Scouting for corn diseases can be a daunting task. Looking out over the top of a corn field suggests that no diseases are present. This differs for the other crops we deal with where can observe a potential problem area and go to see what those problems might be whether it be shorter than normal plants or some other issue.
Foliar diseases are likely present in almost every corn field. However, with that said, the specific disease present as well as the field’s history likely dictates whether or not a foliar disease is present. Also, keep in mind, that certain corn diseases likely manifest themselves in corn that has passed the tassel stages. Most foliar diseases threaten corn as the corn gets closer to physiological maturity. For more information on the particular corn diseases that could be present see: http://www.mississippi-crops.com/2013/04/20/corn-disease-calendar/.
At present I have received only limited telephone calls regarding foliar diseases in corn fields throughout Mississippi, but as drier conditions have allowed more people to scout corn the call numbers have increased each day. Presently, I’ve observed the diseases outlined below and have also observed extremely low levels of southern corn leaf blight in one field in the south Delta.
Anthranose leaf blight
Over the past several weeks I’ve encountered anthracnose leaf spot. Normally speaking we don’t generally observe this disease, but the cooler and wetter conditions. In all of the cases I’ve observed this season the disease has been present on the lower leaves in the plant canopy. Lesions will generally be 3/4 of an inch in length to an inch in length. The lesions will not have parallel margins and most of the lesions may appear to have concentric rings, similar to a target. In addition, lesions may appear to have small pepper grains within the plant tissue that serve as the reproductive structure of the fungus. The disease can increase once tasseling starts, but rarely has an issue been reported with the leaf blight phase in MS. Typically the lower leaves on a corn plant are stressed and low levels of light in the deeper parts of the plant canopy allow the disease to increase on the bottom leaves of the plant. Management practices in the form of a fungicide are not necessary for anthracnose leaf blight.
Holcus leaf spot
Annually, calls/reports of Holcus leaf spot occur throughout the state. Holcus leaf spot is a fairly rare occurrence. Herbicide injury, urea burn, fertilizer burn, or even adjuvant burn associated with a foliar application are a more potential likely observation that widespread Holcus leaf spot. Even if Holcus leaf spot is present in a field at pre-tassel stages, or post-tassel for that matter, no management alternatives will reduce the symptoms associated with the disease.
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.