Recent warm, wet weather has favored development of foliar diseases in corn and other crops. Northern corn leaf blight has been confirmed in southeastern Virginia over the past week, and gray leaf spot has been observed on corn in the region. Corn in much of the state is at or just beyond tasseling, and it is not too late to consider a foliar fungicide application. Several factors increase the risk of corn yield loss to foliar diseases and the chances that application of a fungicide will be profitable.

• Susceptibility of corn hybrid to disease. Varieties have a high turnover rate so check with your local extension office or seed dealer for current information on which varieties have some level of resistance to diseases in the region. Be aware of the specific diseases your hybrid is susceptible or resistant to.
• Yield potential. If yield potential is low, you do not have much to gain and fungicide applications are less likely to be profitable.
• Previous crop and cropping system (e.g. no till). Many pathogens are able to survive on crop residues. Keep in mind that some diseases overwinter on crop debris in Virginia (e.g. gray leaf spot) whereas others require a living plant host and must move in from warmer regions each year (e.g. southern corn rust).
• Crop growth stage and timing of fungicide applications. Diseases are more likely to impact yield at particular growth stages of the crop (typically during development of the grain) so timing fungicide applications accordingly is key.
• Disease pressure. Which diseases, if any, are present, and how widespread are they? Scouting and accurate pathogen/pest identification are critical components of any IPM program. Presence of a disease on lower leaves (2nd or 3rd leaf below the ear) at or near tasseling may indicate the need for a fungicide application. Yield loss can occur if diseases reach the ear leaf prior to grain fill, but the appearance of foliar disease following the dent stage is unlikely to impact yield. A “Corn Disease Scouting and Fungicide Guide” can be downloaded below.
• Weather. Temperature and humidity greatly influence the onset and development of disease. Even if the crop is susceptible and a pathogen is present, the risk of yield loss to disease may be low if environmental conditions are not conducive pathogen growth and reproduction. Warm, humid conditions are favorable for many diseases in our region. In some cases, the micro-climate within a field may be conducive for disease development even when ambient conditions are relatively dry, especially when high plant populations and a dense canopy are present in a field.

Corn Disease Scouting & Fungicide Guide

Southern rust continues to be reported on corn in Virginia. Southern rust was first confirmed August 3 on corn samples from Chesapeake and Suffolk. Since then, reports of southern rust have been made from Sussex and Albermarle counties, and it is likely that the pathogen is throughout much of the state. Yield of corn at or near the dent stage is unlikely to be impacted by the disease, but late planted corn in fields with good yield potential (120 bu/ac +) may need to be protected with a foliar fungicide. Strobilurins are good preventative fungicides whereas triazoles are recommended once sporulation is observed in a field due to their curative activity. A combination fungicide is a good option as long as a strobilurin fungicide has not been applied previously. Wet, warm weather favors disease development. Once sporulation occurs, symptoms of rust are relatively obvious. Lesions start out as raised, blister-like pustules then break open to reveal orange spores. Samples of corn plants with symptoms of southern rust can be submitted to the disease clinic at the Virginia Tech Tidewater AREC (contact Dr. Hillary Mehl, hlmehl@vt.edu).