Side-step Northern Corn Leaf Blight

AK Geldenhuys, Product Agronomist, Western Production Region

Northern Corn Leaf Blight can lead to significant crop losses, especially in the wetter seasons, but by being proactive it can be prevented to a large extent.

What is Northern Corn Leaf Blight (NCLB)?
NCLB is a fungal disease of maize and sorghum caused by the fungus Exserohilum turcicum. It is one of the most common leaf diseases and is prevalent in most of the maize-producing areas in South Africa. The disease is more common under irrigation as it creates the perfect environment for the fungus to thrive in (hot, wet and humid conditions).

Under dryland cultivation it is especially prevalent in the Midlands of KwaZulu-Natal, the Lowveld and in our northern neighbouring countries, where the disease occurs almost on an annual basis.

Under dryland production conditions in the Free State and North West, the disease generally only occurs in the good years with above-average rainfall, when humid conditions with long dew periods occur throughout the season. Consecutive good years increase the inoculum pressure, which makes the disease more common and can then create the perception that hybrids have become more susceptible.

Symptoms

Typical symptoms of NCLB occur on the leaves and are long elliptical grey-green or brown lesions that can be about 2.5 - 15 cm in length (Fig.1). Infection usually begins on the lower leaves, and if conditions remain favourable, it moves rapidly upwards through the canopy.

Within the lesions grey-black spores form that serve as inoculum for the following infections.

Fig.1

To determine if the fungal growth is still active, an infected leaf can be placed in the sun on the dashboard inside a vehicle for about 10 minutes. If a hazy water-soaked margin forms around the lesion, the fungus is still active.

Should conditions remain favourable long enough, the disease can lead to large-scale leaf loss and associated yield reduction. Under severe infections lodging may occur which adversely affects the harvesting process and may result in further yield losses (Fig.2).

Fig.2

Survival and Distribution
The fungus survives as fungal thread-like hyphae (mycelium) and spores (conidia) on crop residues from the previous season. These spores and fungal threads will then multiply in the early summer with favourable conditions and spread by the wind to the lower leaves of the new season's maize plants. The leaves become infected and the lesions form. These new lesions produce spores. If these new infections are not controlled in time it can rapidly lead to epidemic spread of the disease.

NCLB-infection is favoured by moderate temperatures of 18 - 27 °C and long dew periods (leaf wetness). The fungus does not need a wound to infect the plant but can penetrate the leaf surface directly and infects the plant in this way.

Under moderate to high levels of infection, particularly if infection occurs pre-tassel, expected crop losses can easily range from 30% - 50%. If the infection is severe and left uncontrolled, losses can be as high as 70%. This is undoubtedly a disease that should not be underestimated.

Control Measures
Hybrids with natural resistance to NCLB remain the most effective method of control. Since fungi have the ability to mutate, it is possible that hybrids that previously had resistance can be infected by a new mutation or strain of the pathogen. Should such a mutation occur, it can happen that resistance lesions and susceptible lesions occur on the same plant. Fig.3 shows the susceptible lesion on the leaf at the top and the resistant lesion at the bottom. This resistance is the result of natural selection and conventional plant breeding practices and is not a result of genetic engineering (GMO).

Fig.3

Other control measures include a deep ploughing operation whereby the inoculum that survives on plant stubble is buried deep below the soil surface. However, this method is no longer popular as many farmers are set on minimum tillage to take advantage of the associated benefits of moisture conservation, reduced wind erosion and reduced crop damage from sand blasting.

Crop rotation to non-host broad-leaf crops such as soybeans, dry beans or sunflowers is very effective at reducing inoculum levels. Chemical control is also very widely used on a large scale. Various fungicides have been registered for the control of NCLB and work very effectively if fungicides are applied correctly. In severe infestations or under highly conducive environmental conditions, more than one spray may be required.

Through an integrated management program of using resistant hybrids, a crop rotation program and a good fungicide spray program, NCLB can be prevented or controlled to a large extent.

Hybrid choice
At PANNAR, trials are conducted annually to evaluate hybrids for susceptibility to NCLB. This information is indicated in the Pannar brochure to enable sensible hybrid selection. This is especially meaningful in seasons where weather forecasts indicate the probability of a wetter year and thus a greater chance of NCLB infection, or in a maize-on-maize scenario where the previous seasons crop had elevated levels of NCLB infection.

NCLB is a serious disease and differences in hybrid sensitivity are significant. It is not necessarily the hybrid that has always given the highest yield that will perform best, even under favourable climatic conditions. Below are photos of hybrids taken at the same location where NCLB occurred. The differences are very clear and pointed out in the varying yields obtained at harvest in the photos below.

Fig.4 (Hybrid A – 4.7 t/ha)

Fig.5 (Hybrid B – 6.6 t/ha)

Fig.6 (Hybrid C – 8.3 t/ha)

Fig.7 (Hybrid D – 10.4 t/ha)

The photos show the huge impact that a NCLB-infection can have on the plant’s leaf area and the associated reduction in yields. It may be helpful to heed weather forecasts and make decisions ahead of time with your hybrid selection and fungicide spray program to help mitigate potential loss.