Northern Leaf Blight Of Corn – Control Of Northern Corn Leaf Blight
By: Mary Ellen Ellis
Northern leaf blight in corn is a bigger problem for large farms than for home gardeners, but if you grow corn in your Midwestern garden, you may see this fungal infection. The fungus that causes the disease overwinters in debris and proliferates during moderate temperatures and wet conditions. You can manage and prevent the fungal infection or use a fungicide.
Signs of Northern Corn Leaf Blight
Northern corn leaf blight is an infection caused by a fungus that is fairly common in the Midwest, wherever corn is grown. The disease generally only causes limited damage, but it may lead to crop loss under certain conditions. Some varieties of corn are more susceptible, and when the infection sets in early, losses are often greater.
The characteristic sign of corn with northern leaf blight is the formation of lesions on the leaves. They are long, narrow lesions that eventually turn brown. The lesions may also form greyish colored borders around their edges. The lesions begin to form on lower leaves and spread to higher leaves as the disease progresses. During humid weather, the lesions may develop spores that make them look dirty or dusty.
Control of Northern Corn Leaf Blight
Control of this disease is often focused on management and prevention. First, choose corn varieties or hybrids that are resistant or at least have moderate resistance to northern corn leaf blight.
When you grow corn, make sure it does not stay wet for long periods of time. The fungus that causes this infection needs between six and 18 hours of leaf wetness to develop. Plant corn with enough space for airflow and water in the morning so leaves can dry throughout the day.
The fungus overwinters in plant material, so it is also important to manage infected plants. Tilling the corn into the soil is one strategy, but with a small garden it may make more sense to just remove and destroy the affected plants.
Treating northern corn leaf blight involves using fungicides. For most home gardeners this step isn’t needed, but if you have a bad infection, you may want to try this chemical treatment. The infection usually begins around the time of silking, and this is when the fungicide should be applied.
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The disease first appears in the form of small, necrotic spots with halos. These usually expand to become rectangular lesions, about 1/8 inch wide by up to 2 inches to 3 inches long and gray to brown in appearance. Mature lesions usually have distinct parallel edges and appear opaque when put up to the light, but the lesions hybrids vary widely in shape and color. Symptoms can sometimes be confused with northern corn leaf spot, although gray leaf spot lesions are usually limited on the sides by veins.
Corn (Zea mays)-Leaf Spots and Blights
Cause Several fungal leaf spots and leaf blights affect corn. The more important ones in the Pacific Northwest include Northern Corn Leaf Blight (teleomorph: Setosphaeria turcica anamorph: Exserohilum turcicum , = Helminthosporium turcicum , = Bipolaris turcica , = Drechslera turcica ), Northern Corn Leaf Spot (formerly known as Helminthosporium leaf spot) (teleomorph: Cochliobolus carbonum anamorph: Bipolaris zeicola , = Helminthosporium carbonum , = Drechslera zeicola ), and Gray Leaf Spot ( Cercospora sorghi var. maydis and C. zeae-maydis ) . Usually these diseases have not had great economic significance in the Pacific Northwest. The fungi overwinter on infected plant debris, and winds can move spores long distances. Moderate temperatures and high humidity favor the disease. Other grass species are affected.
Symptoms In each of these diseases, the symptoms are long circular to elliptical, grayish green or tan lesions. In moist weather, concentric dark rings may be observed in the lesions. Lower leaves are affected first, and the disease moves up the plant. The plant's premature death resembles frost or drought injury.
- Plant resistant varieties and hybrids if available. The most tolerant hybrids are late maturing.
- Manage irrigation to avoid long periods of high relative humidity in the canopy.
- Practice crop rotation.
- Burying infected plant debris can minimize disease.
- Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. Not a good stand-alone product. May be applied on the day of harvest. 4-hr reentry. O
- Demethylation-inhibiting (DMI) Fungicides (Group 3) are labeled for use.
- Tilt at 2 to 4 fl oz/A on 7- to 14-day intervals. Do not apply within 30 days (field corn forage, grain and stover) and 14 days (sweet corn ears and forage) of harvest. 12-hr reentry.
- Dithane F-45 Rainshield (Group M3) at 1.2 quarts/A on 4- to 7-day intervals, for leaf blight only. Preharvest interval is 7 days for sweet corn and 40 days for field corn. Do not feed treated forage to livestock. 24-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Headline at 6 to 9 fl oz/A prior to disease development on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry except if hand detasseling or hand harvesting, for which the reentry interval is 7 days.
- Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 14-day intervals. Preharvest interval is 7 days. 4-hr reentry.
- Vertisan (Group 7) at 10 to 24 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 7 days for grain and stover 0 days for forage. 12-hr reentry.
- Premixes of fungicides are available for use.
- Elatus (Group 7 + 11) at 5 to 7.3 oz/A for sweet corn for two (2) applications at least 14 days apart. Preharvest interval is 7 days. 12-hr reentry.
- Miravis Neo (Group 7 + 3 + 11) at 13.7 fl oz/A. Early application (V4-V8) late application when disease first appears or at VT or R1. May be re-applied 7- to 14-days later. Do not apply more than two (2) sequential applications. Preharvest interval is 30 days for field corn and popcorn 14 day PHI for sweet corn. 12-hr reentry.
- Quilt (Group 11 + 3) at 7 to 14 fl oz/A or Quilt Xcel (Group 11 + 3) at 10.5 to 14 fl oz/A on 14-day intervals. Preharvest interval of Quilt is 14 days for sweet corn and 30 days for field corn. Preharvest interval of Quilt Xcel is 14 days. 12-hr reentry.
- Stratego (Group 3 + 11) at 10 fl oz/A for sweet corn or 10 to 12 fl oz for field corn and popcorn on 7- to 10-day intervals. Do not apply within 14 days of harvest for sweet corn and 30 days of harvest for field corn or forage. 12-hr reentry.
A look at Exserohilum leaf blight of sorghum
MARYKE CRAVEN, ARC-Grain Crops Institute, Potchefstroom
Leaf diseases are problematic in crops for various reasons, one being that they damage that part of the plant tasked with the production of glucose (sugars). Every square millimetre of the plant’s leaf surface lost due to infection results in less glucose produced.
A second reason mostly unknown to producers, is the fact that when the leaves are damaged to such an extent that they cannot photosynthesise effectively, the plant utilises stored glucose from areas like the roots and stalks to fill the grain. This predisposes the plant’s roots and stalks to infection, resulting in an increase in root rot and lodging, which further reduces the yield potential.
Knowing which leaf diseases are a potential threat is accordingly vital in ensuring optimal yields. Maize and sorghum share a common pathogen in this regard. Exserohilum turcicum is the causal organism of northern corn leaf blight of maize, as well as Exserohilum leaf blight of sorghum (Photo 1).
Although the lesions produced on these two crops are very similar, an interesting fact is that not all isolates from this pathogen can infect sorghum and maize equally well. A specific isolate from a sorghum plant will thus not necessarily be able to infect a maize plant and vice versa.
In both these crops the yield impact associated with the disease is, however, severe in susceptible cultivars if the disease is established before flowering. Although much has been reported on northern corn leaf blight in this regard, information regarding the impact of leaf blight on sorghum is somewhat lacking. During the 2014/2015 season, a trial was conducted at the ARC-Grain Crops Institute (ARC-GCI) to establish the resistance levels of selected cultivars as well as to establish sorghum yield loss associated with E. turcicum infection (Photo 2).
Although the trial had many technical aspects, the basic components of the study were the following: Four sorghum cultivars were included, i.e. PAN 8816, PAN 8906, PAN 8625 and NS 5511. The disease severity levels required to establish the yield loss impact associated with the disease, were created through eleven fungicide treatments which consisted of two fungicides (azoxystrobin/difenoconazole and epoxiconazole/pyraclostrobin) being applied on five different dates (six weeks, six to eight weeks, eight weeks, eight to ten weeks and ten weeks after planting).
A control was included, which did not receive any fungicide application. The trial was inoculated with E. turcicum at approximately six-leaf stage and the resultant disease development recorded at critical growth stages.
High levels of disease severity were achieved (Photo 3) with the final disease severity measured at the hard dough stage, varying between 4,4% and 69,33% over the various cultivars and treatments. Evaluation on the control plots results only, indicated that PAN 8906 as well as PAN 8625 were the most susceptible cultivars measuring leaf blight disease severity of 65,33% and 59,78% respectively (Graph 1).
PAN 8816 had 52% diseased leaf area, whilst NS 5511 demonstrated a high level of disease resistance, with only 8,9% disease leaf area measured at hard dough stage. Both fungicides tested, gave similar control to each other at the various application dates, with the best control being obtained when they were applied twice during the season (eight and ten weeks after planting).
