Drought and disease development in maize

Adverse environmental conditions and disease are placing increasing pressure on maize crops in South Africa, says Dr Belinda Janse van Rensburg, researcher at the ARC-Grain Crops Institute.

White Diplodia mycelial growth infecting the base of the ear with visible black spore-producing bodies. Photos courtesy of ARC-GCI

Maize production is highly dependent on climatic conditions, with temperature and precipitation being the main drivers. Hot, dry weather in the maize production areas of North West, Free State and Limpopo is believed to be caused by cyclones in the Indian Ocean, which absorb the moisture from the subcontinent’s interior. When these conditions prevail, a smaller maize crop will be harvested, which in turn will increase the price of white and yellow maize used as animal feed.

Drought and heat stress often occur simultaneously, exacerbating each another. Under drought stress, stomata in the leaves of maize plants close to reduce transpiration. This can have a negative effect on flowering, pollination and grain fill. A soil moisture deficit of just four days can reduce maize yield by up to 50%. The extent of damage caused to maize plants depends on the duration of the drought stress and the crop’s stage of development. An air temperature higher than 36°C reduces the viability of pollen and hence pollination rate, grain fill and yield. High air temperature and solar radiation also damage maize leaves, reducing the area of chlorophyll production needed for growth and grain fill.

Pests 

Drought also favours insects such as spider-mites and stalk-borers, which can act as vectors of pathogens. Some fungal pathogens (Fusarium spp.) can produce mycotoxins in maize grain. The following maize diseases, which thrive in heat and drought, are economically important.

Ear rot (Fusarium verticillioides)

Fusarium verticillioides is more common in regions with hot, dry growing conditions, especially before or during pollination. The fungus thrives in temperatures above 28°C and can produce mycotoxins called fumonisins. The consumption of maize contaminated with fumonisins can cause mycotoxicoses in animals and has been associated with human oesophageal cancer in South Africa, northern Italy and Iran.

F. verticillioides has a saprophytic (nourished by dead organic matter) as well as a pathogenic stage and may infect maize at all stages of plant development, either through the silk channel, infected seed or lesions. The latter may be caused mechanically or by stalk-borers and birds.

F. verticillioides can also be transmitted to uninfected plants by inoculum from stubble on the land or through airborne conidia (fungal spores), which are abundant in maize lands during the growing season. The most commonly reported method of kernel infection is airborne or water-splashed conidia that land on the silks. Symptoms include the growth of a white-pink cotton-type mould on kernels alongside stalk-borer channels. Cultivar responses to Fusarium infection are inconsistent and there are no fungicides currently registered for the control of Fusarium ear rot.

The only effective way of reducing infection and mycotoxin contamination lies in management strategies on the land. Vectors such as stalk-borers can be controlled by insecticides or Bt-transformed maize. By reducing insect damage on Bt maize stalks, secondary Fusarium infection on plant injuries can be decreased, thus lowering levels of fumonisin. Cultivation practices such as nitrogen fertilisation, timing of sowing and harvesting, insect control and plant density all influence fumonisin contamination in maize grain.

Diplodia ear rot (Stenocarpella maydis) 

Drought early in the season followed by late rain can lead to a diplodia epidemic, especially where a high level of inoculum is present in the stubble. This fungus produces spore-producing structures that can survive on maize stubble during winter and produce spores in spring. The spores will infect plants throughout the growing season.

After rain or during periods of high humidity, the structures release spores into the air that land on maize plants and infect the base of the ear/leaf junction. This spreads upwards into the ear, which becomes entirely overgrown with a white mycelial growth. Black spore-producing bodies at the kernel bases are clearly visible in a cross-section of an infected ear.

Late-season infection may occur when kernel moisture is low, but the symptoms are less obvious. Infection that shows few or no symptoms is locally referred to as skelm (cunning) diplodia. Diplodia ear rot can occur as an epidemic in certain areas. Infected grain is then harvested with the healthy grain, reducing grain quality. Lower grain quality has negative economic implications, as the supplier receives a lower price for the grain. During such an epidemic, when early infections are present, yield loss can be great.

There are no hybrids currently on the market that are immune to diplodia ear rot. But using fungicides to control the disease cannot be economically justified in commercial maize production. However, crop rotation with soya beans, groundnuts, wheat and dry beans can make a difference. Rotation will reduce diplodia-inoculum levels, in turn reducing diplodia ear rot infections. It is best to harvest earlier during a diplodia epidemic to prevent the maize plants rotting on the land. Reducing surface stubble retention reduces the initial inoculum source. However, this should be done carefully and combined with other control measures.

Charcoal stalk rot (Macrophomina phaseolina)

Locally, charcoal stalk rot reduces yield by up to 70%. It occurs at a soil temperature of 30°C to 42°C combined with a low soil moisture level. The disease is driven by heat and stress (drought) and is rare in cooler climates and on irrigated lands. Initial symptoms after flowering include abnormal drying of the upper leaf tissue, stem lodging and premature death. When plants approach maturity, the lower stem internodes (usually the first five) show a typical charcoal, grey-black discolouration, which often gives the entire land a charred appearance.

When the stem is cut open, numerous black specks are visible in the shredded vascular bundles and on the inside of the stem, making the interior parts appear black. Brown, water-soaked lesions that later turn black are present on the roots. The fungus may also infect kernels, blackening them completely. It overwinters as sclerotia (tough, dormant structures) and may penetrate roots and lower stems during the growing season. Soil disturbance during land preparation aggrevates the risk of infection.

Maximum infection occurs in plants subjected to moisture stress during the post-flowering period. Stress at this stage due to a high plant density or drought, combined with heavy nitrogen fertiliser application, hail or insect damage, also promotes disease development. There are a broad range of host plants, and these can be a major source of inoculum that causes infection in the following season.

Treatment

Rotating with a crop such as cotton or small grain (wheat and barley) will reduce the inoculum level in the soil. Stubble debris serves as inoculum for stalk rot, so infected stalks should be removed to reduce inoculum levels. Reducing the effects of stress factors such as drought and unbalanced nutrition can reduce damage. For example, a high plant density should be avoided, as this may contribute to increased plant stress through competition for water, especially during a dry season. Good water management is also crucial, especially at the flowering stage.

A high nitrogen level may also increase the severity of charcoal stem rot. Adequate phosphorus and potassium levels reduce nutrient stress and therefore the severity of the disease. Leaf diseases such as grey leaf spot, northern corn leaf blight and common rust predispose plants to stem rot. Under high leaf disease pressure, photosynthetic leaf area available for grain fill is lost and sugars are diverted from the stalks for grain fill.

Stems are weakened and senesce prematurely, increasing the plant’s susceptibility to colonisation by opportunistic stem rot organisms. For this reason, selecting hybrids with good leaf disease resistance, combined with good leaf disease control, will reduce stress and stem infection.

Improved food security

Adverse climatic conditions are again expected to affect maize production in SA in the coming year. Although the climate cannot be controlled, producers can apply sustainable management practices to reduce yield loss caused by disease. Plant pathologists at the ARC-Grain Crops Institue can assist producers with these disease challenges, thereby improving food security and safety for the end-user.