BELINDA JANSE VAN RENSBURG AND BRADLEY FLETT, ARC-GRAIN CROPS INSTITUTE
During the 2011/2012 season some of the most commonly reported stalk rots are Diplodia, Gibberella and Charcoal stalk rot. We will discuss the economic importance, symptoms, epidemiology and control of these stalk rots.
Diplodia stalk rot
Diplodia stalk rot is caused by the fungus Stenocarpella maydis with the primary host being maize and sweet maize.
This fungus is common in all maize producing areas and in seasons with early rains and persisting late season droughts, this disease becomes very damaging, resulting in lodging and poor grain fill.
Diplodia stalk rot reduces yield by reducing nutrient and moisture uptake to ears during grain fill. This sink (the ear) extracts sugars from the stalk which further predispose the stalk to fungal growth and further reduce nutrient uptake.
This continual sink source cycle reduces yield. The onset of windy conditions whilst plants are drying, results in lodging (Photo 1) and further economic losses as ears have to be picked by hand. Estimated annual yield losses of 5% - 20% may occur due to Diplodia stalk rot and lodging.
Stalk rot symptoms appear several weeks after silking. Leaves of infected plants wilt, become dry and appear greyish-green. Lower internodes become brown and spongy. Small, black fruiting bodies (pycnidia) cluster near the nodes of the rind. The rind may also be covered by a white mycelia growth. The stalk pith discolours and disintegrates (Photo 2) with vascular bundles remaining intact. This weakening of the stalk predispose plants to lodging during strong winds and rain prior to harvest.
The fungus overwinters in a mycelial form in maize stubble (buried or on the soil surface) throughout the winter. Under warm, moist conditions, pycnidia develop which releases spores that are spread by wind and rain. Infection of plants occurs mainly through the crown and roots and occasionally at the nodes between the crown and ear. Infection usually takes place two to three weeks after silking, under favourable conditions. Dry early season conditions followed by rain during silk formation, favours Diplodia ear rot, while a wet early season followed by drier conditions or heat stress is likely to result in more severe Diplodia stem rot.
Crop rotation reduces Diplodia stalk and ear rot. Soybeans are a good alternative crop in a rotation programme, followed by groundnuts, wheat and dry beans. Crop rotation reduces inoculum levels in two ways. Firstly, maize is the only reported host crop of S. maydis which results in a break in the epidemiological cycle of the fungus. Secondly, maize stubble which has been on the soil surface for 24 months, loses its integrity due to saprophytic breakdown resulting in an inability to support pycnidia and mycelial survival.
Stubble removal versus conservation tillage practices
Removal of infected surface maize stubble by ploughing, grazing or in extreme cases burning, reduces the inoculum load in a field. Caution should be exercised when using conservation tillage practices. Crop residues left on the soil surface favour the survival of the pathogen from one season to the next, leading to a build-up of primary inoculum. However, hybrids with some resistance to Diplodia stalk rot can be used in conjunction with these conservation tillage practices.
High N:K (high N; low K) ratios increase the severity of stalk rot whereas a low ratio leads to a reduction in the disease. Stalk rot is reduced with increasing K at low N levels. K does not affect stalk rot at moderate N levels, but increases with K when N is high. Furthermore, when K is low, N levels do not affect disease, but moderate to high K and increased N, enhances stalk rot.
Diplodia stalk rots are stress related diseases and drought stress can be reduced by adjusting planting dates to avoid stress periods at grain-fill. Reduced plant populations also reduce stress levels.
There are no fungicides in South Africa registered specifically against this disease and the economics of spraying for this disease specifically, is questionable. It has, however, been found that a good full protection spray programme for leaf diseases, improves standability since plant sugars from the stalks are not diverted for grain filling.
Gibberella stalk rot
Gibberella stalk rot is caused by the fungus Gibberella zeae (Fusarium graminearum) with the primary hosts other than maize, being oats, barley, rye, sorghum and wheat.
Gibberella stalk rot is a common problem in maize production areas worldwide and also in South Africa. Gibberella stalk rot can cause extensive economic losses due to premature plant death and interference with translocation of water and nutrients during grain fill, resulting in lodging of plants due to weakened stems (Photo 3).
Yield loss depends on a number of factors including genotype, prevailing climatic conditions, fertilisation rates, crop density and cultural practices. Although it is difficult to estimate precise yield loss due to Gibberella stalk rot, during favourable environmental conditions, extensive damage (lodging) is known to occur. Lodging complicates mechanical operations, necessitating the picking up of plants and hand harvesting, which increase time, labour and financial constraints.
Symptoms of Gibberella stalk rot are similar to those of other stalk rots, but it is the pink/red discolouration that is diagnostic (Photo 4). Affected plants wilt, the leaves change from light to dull green and the lower stalks become straw coloured. The internal pith disintegrates, leaving only the vascular bundles intact. The disintegration of stem tissue causes stem lodging and rotting of the root system which leads to root lodging. Small, round, black fruiting bodies (perithecia) may be produced superficially on the stalks, often at the internode.
