Foliar diseases present in most Mississippi corn fields
Northern corn leaf blight (NCLB)
Over the past week I’ve received several calls regarding the presence of NCLB. Similar to last year there are a few key hybrids involved that appear to be more susceptible to the fungal disease. Even though the disease can occur with greater frequency in fields of continuous corn, NCLB can also occur in first year corn fields because the spores are likely airborne over short distances. Remember that NCLB lesions tend to be a 1 inch to 3 inches in length. Rarely does a pattern form on the leaf surface as could be associated with some sort of burn as the result of a foliar applied nutrient. The oldest portion of the lesion will tend to be the part of the leaf where sporulation of the fungus will occur. A 20× hand lens should aid in the diagnosis of the disease. The structures that produce spores will appear dark, almost black, and the spore itself will stick out from the top. Normally, unless the corn leaf tissue is wet from dew or a recent rain, the lesion will appear brown in color when fully mature. Lesions on wet leaves, or leaves deeper in the corn canopy may appear water-soaked or greener in color.
Like the name claims, common rust is a common occurrence in corn fields. Since arriving in MS in 2007, I have not observed a corn field with a tremendous amount of common rust. In addition, I would not suggest a fungicide be applied as a result of observing common rust. To date, southern corn rust has only been reported from FL and GA, therefore I suspect that southern rust is not present in MS. Common rust sporulation appears darker in color than southern rust and is generally more russet or brown than southern rust. Generally speaking, common rust will appear on both the upper and lower leaf surfaces while southern rust will exclusively occur on the upper leaf surface. Remember, common rust pustules that form on the lowest leaves in the corn canopy will oftentimes appear similar to southern rust simply due to a reduced light situation as well as the stressed nature of those corn leaves. At present, southern rust has only been reported from FL and GA and no other states in our region.
Focus on southern rust as the more important of the two corn rusts.
Automatic fungicide applications at tassel (VT)
As I have stated over the past few seasons I am not a proponent of the automatic tassel fungicide application. The main reason for the opposition has to do with the greater data set generated by MSU scientists suggesting that applying a fungicide when a yield-limiting disease occurs (e.g., gray leaf spot, northern corn leaf blight, southern corn rust) is a more economically beneficial scenario. Moreover, many other universities have a similar data set and also make a similar suggestion when it comes to addressing foliar corn diseases of economic importance.
With the extended periods of cooler than normal and wetter than normal weather this year scout your corn for diseases and make a fungicide application decision based on the hybrid’s specific disease package, the disease present, number of years in corn, yield potential, and specific growth stage at time of disease observation. Several years of data suggest this is the most economically beneficial alternative.