Leaf and Stalk Diseases
Part 2, Section 2: Corn Pest Management
Corn Pest Management
Leaf and Stalk Diseases
Leaf and stalk diseases can affect corn from emergence through physical maturity and beyond. Many diseases can reduce yield through parasitizing the plant and reducing grain quality. Stalk rots can further cause yield loss through reduced stalk strength leading to increasing lodging. The following list of diseases, symptoms, and possible control options should be used only as a guide, with diagnosis done either by a local extension agent or sending samples to the Penn State Plant Dis- ease Clinic, 218 Buckhout Lab, University Park, PA 16802.
Northern corn leaf blight is found throughout Pennsylvania and characterized by long (1- to 6-inch), boat- or cigar-shaped, grayish-green to tan lesions. Lesions first appear on the lower leaves, progressing to the upper leaves over time. A few years ago, the incidence and severity of this disease increased with the occurrence of a new race of the pathogen. Higher levels of genetic resistance to the new pathogen have been incorporated into many hybrids, so yield losses have been limited in the last several years. Potentially, however, this disease can result in yield losses. Avoiding inoculum by rotation and/or tillage along with planting resistant hybrids are the best methods of controlling this disease.
Bacterial leaf blight can be prominent in many areas of Pennsylvania. The disease starts as pale green streaks that enlarge and turn brown or necrotic. The lesions often have wavy margins. Many commercial hybrids are resistant, but the disease can be an issue in sweet corn and popcorn. The disease-causing organism overwinters in the corn flea beetle and is transmitted by this insect. To predict the severity of this disease, the sum total of the mean temperatures for the months of December, January, and February indicates how well the flea beetle will survive the winter. If this sum is greater than 85°F, severe levels of bacterial leaf blight may be expected on highly susceptible hybrids. The most practical control for this disease is to plant resistant hybrids. In certain situations, chemical control may be necessary and must be directed toward controlling the corn flea beetle early in the season. Suggested controls of the corn flea beetle are listed in Tables 2.2-22 and 2.2-23.
Gray leaf spot can be severe in fields of continuous no-till corn where air drainage is poor. Fields along streams and rivers are particularly vulnerable to gray leaf spot because of the extended periods of dew and high humidity. Disease symptoms are gray, rectangular lesions that are restricted by the leaf veins. An individual lesion resembles a paper match. This disease produces a toxin that requires sunlight; therefore the disease usually begins on the field edges and moves in. Control measures include rotation, tillage, and planting resistant hybrids. More information, including color photos of lesions and management techniques, can be found at pubs.cas.psu.edu/FreePubs/pdfs/xl0084.pdf.
Anthracnose can occur as early season leaf blight, later season leaf blight, late season top dieback, and/or stalk rot. The early season leaf blight can be found on corn after emergence when exposed to long periods of rain. Depending on the duration of plant stress, favorable plant growth conditions will help the plants recover. Later season leaf blight is usually observed on older (lower) leaves under conditions of high humidity. Symptoms may resemble those of nitrogen deficiency—yellowish-orange leaves with necrotic lesions. Fall symptoms include yellow to brown leaf lesions with wavy margins that spread from bottom leaves to top leaves. Top dieback may be evident later in the season, with many of the tassels broken off. Stalk rot will appear as shiny, black, linear streaks and blotches above the brace roots, leading to increased lodging and harvesting problems. Crop rotation is the best method of control.
Stalk rots are some is one of the most frequently observed corn diseases in Pennsylvania and usually can be traced to stress occurring at some critical point in the growing season. Moisture stress, along with stresses from other diseases, insect damage, and unbalanced fertilization are the most common factors associated with increased levels of stalk rot. Growers should select hybrids that stand well and attempt to minimize as many stresses as possible. There have been mixed results on the reduction of stalk rots using corn with the Bt (Bacillus thuringiensis) gene to decrease insect damage.
Localized outbreaks of the following leaf diseases also have been observed in Pennsylvania:
Eyespot is characterized by round or oval spots with a tan/brown center and a brown or purple margin. The disease has been associated with cooler growing regions and may be controlled by planting resistant hybrids.
Northern corn leaf spot is characterized by long, linear, chainlike lesions. The disease is observed throughout the state and only rarely causes a significant yield loss. Resistant hybrids and crop rotations are the best methods of control.