About Northern Corn Leaf Blight
A fungus called Setosphaeria turcica, sometimes referred to as Exserohilum turcicum, causes northern corn leaf blight. There have been several variations of this fungus found across North America. The fungus overwinters in plant debris of previously infected plants that remain in the field. In the spring, spores are created; these fungal spores travel through water and air that carry the infection over to new host plants. Water infected with spores must be in contact with new leaf tissue between 6 and 18 hours with temperatures between 65 and 80 degrees Fahrenheit for the spread to infect the new host. This is more likely to happen when the weather is humid and wet. The spores will also spread to surrounding fields and plants when infected equipment is used. If weather conditions are advantageous for the fungus, lesions can appear as soon as 7 days following infection. This disease can cause significant yield loss when many of the plants healthy leaves are killed. Keep in mind that this disease is sometimes mistaken for southern corn leaf blight, Diplodia leaf streak, Goss’s wilt, or Stewart’s wilt.
Northern Corn Leaf Blight Identification and Habitat
An outbreak typically starts from the bottom of the plant and then works its way to the upper foliage. However, during severe outbreaks the infection can start on the upper foliage as well. Beginning symptoms will emerge on leaves as thin and long lesions that are tan in color and will correspond with the leaf margins. As time progresses, these lesions will form into more defined shapes, often referred to as ‘cigar-like’ with its oblong appearance. In this stage the blemishes may be tan or gray in color. These spots will produce black fungal spores when levels of high humidity are experienced. The fungal spores can actually be seen with the help of a hand lens. The lesions vary in size, sometimes as short as 1 inch in length, or up to 7 inches. Leaves may have more than one lesion, depending on the severity of the infection. When more than one lesion forms on a leaf, they sometimes merge together. Northern corn leaf blight can also occasionally lead to lodging and stalk rot. There are corn hybrids available that have some resistance to this fungus. In this case, lesions are smaller and not as common; the lesions may appear yellow in color and are unlikely to produce spores.
Northern corn leaf blight prefers weather conditions such as light rain showers that occur regularly, heavy dews, humidity, and modest temperatures. This means that spring and early summer often provide favorable conditions for the fungus to grow. It also means that this disease is more common in tropical highlands and in climates that are known to be cooler. Additionally, spread is more likely to occur in fields that neighbor woodlands or in bottomlands as these areas will remain moist after heavy dew for longer, which promotes infection. Severe damage is caused when corn plants are infected at an early age as this gives the disease time to reach the upper leaves before the tasseling phase of plant development takes place.
Northern Corn Leaf Blight Management and Control Methods
When managing northern corn leaf blight, choose corn hybrids that are somewhat resistant to this fungus. While lesions may still form, they will not be near as severe and will experience a much lower yield loss than non-resistant types. No-till or reduced till fields are at much higher risk for disease development due to the amount of crop residue that remains in the field, which serves as an overwintering site for the fungus. Therefore, using tillage and rotating corn out of the field for at least one year can greatly reduce the chance of disease presence. If a field had been severely infected previously, then a two-year rotation may be necessary. Overall, the weather is the determining factor for how bad the infection is. It is important to note that having satisfactory control is often dependent on using an integrated management system that includes the use of both cultural and chemical methods.
Fungicides are available to help manage northern corn leaf blight. These fungicides will help to minimize yield loss; however, keep in mind other economic factors when determining how valuable this management method would be. The most important area to prevent infection from reaching is the ear leaves and above, which is another factor to consider when deciding whether or not a fungicide is necessary. In this case, scout corn fields right before tassel emergence. If infection is already bad, this is the period in which the fungus can be controlled most effectively. Fungicides with the following active ingredients have proven to be effective for the management of northern corn leaf blight: picoxystrobin (Aproach®), tetraconazole (Domark®), pyraclostrobin (Headline® AMP), azoxystrobin (Quadris®), and propiconazole (Tilt®). Always be sure to carefully read the label for cautions and proper application before use.