Maize (Zea mays) is a cereal of the family Poaceae (Graminae) and also one of the main staple foods of people living in many countries of the world (Komolafe et al., 1979). Maize originated from South America (Mexico) and it is grown in wide range of environmental conditions due to its adoptability. This constrains cause considerable yield reduction. Among different biotic factors, insect pests and diseases play a vital role in affecting the productivity.
Maize is currently the most important cereal grown in the world ahead of Rice and wheat (FAO, 2010) and is particularly important in Nigeria for human and livestock consumption. The value of this crop to man is however reduced by field and storage pest attack. Bacteria and fungi as well as other microbes are known to cause infections in the field. Fungi are particularly important in storage. They rank second as the cause of post-harvest deterioration and loss of maize (Ominski et al., 1994) and could cause about 50-80% of damage on farmers maize during the storage period under favourable conditions (Kossou and Aho, 1993).
SOIL-BORNE PLANT PATHOGENS
Soils contain diverse communities of microscopic organisms that are capable of damaging plants. A detrimental interaction between a soil organism and a plant is often highly specific (Smith et al., 2008). For example, a fungus that causes root-rot of wheat may have no effect on the roots of another plant growing in the same soil. Highly specialized interactions between soil organisms and plants can kill seedlings and even adult trees. Many organisms target younger plants but others appear as problems at later stages in the life of the plant. Other pathogens are able to cause disease in many different plant species. The soil organisms that have the potential to be plant pathogens include fungi, bacteria, viruses, nematodes and protozoa. Some pathogens of the above ground parts of plants (leaves, stems) survive in the soil at various stages in their life cycles (Carpenter-Boggs et al., 2000). Therefore, a soil phase of a plant pathogen may be important, even if the organism does not infect roots.
Nature of diseases caused by soil borne plant pathogens
Disease caused by Phytophthora cinnamomi
Phytophthora cinnamomi causes a serious disease that threatens forests and other ecosystems especially in south-eastern and south-western Australia (Shea et al. 1984). Hundreds of different plant species are killed by this introduced pathogen. Vehicles are often responsible for the widespread distribution of the pathogen by disturbing and transporting infected soil. There is no simple solution to Phytophthora disease in the forest. Quarantine methods have been introduced to limit the spread of the fungus and cleaning of vehicles is mandatory.
Take-all Disease of Wheat
Take-all disease is caused by the fungus Gaeumannomyces graminis var. tritici (Cook, 2003). This pathogen infects the vascular tissue of wheat roots and restricts the transport of water and nutrients within the plant. Severely infected plants have stunted root systems. In addition to root rot, a severe symptom is ‘white heads’ which occurs if plants survive seedling damage and grow to maturity. Such plants form seed heads with poor grain development that are characteristically white. The fungus survives in the soil on decaying plant material and relies on this material as a carbon source to sustain it until it is able to infect new roots in the following wheat crop or alternate hosts.
Crown gall occurs on many genera of plants and is characterised by the formation of root tumours caused by the bacterium Agrobacterium tumefaciens. The bacteria infect the root and induce plant cells to divide; a tumour-like swelling is formed that contains infected cells around its outer surface. Bacterial DNA is transferred to the host plant (Viss et al., 2003).
Root Knot Nematode Disease
Root knot nematodes cause disease on hundreds of plant species, especially horticultural species, in warmer climatic zones (Trudgill and Blok, 2001). Species of nematodes in the genus Meloidogone induce the formation of numerous galls throughout the root system. The damaged roots also have malfunctioning root tips which reduce root growth, resulting in considerable yield losses.
These diseases are caused by a diverse group of fungi and related organisms. The most important genera include Pythium and Phytophthora, Rhizoctonia, Cylindrocladium and Armillaria. These diseases are characterised by a decay of the true root system; some pathogens are generally confined to the juvenile roots while others are capable of attacking older parts of the root system. Symptoms that are observable include wilting, leaf death and leaf fall, death of branches and limbs and in severe cases death of the whole plant.
The word smut means a sooty or charcoal-like powder. The affected parts of the plant show a black or purplish-black dusty mass. These symptoms usually appear on floral organs, particularly the ovary but they can also be found on stems, leaves and roots.
