Dean Malvick Assistant Professor and Extension Specialist in Plant Pathology Phone: (217) 265-5166 E-mail: dmalvick@illinois.edu

The term "foil" means to keep from being successful, and it has often been used in detective stories where a detective foils a crime by outwitting a crafty criminal. The term "foil" also applies well to our challenges as agricultural professionals in preventing crop diseases. Note here that the key word is "prevention." While there are curative treatments available for some plant diseases, the best approach for reducing most diseases is making an up-front effort to keep them from being successful. Disease prevention requires a basic understanding of the disease and the factors that favor disease development.

In the summer of 2001, nearly 100 percent of the corn disease samples and 57 percent of soybean samples submitted to the UIUC Plant Clinic were foliar and stem diseases. Stem and foliar diseases are commonly reported because they are easily seen and readily spread (unlike most root diseases). Many are considered compound-interest diseases that spread in numerous cycles of infection in a single season. That is one reason why most severe plant disease epidemics in history have been foliar and stem diseases. In this presentation, key information and management strategies for several common foliar and stem diseases will be discussed.

Gray Leaf Spot of Corn

Illinois has the dubious distinction of being the first place gray leaf spot (GLS) was reported (in 1925). Fortunately, this disease was not a major problem in Illinois until recently, when it became important, in part, due to widespread use of conservation tillage systems that leave infested corn residue on the soil surface . GLS infections, caused by the fungus Cercospora zaea-maydis, begin as small brown spots with yellow halos and expand to tan/gray rectangular lesions that are bordered by leaf veins . The disease primarily occurs following conditions of dew, fog, and cloudy weather because the pathogen requires at least 95 percent humidity to infect.

Gray leaf spot was observed in many parts of Illinois in the summer of 2001, but, fortunately, it was minor in many areas and caused minimal yield loss. This observation indicates that there may be large amounts of inoculum in many fields, creating a situation favorable for GLS next year if environmental conditions promote the disease. Hybrids with high levels of resistance should be chosen to manage this disease. Plowing and burying residue may be helpful in some fields, but in areas where disease has been common in recent years, this may not bring significant benefits. Foliar fungicides can provide control and may be economical when conditions are favorable for disease and high-yielding, susceptible hybrids are grown.

Corn Anthracnose

In the summer of 2001, the leaf blight and stalk rot phases of anthracnose, caused by the fungus Colletotrichum graminicola, were reported in Illinois. This pathogen causes small, elongated water-soaked spots primarily when plants are most susceptible at the seedling and mature stages. The spots become enlarged with tan centers, and small, dark, hair-like spots often develop in the middle of lesions. Lesions can spread to cover much of a leaf and top dieback can occur, causing yield loss. This is the only common leaf-blight pathogen that also causes a common stalk rot. Anthracnose stalk rot usually occurs late in the season, just before normal senescence, and is recognized due to the shiny black color that develops on stalks. Unlike other common stalk rots, anthracnose often affects more than one internode.

The anthracnose pathogen overwinters on corn residue, and the spores produced on residue in the spring are carried by wind or splashing rain to initiate the leaf blight phase. Warm temperatures and cloudy, wet weather favor this disease. Spores produced on leaf lesions may spread to and infect stalks at leaf sheaths or wounds. Buried, infested residue may also initiate anthracnose stalk rot infection. Resistant hybrids are a key to control of anthracnose; however, resistance to the stalk rot phase is poorly correlated to the leaf blight phase. Crop rotation is primarily of value for the seedling phase of this disease, and control of corn borers may be important in reducing stalk infection via feeding wounds.

Green Stem Syndrome of Soybeans

Green stem syndrome was widely reported in Illinois in 2001. Green stem syndrome refers to scattered plants that maintain green stems and leaves in fields of brown and mature plants. Green stem is a problem in many areas of Illinois that appears to have increased in the past 10 years. Yield loss from green stem occurs due to reduced pod numbers and seed numbers per pod. In addition, reduced seed quality due to mottled discoloration has been associated with green stem. Green stem can also cause delays in harvest and difficulties with harvesting.

Green stem varies in severity and incidence among different locations and from year to year. The incidence of green stem differed among varieties and locations in Illinois this year. The cause of green stem is under investigation but remains unknown. This is a complex disease that appears to have more than one cause, such as plant viruses, phytoplasmas, high soybean population density, genetic mutations in soybean, and insect damage. Plant viruses are commonly associated with green stem. Bean pod mottle virus (BPMV, transmitted by the bean leaf beetle) has often been associated with green stem. Soybean mosaic virus has also been linked to this disorder. Specific viruses such as BPMV, however, are not always detected in plants with green stem, and plants with specific viruses do not always have green stem.

White Mold

White mold, also called Sclerotinia stem rot, has been a minor problem recently in most areas of Illinois. This disease, however, may return as a major problem in some areas when cool, wet weather conditions occur during flowering and early pod fill. This disease varies widely in severity and incidence from year to year and from location to location. White mold is caused by the fungus Sclerotinia sclerotiorum. It can survive for several years in the form of hard black sclerotia in soil or stem tissue. The sclerotia germinate near the soil surface during wet conditions after the canopy has closed. Small, mushroom-shaped structures called apothecia grow from the sclerotia, which produce ascospores that are spread by wind to infect dead flower petals. Initial lesions usually develop at stem nodes during or after flowering. The lesions expand and the tops of the plants initially become discolored before they wilt and die. Infected areas become soft and are often covered with white moldy growth in moist conditions. The sclerotia develop on or inside infected stems and pods as the disease progresses.

White mold is favored by cool (<85°F) moist weather, poor air circulation under the soybean canopy, narrow rows, and early planting. This disease frequently occurs in fields with a history of white mold. High fertility and high plant populations (which equal high yield potential) also encourage this disease. Management of white mold depends in part on planting clean seeds of the least susceptible soybean varieties. Foliar fungicides may be beneficial but may not be economically viable.

Frogeye Leaf Spot

Frogeye leaf spot, caused by the fungus Cercospora sojina, is an increasingly common foliar disease of soybean in Illinois. This disease has been managed with resistant varieties, but new races of the fungus have been known to overcome resistance. More than five races are known in the United States. Recently, frogeye has caused minimal yield loss in Illinois but has been damaging in some southern states. This pathogen overwinters in soybean residue and seeds. Symptoms are most visible on leaves, where lesions (1/16- to 1/8-inch diameter) develop as nearly round brown/gray spots surrounded by a thin reddish ring. Young leaves are most susceptible, and infection occurs all season, but the disease tends to become most severe in mid to late summer.

Frogeye leaf spot is favored by warm temperatures, wet weather, and high humidity. The key to its management is resistant soybean varieties. It may also help to bury infested residue where disease has been severe, use seeds free of the pathogen, and rotate with a non-bean crop. Seedmoist treatments reduce seed transmission of the pathogen, and foliar fungicides can provide some control.

Future Considerations

Despite our best efforts, we cannot completely foil these or most other crop diseases. Likewise, we cannot predict which diseases will be important next year. Weather and other factors that influence disease remain unpredictable, and pathogen populations and races change.

We can, however, look at disease history as well as moisture and humidity conditions for fields, and develop a plan to reduce disease damage by choosing the best varieties, managing fields appropriately, planting clean seed, and applying fungicides where their use is warranted. Although some yield losses will frequently occur, these and other preventative measures can significantly reduce the risk of severe crop losses due to disease.

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