Currently, there has been no report of Northern Corn Leaf Blight (NCLB) in Arkansas, but unusually cooler weather conditions and frequent rains have been favorable for this disease. Therefore, it is a good time to review risk factors for disease development and management options, if necessary.
NCLB is caused by a fungus, Exserohilum turcicum, which can also infect sorghum, Johnson grass and Sudan grasses. Leaf lesions are large (2 to 4 in.) gray green in color and cigar-shaped (Fig. 1). The fungus overwinters on corn stalk stubble, thus NCLB most commonly occurs in fields with minimum tillage and continuous corn production practices. When conditions are favorable, the pathogen produces spores (conidia) that are dispersed onto nearby plants or disseminated by wind to another field.
Early detection is beneficial to determine management options and scouting high-risk fields increases the likelihood of early detection. A high-risk field would have three main components: A history of disease, a highly susceptible or susceptible hybrid, and corn stubble from the previous cropping season. A low-risk field would be planted in a resistant hybrid along with pathogen-reducing tillage and rotation practices.
Though host plant resistance is the most economical way to manage NCLB, a few fields are planted each year with a highly susceptible hybrid (Fig. 2). Fungicides are beneficial on susceptible hybrids because they are frequently the only option for disease control. Fungicides do add additional cost to production, which should be considered when making a fungicide application for NCLB management. Most frequently, fungicides are applied at VT/R1 because leaf loss at these stages of growth contributes to a higher yield loss. Automatic fungicide applications at VT/R1 are NOT recommended in the absence of yield-limiting diseases because of additional production cost and risk of selecting fungicide-resistant strains of NCLB. Additionally, and maybe more importantly, is the potential risk of needing a 2nd fungicide to finish the season, which again adds to total production costs. For the past few seasons, southern rust has arrived later in the growing season (mid- to late-June) and given the current growth stages of corn, southern rust may be a serious threat in some fields. Therefore, deciding when to make a fungicide application should be based on managing yield-limiting diseases present and risk of southern rust, rather than growth stages alone. Common rust is not considered a yield-limiting disease but can be found in almost all corn fields in the state.
Currently, there are no fungicide thresholds specifically for NCLB management; however, researchers have determined it is important to protect the ear leaf and upper leaves as the plant matures through the reproductive stages of growth. Fungicide thresholds are complicated by the differences among hybrid susceptibility to NCLB, time required for infected corn to reach physiological maturity (i.e. black layer), and most importantly, unpredictability in forecasting the weather. To provide some guideline, consider a fungicide on a susceptible hybrids when there is at least one lesion on 50% of the plants prior to or near VT/R1, AND weather conditions are favor disease development. Such conditions are moderate temperatures (64 to 81°F) with several days of light rain or heavy dew for NCLB. Hot and dry weather will suppress NCLB development.
For those considering a fungicide to manage foliar diseases of corn refer to the fungicide efficacy table (note NCLB is Northern Leaf Blight on table) for fungicide options. Because prices vary due to promotions by agricultural chemical companies, fungicide cost should be considered along with fungicide efficacy. Among the fungicides labeled for corn diseases, strobilurin fungicides (FRAC code 11) are more effective before disease development (early detection), whereas triazoles fungicides (FRAC code 3) are effective before and after disease development. Alternating chemistry (i.e. FRAC codes) from one year to another is recommended to prolong fungicide efficacy and minimize selection of fungicide-resistant corn pathogens. A higher volume of water (5 to 7 gal/ac) will improve coverage needed to protect the ear leaf and adjacent leaves at mid-canopy.