Two diseases are summarized under the name Corn Leaf Blight. In the moderate and cool climate we will find the Northern Corn Leaf Blight caused by the disease Helminthosporium turcicum. Whereas it warmer climate we might find the Southern Corn Leaf Blight caused by the pathogen Bipolaris maydis (Nisikado & Miyake) Shoemaker, teleomorph Cochliobolus heterostrophus (Drechs.) Drechs. This disease became famous when it has led to very high losses in the year 1970 in which a very warm summer climate combined with a very successful parent genome widespread in the corn grown in the US at this time.

Southern corn Leaf Blight (Bipolaris maydis)

• SCLB: Biology and Disease Cycle
• SCLB: Infection Model

Southern Corn Leaf Blight is a disease caused by the pathogenic fungus Bipolaris maydis. This was a minor disease of corn for many years, with no economic effect on yield. In 1970, a highly virulent strain called Race T appeared on corn hybrids with Texas male-sterile cytoplasm. Losses were severe.

Symptoms

Symptoms of Race lesions are spindle-shaped with yellow-green halos. They will later have dark, reddish-brown borders and occur on leaves, stalks, leaf sheaths, husks, and shanks. Cob Rot ear occurs as well as ear drop. The fungus overwinters in corn debris and on the seed. Wind and splashing water spread the spores rapidly in the field under ideal conditions, cycling in about 72 hours. Control with resistant hybrids.

Biology and Disease Cycle

Southern corn leaf blight (SCLB) is favoured by warm temperatures (68-90°F or 20-32°C) and high humidities. Thus, it tends to be a problem in the Southern half of Illinois, although it can be found farther north if weather conditions are favourable. Frequent rainy periods enhance disease development.

The symptoms of SCLB are leaf lesions ranging from minute specks to spots of one-half inch wide and one and one-half inches in length. They are oblong, parallel-sided, and tan to greyish in colour. A purplish to brown border may appear on the lesions depending on the genetic background of the plant. Early and severe infections in susceptible plants predisposes them to stalk rots.

The fungus overwinters in corn debris as spores or mycelium. Spores are spread by wind or splashing water to growing plants. After infection and colonization, sporulation from these primary lesions serves as the source for secondary spread and infections as long as weather conditions are favorable for disease development and living tissues are present. The disease cycle may repeat every few days under ideal conditions. Germination of spores and penetration into the plant can occur within six hours when free water is present on the leaf surface and temperatures are between 15 and 28°C. Control of SCLB is easily accomplished with resistant hybrids. Although some slight flecking may be found in some hybrids, this is simply a part of the resistant reaction and does not lead to any economic loss.

Burial of crop residue is helpful where erosion is not a problem. Crop rotation is especially suggested where no-till is used or where heavy crop residues are found. Since this fungus overwinters on debris, the planting of corn into such residues may result in earlier infection and poor seedling performance. Foliar fungicides are useful in seed production fields. For optimal control, it is important to control foliar disease during the period from 14 days before tasselling to 21 days after tasselling. Research has shown that this four-week period is the most critical for leaf blight damage and that yields and quality are most affected if susceptible inbred are not protected at this time.

Infection Model

This disease needs free water (leaf wetness) to infect plant tissue. Therefore conditions for starting an infection are: leaf wetness and temperatures warmer than 15°C. The infection will be finished after we can accumulate 100 degree hours (temperature and leaf wetness hours).

START SCLBinfection: if leaf wetness > 0 and air temperature >= 15 then begin

INCREASE of INFECTION: SCLBinfection actual hour =SCLBinfection former hour + (air temperature * leaf wetness period in minutes)

STOP: if conditions are not given (no leaf wetness, temperature below 15°C), or if SCLBinfection > 100*60 °C (= degreeminutes) then SCLBinfection = 100*60 (in sum 6000 degree minutes or 100 degreehours (wich is for example about 6hours leaf wetness at 15°C or 5hours leaf wetness at 18°C), of leaf wetness at a temperature higher than 15°C is the maximum).
(If Temperature is higher than 15°C and leaf wetness for more than 100*60°C accumulated Temperature per minute infection is complete)

These models have been designed following the Report on Plant Diseases 202 – Common Leaf Blights and Spots of Corn by Mohammad Babadoost.

Extension Specialist in Fruit and Vegetable Diseases, Department of Crop Sciences, the University of Illinois at Urbana-Champaign.

Northern Corn Leaf Blight (Exserohilum turcicum)

Northern corn leaf blight (NCLB), caused by the fungus Exserohilum turcicum previously called Helmithosporium turcicum, can cause yield losses in humid areas where corn is grown. In moderate humid climate the disease can occur everywhere, but usually does not appear in fields before silking. This disease rarely causes significant yield losses during dry weather, but during wet weather periods it may result in losses of over 30%, especially when established on the upper leaves at the silking stage of maize development. If leaf damage is only moderate or is delayed to about 6 weeks after silking, yield losses are mostly minimal. Northern corn leaf blight also predisposes corn to stalk rot by increasing stress on the plants.

Infection model

Exserohilum turcicum is able to infect the leaves of the corn plant when temperature and leaf wetness conditions are given. The time span to infect plant tissue is depending on the actual air temperature and leaf wetness period. The infection can take place, if temperature is ranging from 13°C to 28°C. At a temperature of 18°C it will need 18 hours for infection. If the temperature is lower it will need even longer time and if it is warmer it will need a shorter leaf wetness period.

START of the calculation of a NCLBinfection: If (leaf wetness > 0) and (air temperature >= 13) and (air temperature <= 28) INCREASE: NCLBinfection actual:=NCLBinfection of the former hour +((air temperature-13)*leaf wetness duration in minutes). STOP: no leaf wetness, temperature not in the optimal range or if NCLBinfection > (18-13)*18*60 then NCLBinfection = (18-13)*18*60. That means that in sum 5400 degreeminutes (= 90degreehours) could be reached then an infection is fulfilled (= (18°C- 13°C)* 18hours leaf wetness in minutes (18*60). At 18°C it needs 18hours of leaf wetness for an infection. If the temperature is higher or leaf wetness periods are longer it is still 5400 (all values above are 100% infection).

These models have been designed following the Report on Plant Diseases 202 – Common Leaf Blights and Spots of Corn by Mohammad Babadoost.

M. Babadoost is an Extension Specialist in Fruit and Vegetable Diseases, Department of Crop Sciences, the University of Illinois at Urbana-Champaign.