By Jaime Cummings, Joe Lawrence.et.al.
Reports of Southern Corn Leaf Blight, have been confirmed by our neighbors near Erie, PA this past week. This is on our radar, because that area shares latitude with some of our corn acreage in our southern tier and Hudson Valley region. Therefore, you may want to keep an eye out for atypical corn foliar disease symptoms as the season progresses.
Most corn growers are unfortunately familiar with many of our common foliar diseases, including northern corn leaf blight, gray leaf spot, and eyespot. Sure, at first they all look alike as all young lesions start out as small chlorotic spots. But, as the disease progresses, and as the lesions mature, each disease has fairly distinctive lesion types that a trained eye could possibly identify even from the window of the truck on a drive-by scouting effort. But, throw an unfamiliar leaf spot into the mix, and it might get a little more confusing.
Southern Corn Leaf Blight (SCLB), though not common in NY, was confirmed in 2018 on Long Island, and may be appearing again in 2019. Suspicious samples have been submitted for ID. SCLB lesions may not be as distinctive or easy to identify, because they are somewhere intermediate in size and shape between gray leaf spot and northern corn leaf blight, and they also resemble lesions of the northern corn leaf spot disease. With so much overlap in symptoms, it’s best to get an accurate diagnosis before making any management decisions.
SCLB typically appears on corn leaves between VT and R4 growth stages as irregular tan lesions with vaguely reddish margins. Lesion shape and size may vary among hybrids. There are different races of this pathogen (races T, O, and C), but race O is most common in North America and is restricted to leaf infections. However, race T also exists in the US, and can infect leaves, stalks, and ears. As with most of our corn diseases, the fungus overwinters on corn debris, and can be further disseminated by wind or rain within and among fields in subsequent seasons (Fig. 1). There can be multiple cycles of this disease in one season if conditions are favorable (warm and wet). However, this hasn’t been a major disease of concern since the 1970’s, and we don’t anticipate it to be a chief concern here in NY compared as compared to our regional issues with northern corn leaf blight and gray leaf spot.
Figure 1. Typical disease life cycle of a corn foliar pathogen such as southern corn leaf blight.
As with all corn foliar diseases, the incidence and severity of the lesions and the level of epidemic in the field will determine its impact on yield, because all foliar diseases affect photosynthesis and may leave plants more susceptible to stalk rots. The first and best option for managing SCLB is through genetic resistance. However, since this is such an uncommon disease in NY or other northern production areas, resistance ratings specifically for this disease may not be widely available in seed catalogs when making hybrid selections. And, as for many of our common foliar diseases, an integrated management approach will work best. Reducing primary inoculum through residue management and crop rotations, in combination with genetic resistance and use of fungicides only when necessary will successfully minimize losses from southern corn leaf blight. Please remember, research has shown that fungicides are most cost-effective with a single application at VT/R1 when disease pressure is >5% throughout the field, and when the disease reaches at least the ear leaf by tasseling on susceptible hybrids when the weather is expected to be conducive for the disease to spread (Fig. 2).
Figure 2. Corn fungicide timing and disease severity trials summary of hundreds of trials in 2013 by Dr. Kiersten Wise of University of Kentucky show that a single fungicide application at VT/R1 with disease severity >5% resulted in the best yield response.
A corn field in Chautauqua County with suspected southern corn leaf blight infection. Samples submitted and awaiting diagnosis.