This is shaping up to be another “interesting” year for corn and other field crops in Missouri and most of the Midwest. Prolonged periods of wet weather and then flooding delayed planting or led to replanting. Overall the corn crop is behind normal although not as far behind normal as it was at this time last year. There is also a wide range in growth stages of corn across the state. We have not received many samples or calls related to corn foliage diseases but with the most recent bout of wet weather, it is likely that corn foliage diseases may begin to show up in fields.
So far the samples that have been submitted have had anthracnose leaf blight on the lowest leaves, especially in fields with corn on corn (see May 24 issue of the Integrated Pest & Crop Management Newsletter for more detailed information on anthracnose of corn). However, corn growth over the last week to 10 days has been incredible. Plants that were at the 2-3 leaf stage around June 5 are now 3-4 feet tall. The anthracnose lesions which were so evident on the first true leaf and the oldest, smallest leaves on plants in early June are in some cases now hard to find. The small leaves at the very base of the plant have sloughed off or are sloughing off and lesions are not very prevalent on the leaves that have emerged with the rapid growth over the last week to 10 days.
Gray leaf spot, common rust and southern rust are the foliage diseases most likely to occur on corn in Missouri over the next few weeks. Northern corn leaf blight does not occur every year but may occur in wet or cool, wet years so that would be another foliage disease to look for when scouting fields. I have had some questions about the bacterial leaf spot, Holcus leaf spot. Although this disease would be likely to occur after hail storms or rain storms with hard, driving winds, I have not seen any samples that were positive for Holcus leaf spot yet this year.
Generally speaking with the corn foliage diseases, the later in the season (especially the longer after pollination) that the foliage disease becomes established, the lower direct yield losses will be. Highest yield losses occur if diseases such as rust or gray leaf spot develop prior to pollination. Also, most of the corn foliage diseases are favored by extended periods of free moisture on the leaf surfaces. This moisture can be from rain, overhead irrigation or heavy dews that stay late in the day. Fields with poor air movement, river bottom fields or shaded portions of fields may also have higher levels of corn foliage diseases.
Most of the control recommendations for minimizing losses due to corn foliage diseases are preventative measures such as planting resistant hybrids, rotating crops so the corn doesn’t follow corn in the same field or tillage to reduce the amount of infected residue left on the soil surface.
Several fungicides are labeled for use on corn to control foliage diseases. See the 2011 Missouri Pest Management Guide: Corn, Grain Sorghum, Soybean and Winter Wheat M171 for fungicides labeled for use on field corn.
In making the decision on whether or not to apply a foliar fungicide to corn it is important to consider the yield potential of each individual field. If fields are uneven and struggling because of wet conditions, foliar fungicides are less likely to give significant increases in yield. If nitrogen loss is a problem again because of wet conditions, it may be more beneficial to correct the nitrogen deficiency than apply a fungicide. Foliar fungicides may give greater yield increases on susceptible hybrids than on hybrids with resistance to the foliage disease present. With foliar fungicides it is important to be scouting fields so that products are applied before the disease has built up to high levels. Later planted fields which will have plants at earlier growth stages later in the season may also benefit more from fungicide application if diseases are occurring than early planted fields which are at more advanced growth stages.
With the wide range in planting dates and plant vigor across the state it is impossible to make blanket assessments of the incidence or severity of corn foliage diseases or to make statewide management recommendations. Individual fields need to be monitored for stand, vigor and yield potential, for other issues such as nitrogen loss or weed escape problems, for the foliage diseases present and the severity of those diseases as well as for the forecast weather conditions in the area before deciding to apply a foliar fungicide.
Fields with high levels of various foliage diseases may also show higher levels of stalk rot this fall. As harvest approaches, check fields which have had foliage disease problems for stalk rot and try to harvest problem fields promptly.
Symptoms of Common Corn Foliage Diseases
Anthracnose (Colletotrichum graminicola)
Infection is most common on lower leaves of young plants but may occur on upper leaves of maturing plants too. Anthracnose lesions tend to be brown, spindle-shaped lesions with yellow to reddish-brown borders. Concentric rings or zones are sometimes apparent within the diseased areas. Stalk symptoms appear as black linear streaks on the surface of lower internodes late in the season.
Holcus Leaf Spot (Bacterial leaf spot)
Lesions usually are oval to rectangular in shape. Initially, they are dark-green and water soaked. Later they become dry and turn light brown with a reddish margin. The lesion resembles parchment paper. Holcus leaf spot may occur a few days after a rain storm but does not usually cause serious losses.
Common Rust (Puccinia sorghi)
Circular to elongate, golden-brown to reddish-brown pustules develop on both upper and lower leaf surfaces. As plants mature, the pustules become brownish-black in color. The pustules rupture, revealing powdery brown spores.
Southern Rust (Puccinia polysora)
Light, reddish-brown, circular to oval pustules develop primarily on the upper leaf surface. Eventually pustules rupture to reveal powdery spores. Later a brownish-black spore stage often forms in rings around the initial pustules.
Gray Leaf Spot (Cercospora zeae-maydis)
Lesions on maturing corn are pale brown to reddish brown and blocky to rectangular in shape when compared to other corn leaf blights. The lesions typically are restricted by leaf veins giving the lesions parallel edges. Older lesions have a gray cast. Lesions may merge, resulting in large areas of dead leaf tissue. Lesions usually develop first on lower leaves but under favorable weather conditions, extensive leaf blighting over the entire plant may occur.
Northern Corn Leaf Blight (Exserohilum turcicum)
Long, elliptical, grayish-green or tan lesions ranging from 1.0-6.0 inches in length develop on the lower leaves. As the season progresses, nearly all leaves of a susceptible plant may be covered with lesions, giving this plant the appearance of having been injured by frost. During damp weather, dark olive-green to black spores may be produced across surface of lesions.