None of the applications significantly reduced the leaf blight severity with the resistant NS 5511 when compared to its control treatment, indicating that even under such severe inoculum pressure, it was not economically viable to spray this specific cultivar in order to control the disease. Regression analyses were conducted on the different cultivars and fungicides used in order to establish the yield loss impact. The best fit with regard to regression analysis was obtained with PAN 8625 (azoxystrobin/difenoconazole treatments).
Yield loss of 7,6% was observed at flowering for each 10% increase in disease severity (Graph 2). For the same cultivar, a yield loss of 5% for every 10% increase was observed at soft dough stage (R2 = 80,2%, SE = 0,231). Although more research is required to confirm the findings of the current study, these figures are quite similar to that which have been reported internationally for northern corn leaf blight on maize, i.e. a 2% to 8% yield decline for every 10% increase in disease severity.
Two important aspects highlighted by the preliminary study should accordingly be taken note of. Firstly, the importance of monitoring disease development or taking preventative steps to control the disease, as any level of disease development has a yield implication associated with it. The earlier the disease development occurs, the greater the loss.
The second aspect is knowing the resistance level of the cultivar. Resistance cultivars are available in the market and producers should contact their seed suppliers in this regard. The utilisation of resistant cultivars is a viable method to reduce inoculum levels, as well as impute costs associated with fungicide application.
Producers are welcome to contact Dr Maryke Craven at 018 299 3646.
Publication: September 2016
Northern Corn Leaf Blight
Northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum, is one of the most frequently occurring foliar diseases of corn in Ohio and the Midwest in general. Since the early 2000s, both the prevalence and severity of this disease have increased, but in most years plants only becomes severely diseased well after silking. During wet weather yield losses may be as high as 30-50% if the disease becomes establishes before tasseling. However, if leaf damage is only moderate or is delayed until 6 weeks after silking, yield losses are minimal. Severe damage caused by NCLB also predisposes plants to stalk rot and lodging, which may further reduce yield and grain quality.
|Fig. 1. Typical cigar-shaped lesion of northern corn leaf blight|
The characteristic symptom of northern corn leaf blight on a susceptible hybrid is one-to-six inch long cigar-shaped gray- to tan-colored lesions on the leaves (Fig. 1). However, before lesions are fully developed, they first appear as small light-green to grayish spots approximately 1-2 weeks after infection. As the disease develops, the lesions spread to all leafy structures, including the husks, and produce dark gray spores, giving lesions as dirty appearance. The lesions may become so numerous that the leaves are eventually destroyed, causing major yield loss due to reduction in the available of carbohydrates to fill the grain. The leaves then become grayish-green and brittle, resembling leaves killed by frost.
Lesion characteristics may vary among hybrids based on their resistance and interaction with different races of the pathogen. Several physiological races of the fungus are known to occur, including races 0, 1, 2, 12, 23, 23N and 123N. Two types of resistance are available to protect against these races: partial resistance (non-race specific), which is controlled by multiple genes and protects against all known races of the fungus, and race-specific resistant, which is controlled by single Ht genes (Ht1, Ht2, Ht3, and HtN) and, as the name suggests, protects against specific races of the pathogen. Partial resistance and resistance conferred by HtN are expressed as a reduction in the number and size of the lesions, whereas resistance conferred by Ht1, Ht2, and Ht3 is expressed as small chlorotic lesions. A compatible interaction between Ht genes and races of the pathogen results in a susceptible lesion type—large necrotic cigar-shaped lesions (Fig. 1)—whereas incompatible interactions result in small chlorotic lesions (resistant lesion type). For instance, race 1 of the fungus causes large necrotic cigar-shaped lesions on hybrids with Ht1, but small chlorotic lesions on hybrids with Ht2, Ht3, or HtN.
Disease Cycle and Epidemiology
The fungus causing NCLB overwinters as mycelia and conidia on corn residues left on the soil surface. Conidia may also be transformed into thick-walled resting spores called chlamydospores. During warm, moist weather in early summer, new conidia are produced on the old corn residue and carried by the wind or rain to lower leaves of young corn plants. Infection and disease development are favored by heavy dew, frequent rainfall, high humidity, and moderate temperatures. Infection by germinating conidia occurs when free water is present on the leaf surface for 6-18 hours and the temperature is between 66 and 80°F (18-27°C). Under favorable conditions, lesions develop and produced a new crop of spores within 7-12 days on susceptible hybrids, causing the disease to spread rapidly. Secondary spread from lower to upper leaves and among plants within a field results primarily from spores being splashed around by rain, whereas wind is responsible for long-distant spore movement and spread of the disease from one field to another.