days between the time of initial infection and the appearance of symptoms; however, severity can increase suddenly when conditions are favorable.

Common Rust
Common rust has been widely observed in the state for several weeks now.  In general, most yellow dent corn hybrids have an adequate level of resistance to common rust, and management of common rust with foliar fungicides in yellow dent corn hybrids is not generally needed.

Southern Corn Rust has been confirmed in Webster County.   Since most hybrids are susceptible to southern rust, it is important to be able to be able to differentiate southern rust from common rust.  The timing of southern rust’s appearance in Kentucky relative to corn growth stage is critically important in regards to risk of yield loss.  From Dr. Bradley’s research trials, research results showed the following (in general): if southern rust begins to appear after the R3 (milk) stage, yield loss caused by southern rust will be negligible (although disease levels can still appear to be very severe).
When making decisions about applying a foliar fungicide to corn keep in mind scouting observations, disease risk assessments, and economical considerations.  Also, remember that because of current weather conditions and the amount of precipitation we have had in McLean County in July, conditions are favorable for the development of corn diseases.

For more information on corn diseases, contact the McLean County Cooperative Extension Office at (270)273-3690.  Educational programs of the University of Kentucky Cooperative Extension Service serve all people regardless of race, color, age, sex, religion, disability or national origin.


Upcoming Events
July 28th UK Corn, Soybean & Tobacco Field Day, Princeton Research Station
July 29th Extension Blood Drive, 11am-4pm
July 30th Farm to Fork Dinner, 6pm at Myer Creek Park
August 4th Forestry Program, Noon
August 9th McLean Co. Beef Field Day, 9am at Cody Robertson’s Farm
August 10th Hay Testing Blitz
August 11th UK Hemp Field Day