Under warm, wet conditions, the perithecia produce ascospores that are disseminated by wind and may infect maize plants. Inoculum may also be produced as conidia during the summer. Stalk infections usually occur shortly after pollination, developing at the origin of the leaf sheaths or around the brace roots. The fungus may also enter through the roots and grow up into the lower stem.
Crop rotation with legume crops or sunflowers will allow stubble to break down, without providing a host on which to survive, thereby reducing inoculum.
Hybrids with resistance to other stalk rots, such as Diplodia, are also resistant to Gibberella stalk rot. Although a lot of efforts have been made to select hybrids with stalk rot resistance, the main consideration is still yield. Genotypes with higher yields tend to have bigger ears, which act as large sinks for carbohydrates in the plants. The larger ears result in reduced carbohydrate levels in the lower stem, predisposing the plant to stalk rot. Therefore the balance between breeding for resistance to stalk rot and breeding for high yield is delicate.
Nutrients and stress reduction
Cultural practices that reduce plant stress also reduce incidence of stalk rot. Common stress conditions include: high nitrogen, low potassium fertility, high moisture in the mid to late season after a dry season, moisture stress early in the season and during grain fill as well as high incidence of leaf diseases. Physical damage that creates wounds (insect, hail) allowing the pathogen to enter the maize plant may also predispose the plant to stalk rot.
There are no fungicides available for the control of Gibberella stalk rot. However, fungicide applications for the control of leaf diseases may be beneficial in reducing stress on the plant, thus lessening stalk rot severity and ultimately lodging.
Charcoal stalk rot
Charcoal stalk rot is caused by the fungus Macrophomina phaseolina with a broad host range including soybean, sunflower, common bean, sorghum, maize, cotton, tobacco and a range of vegetable crops.
Charcoal rot is a common stalk rot disease in warm, dry areas. Yield losses as high as 70% have been documented in Africa. The disease is particularly prevalent in drought years and in arid regions where maize is regularly cultivated with other host crops. This disease is heat and stress (drought) driven and is therefore rare in cooler climates and irrigated fields. High incidence of this disease was recorded in the 2007/2008 season in the Free State and North West Province.
In maize, early symptoms are similar to those of Gibberella and Diplodia stem rot. After flowering, initial symptoms are the abnormal drying of the upper leaf tissue, stem lodging and premature death. When plants approach maturity, the lower stem internodes (usually limited to the first five nodes) show a typical charcoal, grey-black discolouration (Photo 5) which often gives the whole land a black appearance.
When the stem is cut open, numerous black specks are visible in the shredded vascular bundles and on the inside of the stem, giving the interior parts of the stem a charred appearance. Brown, water-soaked lesions which later turn black, are present on the roots. The fungus may also infect kernels, blackening them completely.
Charcoal stem rot is favoured by soil temperatures of 30°C - 42°C and low soil moisture. This fungus overwinters as sclerotia and may penetrate roots and lower stems during the growing season. The main risk of dispersal is by the movement of soil (contaminated with microsclerotia) by tractors, ploughs and other farm machinery.
Maximum infection occurs in plants subjected to moisture stress during the post-flowering period. Post-flowering stresses due to high plant populations or drought coupled with heavy applications of nitrogen fertiliser, hail or insect damage, promote disease development. The broad range of host plants is a major source of inoculum causing infection in the following seasons.
Rotation with a poor host such as cotton and small grains (wheat and barley), will reduce inoculum levels in the soil.
Hybrids with good resistance/tolerance to Diplodia and Gibberella stalk rot generally have good resistance to Charcoal rot.
Stubble debris serve as inoculum for stalk rots and removal of infected stalks will reduce inoculum levels.
Nutrients and stress reduction
Avoidance of stress factors, such as drought and unbalanced nutrition, may help reduce damage. High plant populations should be avoided as this can contribute to increased plant stress through competition for water, especially during a dry season.
Good water management is therefore important in managing this disease, especially when plants reach the flowering stage. High nitrogen levels may also increase charcoal stem rot severity. Adequate levels of P and K reduces nutrient stress and therefore severity of this disease.
Leaf diseases such as grey leaf spot, northern corn leaf blight and common rust, predispose plants to stem rots. Under high leaf disease pressure, the photosynthetic leaf area available for grain fill is lost and sugars are diverted from the stalks for grain fill. Stems are weakened, senesce prematurely, and susceptibility and subsequent colonisation by opportunistic stem rot organisms is increased. Thus, selecting hybrids with good leaf disease resistance and good leaf disease control will also reduce stress and therefore stem infections.
For further information regarding these stalk rots of maize or any other maize diseases, please contact Belinda Janse van Rensburg or Prof Bradley Flett at (018) 299-6100.