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Disease Development. Anthracnose is caused by the fungus Colletotrichum graminicola which overwinters on corn residue. Spores spread to growing plants by windblown rain and rain splash. Disease severity can be increased during extended periods of low light intensity (overcast conditions) and high humidity.
High yield potential and/or other stresses are often associated with stalks rots. This is because roots and stalks may be forced to remobilize their stored nutrients to provide for the grain which is the primary sink. High yield potential creates a larger sink. Stresses such as foliar diseases, insects, drought and cloudy weather decrease the amount of energy and nutrients the plant produces for grain fill. Consequently, the plant must pull or cannibalize carbohydrates from stalks and roots, making the plant more susceptible to stalk rot.
Leaf Blight Phase. Lesions of the leaf blight phase are nondescript, oval-to spindle-shaped necrotic areas that may appear water-soaked or chlorotic (Figure 1). Lesions are often found on the bottom leaves first and can progress to the upper leaves. Small, black, hair-like fungal structures called setae often occur in necrotic tissues and can be seen with the aid of a hand lens. Lesions are usually tan to brown with yellow to reddish-brown borders. Heavily infected leaves wither and die.
Top Die-Back. In fields with heavy anthracnose stalk rot pressure, it is common to observe that a portion of the plant above the ears dies prematurely while the lower plant remains green. This symptom, known as “top die-back”, may appear as early as 1 to 3 weeks after tasseling (Figure 2).1 As the stalk rot phase progresses, the pith and the vascular system becomes rotted, reducing the water translocation to the top leaves. In cases where water availability is reduced in the soil, that top leaves tend to dry down and die because of reduced water supply.
Stalk Rot Phase. Disease onset usually occurs just before plants mature. Usually, the entire plant dies and several nodes are rotted. Late in the season, after plants show signs of early death, a shiny black discoloration develops in blotches or streaks on the stalk surface, particularly on lower internodes. Internal stalk tissue may become discolored and soft, starting at the nodes (Figure 3). Stalks may also have discolored pith while the rind remains green. Lodging typically occurs higher on the stalk than with other stalk rots.2
Mid-Season. Some fungicides are labeled to help control the leaf blight phase of anthracnose. It is important to read the fungicide label to make sure that it is for the control of anthracnose as well as the proper application rate and timing restrictions. Generally, fungicides do not control the stalk rot phase of anthracnose. However, fungicides can help maintain plant health, which can cause the corn plant to be less susceptible to stalk rot pathogens.
Prior to Harvest. Plants severely damaged by the stalk rot phase may become lodged before the normal harvest period. Therefore, preparations should be taken to harvest problem fields early. Although high grain drying cost may be a concern when harvesting wet grain, this expense may be a better option when compared to the potential loss of yield due to increased lodging later in the harvest. Scouting fields for potential stalk lodging can be broken down into two methods. Two methods for determining potential stalk lodging are:
Tillage. Burying infected residue can help decrease the amount of disease inoculum.