A look at the most important ear rots in maize production
BRADLEY FLETT, ARC-Grain Crops Institute, Potchefstroom
The major maize ear rot diseases occurring in South Africa are Diplodia, Fusarium and Gibberella. The diseases have been identified as recurring problems throughout maize producing areas. Maize ear rots result in grain quality reduction, yield losses, livestock and potential human toxicity problems.
The fungi causing these maize ear rots can also cause maize stalk rots which result in lodging of plants. It is important to remember that environmental conditions favourable for ear rots do not correspond to conditions causing stalk rots, even though the same fungi are involved.
Diplodia, Fusarium and Gibberella must be seen as individual diseases as climatic and/or environmental conditions for the development of each of these diseases varies. Therefore, these diseases will be discussed separately.
Diplodia ear rot
Diplodia ear rot symptoms associated with infections during early ear development are yellowing and drying of husk leaves while stalks and leaves remain green (Photo 1a and Photo 1b). Infection generally begins at the ear base and ramifies upwards. The entire ear becomes overgrown with a white mycelial growth (Photo 2).
A cross section of an infected ear shows black spore-producing bodies at the kernel bases (Photo 3). Late season infections may occur when kernel moisture is low and symptoms are less obvious. Embryos become infected and slightly discoloured but no ramification of the rest of the ear occurs. Such symptomless infections are locally referred to as
Yield losses caused by Diplodia ear rot have not yet been quantified because harvest method, make of harvester, harvest speed and harvester settings all affect the percentage rotten kernels that remain in the grain bin or trailer.
Rotten kernels that are light are blown out during the harvesting process, the percentage depending on abovementioned factors. This implies that where Diplodia ear rot infections are serious, damage is twofold.
Firstly, if a low percentage of rotten kernels are discarded during the harvesting process, grain quality reductions are noted. Secondly, where a high percentage of rotten kernels are discarded in the harvest process this will improve grain quality, but manifest itself as yield loss.
Diplodiosis, a nervous disorder of cattle and sheep, results from the ingestion of ears infected by S. maydis (Diplodia ear rot). Cases of diplodiosis occur from six days to two weeks after the animals are placed on fields with infected maize ears. The disease is characterised by reluctance of the animals to move, a wide-based stance, incoordination,
tremors, paralysis and death.
Myelin degeneration (status spongiosis) is the major histopathological change observed in affected animals. The disease also causes abnormal foetal development and foetal death. Field outbreaks of diplodiosis in southern Africa are favoured by late heavy rains and occur during the late winter months (July to September).
The practice of using harvested maize fields for winter grazing is a major contributing factor to outbreaks of this mycotoxicosis. Mycotoxins are secondary metabolites produced by certain fungi and are toxigenic to animals or humans. In addition to diplodiatoxin, new metabolites, namely dipmatol, diplonine and chaetoglobosins K and L, have been isolated recently from S. maydis infected crops.
To date, none of the pure metabolites has been administered to ruminants in order to reproduce the disease. Until such time that the toxic metabolite(s) responsible for this disease are fully understood, diplodiosis will continue to be reproduced experi mentally only by feeding naturally infected maize or pure cultures of the fungus or their extracts to ruminants.
Not all fungus isolates are toxic. For example, two isolates from the same field may be fed to ducklings with one being toxic and the other not. This makes decisions difficult on whether or not to use Diplodia-infected grain for feed. Milling infected grain is thought to reduce the heat-sensitive toxin and ensure that total Diplodiainfected grain content of feed rations is lower than 10%.
Animals, particularly cattle, being fed Diplodia-infected rations must be monitored. As soon as symptoms of reluctance to move, standing with wide-based stance, poor co-ordination, walking stiff legged with a high stepping gait, falling, paralysis, constipation, salivation and tremors are observed, animals must be removed immediately and fed on healthy rations. Recovery rates are high, but animals will die if kept on the infected ration.
Life cycle and epidemiology
S. maydis spores are transmitted by air, seed and soil. Airborne spores result in heavy infections up to 10 m from the inoculum source and the number of successful infections is reduced with distance from the source.
Single spores travelling long distances may lead to trace infections which may then develop into an epidemic focal point. Infected maize seed is an important inoculum source which may result in seedling and crown rot diseases. However, the majority of Diplodia infected kernels do not germinate.
Spores land behind leaf or ear sheaths where they germinate and infect stalks or ears. Spore germination is inhibited by exposure to sunlight and desiccation. Free water is necessary for germination. Germination may take up to seven days after spore germination. Germinated spores may enter tissue and lie dormant until conditions are favourable for fungal growth through the tissues.
Mycelial colonization is accompanied by cell wall degradation ahead of the growing pathogen due to enzymes that are secreted by the fungus. Fungal ramification of maize ears generally begins at the shank. Cob tissue colonisation begins at the attachment with embryonic tissues and proceeds into the endosperm. Similarly stalk ramification may also occur. These infected tissues develop fruiting bodies (pycnidia), which produce spores during the subsequent season.