The main species of fungi that cause these diseases are Fusarium oxysporum and Verticillium spp. The symptoms of these diseases include wilting of the foliage and internal necrosis of the vascular tissue in the stem of the plant. Some species of bacteria can also cause similar types of diseases.
Seedling blights and damping-off diseases
Various common names are used for diseases of seedlings such as seedling blight and damping-off. The fungi that commonly cause seedling death include Pythium, Phytophthora, Rhizoctonia, Sclerotium rolfsii and less commonly Fusarium spp. These fungi can infect the seedling during the germination, pre-emergence or post-emergence phases of seedling establishment. Environmental factors which inhibit germination and emergence usually increase disease severity. Thus cold conditions, dry or very wet soils or a hard soil surface commonly lead to increased seedling disease. In northern Vietnam, Pythium, Rhizoctonia and Sclerotium rolfsii are commonly associated with seedling death of vegetables such as beans, cabbages and other cruciferous crops, cucurbits and tomato (Viss et al., 2003).
SOIL-BORNE FUNGAL PATHOGENS
Soil borne fungal pathogens are causal agents of legume diseases of increasing economic importance such as root rots, seedling damping-off and vascular wilts. In comparison to plant responses to foliar pathogens relatively little is known about responses to root infecting pathogens, primarily due to the difficulty in observing the early stages of the interaction and attaining synchronous infection for gene expression studies. Often soil borne fungal pathogens work in disease complexes resulting in plants being infected by multiple pathogens at once. In order to study legume defenses against these pathogens, inoculation systems have been developed to enable efficient infection by individual fungi.
Soil-borne fungal pathogens cause serious crop losses in both tropical and temperate regions, with each climatic zone tending to favor a different suite of species. The fungi can build up in the soil slowly and insidiously over many years. Diagnosing soil-borne pathogens, identifying them to species level, and testing for pathogenicity is generally much harder than for fungi causing leaf infections. Soil-borne fungal infection may cause very general symptoms to the parts of the plant above ground - such as reduced yield, wilting or leaf fall - which may not be obvious to inexperienced observers as signs of infection. Fungi are common in soil, in air (mainly as spores) and on plant surfaces throughout the world in arid, tropical, temperate and alpine regions. The diseases that are caused by fungal pathogens which persist (survive) in the soil matrix and in residues on the soil surface are defined as ‘soil-borne diseases’. Thus the soil is a reservoir of inoculum of these pathogens, the majority of which are widely distributed in agricultural soils. However, some species show localised distribution patterns. Damage to root and crown tissues is hidden in the soil. Thus these diseases may not be noticed until the above-ground (foliar) parts of the plant are affected severely showing symptoms such as stunting, wilting, chlorosis and death.
EFFECTS OF SOIL-BORNE PATHOGENIC FUNGI ON MAIZE
Effect of Ustilago maydis on maize: Corn smut (Ustilago maydis) is a pathogenic plant fungus that causes smut disease on maize. The fungus forms galls on all above-ground parts of corn species, and is known in Mexico as huitlacoche; it is eaten, usually as a filling, in quesadillas and other tortilla-based foods, and soups.
Disease Management: The use of high quality seed treated with protectant fungicides may decrease the severity of disease. Self improved modern commercial hybrids and resistant varieties to grow rather than old varieties of maize.
Effect of Gibberella fujikuroi on maize: The occurrence of perithecia belonging to the G. fujikuroi on maize stubble in Northern Vietnam has been recorded. The implications of this discovery relate to potential mycotoxin contamination and the subsequent end-use of maize products. The economic losses for maize from this disease are not known. Mating population A does however produce the mycotoxin fumonisin. Fumonisins have been shown to cause equine leukaencephalomalacia and porcine pulmonary edema and are hepatocarcinogenic in rats. The World Health Organisation (WHO) classifies fumonisins as Class 2B carcinogens. This pathogen is responsible for maize root, stalk and cob rot (Summerell et al., 1998)
Disease management: Select improved hybrids and resistant varieties to grow. It is thought that the stalks sampled were on old varieties of maize rather than the modern commercial hybrids. Balanced soil fertility, avoiding low potassium and high nitrogen also helps prevent disease. A lower planting density is also recommended. Avoid harvesting the corn during wet weather to prevent postharvest rots.
There are numerous soil-borne pathogenic fungi of maize not mentioned here. It has been found that if this infected maize are consumed it could cause liver problems.
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