Part 2, Section 2: Corn Pest Management
Corn Pest Management
DISEASES
Leaf and stalk diseases
Corn diseases are strongly influenced by weather conditions and are very difficult to predict. They are controlled best by using resistant hybrids and a balanced fertility program. Since major corn pathogens can overwinter in Pennsylvania, tillage and rotation are common cultural practices for reducing sources of disease-causing organisms, particularly if disease was a problem the previous year.
In no-till plantings of continuous corn, extra care must be taken to select disease-resistant hybrids. A high incidence of leaf blight causes early death of plants and moves harvest dates ahead of the norm; stalk rot increases in blighted fields. Delayed harvest leads to greater field losses. Harvest dates should be determined by crop maturity, not by the calendar.
Disease prevention or control with fungicides other than seed treatments usually is not economical and should be considered only on an individual basis.
Northern leaf blight. Found through-out Pennsylvania, this disease is characterized by long (1- to 6-inch), boat-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 race have been incorporated into many hybrids, so disease-caused 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. Prominent in many areas of Pennsylvania, the organism causing the disease overwinters in the corn flea beetle and is transmitted by this insect. 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 2.2-21 and 2.2-22.
Gray leaf spot. Most severe cases of gray leaf spot occur 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. Disease symptoms are gray, rectangular lesions that are restricted by the leaf veins. An individual lesion resembles a paper match. Control measures include rotation, tillage, and planting resistant hybrids.
Stalk rot. The most frequently observed corn disease in Pennsylvania, stalk rot usually can be traced to stress occurring at some critical point in the growing season. Moisture stress, stresses from other diseases, and improper 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.
Localized outbreaks of the following leaf diseases also have been observed in Pennsylvania:
Eyespot has been associated with cooler growing regions and may be controlled by planting resistant hybrids.
Northern leaf spot is characterized by long, linear, chainlike lesions. The disease is observed throughout the state and probably only rarely causes a significant yield loss. Resistant hybrids are the best method of control.
Anthracnose usually is observed on older (lower) leaves under conditions of high humidity. Symptoms may resemble those of nitrogen deficiency—yellowish-orange leaves. Anthracnose also may cause seedling death early in the season. Fall symptoms include leaf lesions that spread from bottom leaves to top leaves.