Control measures include reduction of infested surface stubble by means of grazing, burning, baling or ploughing in of surface maize stubble. As the fungus (Stenocarpella maydis) survives on maize stubble (Photo 4) and survives poorly in soil, any management practice that reduces levels of infected surface stubble will reduce inoculum concentrations in the field.
The removal of stubble for a single season and then resorting back to stubble retention practices only reduces Diplodia ear rot for that specific season. Where stubble is present the following season, the risk of Diplodia ear rot may increase to its original level, should weather conditions be favourable.
Crop rotations reduce Diplodia ear rots by reducing inoculum levels in two ways. Firstly, a non-host for the fungus will not allow the fungus to persist for the season where maize is not grown. Secondly, a greater period (a season or two) between maize crops allows for natural breakdown of maize stubble, which again reduces survival of the fungus. Leguminous crops such as soybeans, dry beans, groundnuts and cowpeas are very good rotation crops.
Other rotation crops that reduce diplodia ear rots are wheat and oats. Sunflowers do not significantly reduce diplodia ear rots under experimental conditions, but the reason has not yet been determined.
Early harvesting will reduce diplodia ear rots as it reduces time available for the fungus to grow on the ear. The fungus (Stenocarpella maydis) can grow on maize ears in the field until 11% moisture. Late or winter rains keep ears wet and increase the chance for fungal growth. In certain cases it would pay to harvest early at higher moisture levels and artificially dry grain. This is one reason why Diplodia ear rot may not be considered the major ear rot problem in the USA where maize is harvested early and dried artificially.
Selection of cultivars is very important in control of Diplodia ear rots. However, it appears that there is widespread confusion regarding resistance and use of resistance. There are no hybrids on the market that do not get Diplodia ear rot at all; however, some get more than others under specific climatic conditions. This reaction is affected by different climatic conditions which is important to consider when selecting resistant hybrids.
Fusarium ear rots
Fusarium ear or kernel rot, is caused primarily by the fungus Fusarium verticillioides as well as F. proliferatum and F. subglutinans. F. verticillioides also causes stalk and root rot, as well as seedling blight of maize. Two major symptom types of this ear rot can be noted in the field.
The first are symptoms observed in association with maize stalk borer feeding channels (Photo 5a and Photo 5b). F. verticillioides, in particular, is generally associated with insect or bird damage on maize ears. The fungus appears as pink/white mycelial growth on damaged kernels. The second symptom type is evident as pink or streaked kernel discolouration not related to kernel damage (Photo 6). F. verticillioides may infect kernels without showing any visible symptoms. It has been known for clean (first grade) grain samples to have symptomless infections of up to 90%.
Fusarium ear rot can result in yield and grain quality reductions. Infections associated with ear damage are often localised on ears and infected grain is harvested during the harvesting process. The symptomless nature of certain infections by these fungi results in infected grain passing unnoticed. The major economic implication of Fusarium ear rot is the ability of these fungi to form mycotoxins in infected maize. The most important being fumonisins which are toxic to chickens, pigs and horses.
Horses are extremely sensitive to fumonisin and a level above 5 parts per million (ppm) in their feed will result in a fatal disease called Leukoencephalomalacia. Guidance levels for fumonisins in pig and chicken feeds are set at maximum allowable (safe) levels of 10 ppm and 50 ppm, respectively.
Research has also implicated this mycotoxin in causing human oesophageal cancer, which is common in certain regions of Africa, Europe, China and the USA.
Life cycle and epidemiology
F. verticillioides is a ubiquitous fungus which is widespread throughout the South African maize production area. The fungus survives on crop stubble in or on the soil surface. The ability to infect endophytically and symptomlessly, gives this fungus a further survival advantage over other fungi. This fungus may overwinter in seed in the pedicel, endosperm and/or embryo.
F. verticillioides is transmitted by seed, air and insects. Seed infection levels of up to 100% have been recorded. The fungus has been shown to grow systemically from the roots to the stalk, and into the ear. Airborne spores are carried great distances by wind currents and the small size of the spores enables them to be spread widely. Insect transmission is primarily due to the stalkborers, Chilo partellus and Busseola fusca. Stalkborers feed on infected tissue, move to new plants or plant parts and continue feeding, while leaving the fungal spores in their frass.
Under favourable environmental conditions, the fungal spores germinate on the plant surface and infect maize stalks or ears directly, or through wounds caused by hail, insects or birds. F. verticillioides is favoured by dry, hot climatic conditions such as those prevailing primarily in the North-Western parts of the South African maize-producing areas. Climatic conditions play a major role in the severity of both Fusarium ear rot as well as mycotoxin production.
