Northern Corn Leaf Blight on Corn– (Gregory Shaner)
- An old foe is making a comeback.
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Northern corn leaf blight is one of several diseases that kill corn leaves prematurely. Northern corn leaf blight can be found some fields nearly every year, but usually it is not severe enough to be of concern. This year I have seen, and have received several reports of extensive blighting of leaves on some corn hybrids due to northern corn leaf blight.
Lesions of northern corn leaf blight on a susceptible hybrid are fairly easy to recognize. They are large—up to 6 inches or more long and 1 inch wide—and taper at each end. They are tan, but when air is humid may have a dull green cast.
Like several other common leaf blighting diseases of corn, the northern corn leaf blight fungus, Exserohilum turcicum, overwinters in residue of corn infected the previous year. In the spring and early summer, the fungus produces spores on this residue. Wind blows these spores around and those that land on growing corn can infect and cause disease. Initial lesions develop on lower leaves. Once a dead lesion develops on a leaf, the fungus will produce spores there, and these can infect more leaf tissue. Over time, the disease progresses up the plant, killing tissue on leaves above the ear. The dull green cast on lesions is because of the spores of the fungus on the lesion surface. Moderate temperatures (65 to 80 ∞F) and long dew periods favor infection.
Most hybrids have a partial resistance to northern corn leaf blight that restricts lesion size and reduces the number of spores the fungus can produce on a lesion. This slows down the spread of the blight so that the amount of leaf tissue destroyed is not enough to reduce yield much if any. This is why, even though the fungus is widespread in Indiana, we don’t usually see much damage from northern corn leaf blight.
This year, a lot of leaf tissue on some hybrids, particularly in central and northern Indiana, is blighted. These hybrids evidently do not have enough resistance to retard the spread of the disease. If grain in these fields is now in the dough stage, the reduction in yield may not be great, but there will probably be some loss. Damage from northern corn leaf blight depends on how much leaf tissue is destroyed at a given stage of kernel development. Kernels on severely affected plants may not fill completely.
It’s far too late to consider any remedial action, i.e., application of a fungicide. Nonetheless, growers should check their fields for leaf blight. If it is severe and if yield and grain quality are not what would be expected, a grower might want to use a more resistant hybrid in the future. It is impossible to say whether northern corn leaf blight will be a problem next year, but if a field is severely diseased this year, there will be a lot of overwintering fungus, and so a lot of inoculum next year.
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White Mold in Soybean – (Andreas Westphal, Gregory Shaner, and Scott Abney)
Full-season soybean crops around Indiana are approaching maturity. Late season diseases show-up at this time. These include sudden death syndrome, brown stem rot, Phytophthora stem rot, damage by plant-parasitic nematodes, and white mold. White mold, also called Sclerotinia stem rot, is mainly a problem in northern Indiana. White mold is first evident as scattered wilting plants among healthy plants. Leaves of infected plants are entirely wilted, but remain attached to the stem (Fig. 1). The soybean stem will have bleached lesions 1 to 5 inches long, often centered on lower nodes (Fig. 2). These lesions are often covered with cottony mycelium of the fungus. Embedded within this mycelium are sclerotia—hard, black structures of 0.1 to 1 inch long (Fig. 3). Greater numbers of sclerotia may be found within the stem. These sclerotia can survive in soil for several years. They are also an important means of dispersal.
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Figure 1. Soybean plant with white mold infection. Leaves are heavily wilted. |
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Figure 2. Stem lesion, typical white covering on soybean plant. |
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Figure 3. Sclerotium on the
outside of soybean stem. |
Yield reduction depends on when white mold first affects a soybean plant. A girdling stem lesion largely prevents movement of water and nutrients from the root system to the foliage. If the soybean has not matured by the time a girdling stem lesion develops, seeds may remain small and be lost during harvest. Seed of plants affected later may be of more normal size. Yield loss is related to time of infection and how much of a field is affected.
A fungus known as Sclerotinia sclerotiorum causes white mold. The pathogen may be introduced into previously non-infested fields with soybean seed contaminated with sclerotia. Sclerotia are similar to soybean seed in size and density, and can be mixed with seed from infested fields when harvesters do not separate them from the seed. Modern seed cleaning equipment does a good job of removing sclerotia. The fungus can be spread within a field during harvest, when sclerotia are blown out the back of the combine with chaff.
Sclerotia in the top inch of soil will germinate under favorable conditions. A fruiting structure termed an apothecium emerges from the sclerotium and rises slightly above the soil surface. The apothecium resembles a small golf tee. Within its cupped surface, it produces spores. Wind disperses these spores. Spores that land on soybean plants will infect and cause white mold. Don’t confuse apothecia of Sclerotinia with the fruiting bodies of bird’s nest fungi. Bird’s next fungi are common in corn and soybean fields. They produce a small, leathery cup. They grow on dead plant residue and do not cause disease on soybean.
Most spores of Sclerotinia sclerotiorum land close to where they were produced. When apothecia develop within a field of soybean, many spores will land on the base of plants. There, they can germinate and infect. Infection seems to occur most readily from spores that land on senescing flower petals. From there they infect the stem and causes its typical lesions.
While the exact conditions necessary for the pathogen to infect are not known in detail the disease is often observed in the northern Indiana. Cool temperatures and extended periods of leaf wetness during the early stages of flowering are critical for infection and disease development. These conditions are more common in the northern part of the state, especially in narrow-row beans that reach canopy closure by the time flowering begins. The year 2004 has been much cooler than usual and leaf wetness periods during flowering may have been conducive in more areas of Indiana than in previous years. It may be worthwhile to inspect early dying soybean fields for white mold.
An infested field needs special management considerations. If there are only a few affected areas in a field, it might be better to not harvest these, to avoid dispersing sclerotia. The number of viable sclerotia in a field will decline over time if there are no susceptible hosts in a field. However, sclerotia can survive for several years. Corn and small grains are not hosts for this fungus, but many broadleaf weeds can be infected. These include pigweeds, ragweeds, and several mustards. Therefore, good weed control, regardless of the crop in a field, is necessary. Many vegetable crops are also susceptible, which may also be why the disease is more common on soybean in northern Indiana, where susceptible vegetables are produced. Wider row spacing (30 inches) may reduce severity of white mold. This is because at the time flowering begins, the canopy has not closed, so microclimate conditions at the soil surface may be less conducive for production of apothecia and spores, and for infection. A grower should choose a less susceptible soybean cultivar for production in high-risk fields.
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