Due to the common occurrence of these fungi in nature, the use of sanitation practices have not been very successful in disease reduction. With the use of climatic modelling, Fusarium ear rot and fumonisin predictions can be made for areas with favourable climatic conditions.
These models still require confirmation and further research before being used. Recent research results indicate that timely control of stalkborers using insecticides or Bt maize helps to reduce F. verticillioides ear rot infections and fumonisins. The literature indicates that hybrids vary in their susceptibility to F. verticillioides infections and fumonisin production.
Local hybrids therefore need to be screened before hybrid selection will play a role in controlling Fusarium ear rot and fumonisin contamination levels. Research on hybrid screening and breeding of resistance to fumonisins is on-going at the ARC-GCI.
Graminearum ear rot
Graminearum ear rot, also known as Gibberella or red ear rot, is caused by the Fusarium graminearum species complex which also causes stalk rot, root rot and seedling blight of maize. In South Africa the primary pathogen causing ear rot in the Fusarium graminearum species complex is F. boothii.
Disease symptoms are dark red discolouration of the whole or part of the maize ear (Photo 7 and Photo 8). Early infections result in complete ear rotting, with husks adhering tightly to the ear. Graminearum ear rot usually progresses from the tip of the ear downward.
Graminearum ear rot is increasing in economic importance in the South African maize production areas and may become a major threat to the maize industry. Previously reports from certain areas where cool, wet, late season conditions were experienced, imply sporadic and localised outbreaks of this ear rot disease.
Recent studies have indicated an increased spread and severity of this disease as well as the stalk rots caused by the Fusarium graminearum species complex. In certain cases severe yield and quality reductions were observed. The major concern is toxicity, associated with this disease. F. graminearum is known to produce a number of important mycotoxins, which cause major problems, especially for pig producers.
Recently, high levels of these mycotoxins in dog food have resulted in feed refusal and mycotoxicoses in dogs. These mycotoxins are zearalenone (ZEA), deoxynivalenol (DON) and nivalenol (NIV). Toxigenic symptoms in pigs, range from hormone induced syndrome caused by ZEA, which reduces the reproductive performance of the animals, to feed refusal due to high levels of DON and/or NIV.
Cattle appear to be much more resistant than pigs to the hormonal effects of ZEA, whereas chickens do not seem to be affected. The most conspicuous changes in pigs due to ZEA are enlargement of the uterus and mammary glands and atrophy of the ovaries.
Feed refusal is a result of the unpalatability of the feed and may be reflected in decreased weight gains and slower growth rates. Vomiting may occur in animals that consume small quantities of infected grain. Maize containing more than 5% infected kernels should not be included in rations for pigs, although it may be diluted with sufficient quantities of first grade maize.
Life cycle and epidemiology
The species within the Fusarium graminearum species complex survive primarily on the surface of maize stubble throughout winter. Species within the Fusarium graminearum species complex have also been reported to survive on stubble or organic matter of other crops previously planted as saprophytes. Survival structures (perithecia) may develop and mature on maize stalks under warm wet conditions. Ascospores are exuded from the perithecia and are taken up into air currents where they can be transported long distances, from where these spores are then deposited on, and infect other maize plants or other susceptible crops.
The species within the Fusarium graminearum species complex also infects various other tissues and cereals such as wheat and barley, which may help the pathogen to overwinter, causing even larger disease outbreaks the following season. F. boothii infects maize seed and infection levels of up to 66% have been reported.
Seed to seed transmission, however, has not been clearly shown. These spores infect the maize silks and grow down into the point of the ear. The pathogen has also been reported to be transmitted by birds and insects.
Graminearum ear rot severity is favoured by cool, wet weather within three weeks of silking. This disease is common under irrigation conditions in South Africa. Regions affected by sporadic outbreaks of this disease are generally KwaZulu-Natal and Mpumalanga and irrigation fields in the Limpopo and North West Province.
Rotation of maize with non-graminaceous crops decreases the incidence of Graminearum ear rot. It is however important to note that certain species within the Fusarium graminearum species complex are able to utilise organic matter and other crop residues to overwinter as saprophytes. More research on the efficacy and ability of these species to survive on alternate sources of organic matter need to be researched intensively to determine the potential of crop rotation on reducing inoculum levels.
As the fungus overwinters on maize stubble retained on the soil surface, the removal of maize residues will reduce disease incidence the following crop season. The ability for species within the Fusarium graminearum species complex to survive on other sources of organic matter and stubble of other crop species will also influence the role that removal of maize stubble will play in reducing inoculum sources of this disease.
Reports in the literature indicate that hybrids vary in susceptibility to the disease. Although local hybrids are presently in the process of being screened for resistance to this disease no results are yet available. Field observations have resulted in the identification of highly susceptible hybrids.
Publication: February 2016
Section: On farm level