Thursday, 10 April 2014

EFFECTS OF SOILBORNE PLANT PATHOGENS ON MAIZE

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.

P. cinnamomi and related fungi also cause disease of horticultural plants such as azalea, pineapple and avocado. In horticultural systems, organic mulches have been used to stimulate the community of soil organisms and reduce the negative impact of the pathogens.

                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.

The take-all fungus is a major root disease world-wide and is estimated to cause millions of dollars in lost wheat yield each year. The main method for control is by crop rotation with non-host plants and removal of weeds that may act as alternate hosts. There has been little success in breeding for resistance in wheat to the fungus Gaeumannomyces graminis var. tritici (Cook, 2003). Under some conditions, soils become naturally suppressive to the fungus. This has been observed in long-term monocultures of wheat. Reduced disease is likely to be due to changes in activity of other soil microorganisms.



                Crown Gall

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.

                Root rots

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.

                Smuts

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.





                Stem, collar and head rots

These diseases are also caused by a diverse group of pathogens including species of Phytophthora, Sclerotium, Rhizoctonia, Sclerotinia, Fusarium and occasionally Aspergillus niger. The most obvious symptom of these diseases is the decay of the stem at ground level. Quite often this decay can lead to symptoms of wilting, death of leaves and to death of the plant. In tropical agricultural ecosystems these fungi can cause stem, foliage and head rots during warm, wet conditions. Phytophthora spp. for example, can cause diseases such as heart rot of pineapple, blight of potato and tomato and some fruit rots in these conditions. Similarly Rhizoctonia spp. can cause leaf blight in maize and head rot of cabbage in warm wet weather.

                Wilt diseases

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).

Rhizobacteria of maize isolated from various soils with a long history of maize cultivation and which exhibited antagonism invitro to Fusarium moniliforme were screened in a greenhouse for their ability to colonize maize roots. When used as seed coatings, the root “colonizing potential” of the antagonists differed significantly between strains and they also varied depending on the soil type used. For non-sterile soil, larger populations of the inoculant strain developed on maize roots growing in a Darling Downs vertisols than in a silt loam soil from Hawkesbury. However, the reverse was true when plants were grown in the silt loam Hawkesbury soil sterilized by γ-irradiation. The varying response to inoculation measured in both soils was apparently due to the diverse range of competing soil bacteria rather than the total microbial population per se. Of the 16 rhizobacteria screened for their colonizing ability, Pseudomonas cepacia strains 526 and 406 and Enterobacter agglomerans strain 621 formed consistently larger populations on the roots of both seedling and mature maize plants than the other strains. The seed inoculated P. cepacia strains were highly competitive and on 2 week old plants formed 10–80% of the total root-colonizing bacterial populations cultured on nutrient agar, and 1% after 2–3 months of plant growth. In montmorillonite soil, an inoculum density as low as 10 P. cepacia seed−1 resulted in 104 bacteria g−1 dry wet root. Nearly 70% of the P. cepacia strains tested were positive for pectinase and lipase activity. It is postulated that production of antifungal substances, rapid growth rates, the capacity to utilize a wide range of carbon sources exuded by maize roots and to a lesser extent the production of extracellular enzymes may facilitate P. cepacia strains to colonize maize roots (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.



These diseases are difficult to control because they are caused by pathogens which can survive for long periods in the absence of the normal crop host, often have a wide host range including weeds, chemical control often does not work well, is not practical or is too expensive and it is difficult to develop resistant varieties of plants. These diseases are often very difficult to diagnose accurately and the pathogens may be difficult to grow in culture and identify accurately. The fungi have many important functions, largely unrecognized, in the biosphere. Although many species are beneficial, a considerable number are detrimental to our interests. There are approximately 100,000 species of fungi described in the literature and there are many, many more yet to be described. Many of these live saprophytically on dead organic matter on or in soil where they are regarded as the most important decomposers of plant residues and other organic matter. Many species produce the enzymes needed to degrade the lignin and cellulose in plant residues and so initiate the decomposition of these complex compounds. More than 8,000 species of fungi are known to cause diseases of plants and most plants are susceptible to some fungal pathogens (Chatel et al., 1973). Some species of fungi, the mycorrhizae, live symbiotically on or in the roots of many plants. This relationship is basically parasitic but in many situations is probably beneficial to both the plant and the fungus. The growth of the plant is promoted by the improved uptake of some mineral nutrients while the fungus gains access to organic nutrients and shelter.

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 Symptoms

The symptoms are generally rotting of the roots, plant base and lower internodes. Rot normally begins soon after pollination and becomes more severe as the plant matures. A white-pink, salmon or purple discolouration of the pith, stalk breakage and premature ripening occurs. These symptoms are also characteristic of G. zeae infection, but this stalk rot has a red discolouration rather than the salmon. Superficial perithecia are also produced on the stalks. Infection is favoured by a dry early season, with warm 28-30°C wet weather, 2-3 weeks after silking. This disease occurs mostly in summer-autumn crops in the Northern part of the country and the wet season in the southern parts.
Control

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.
                  Effect of Gibberella zeae on maize

Gibberella zeae (Schwein.) Petch is responsible for major losses in corn, largely due to stalk rots and cob rots. G. zeae infection can lead to mycotoxin contamination of grain. When female pigs consume the contaminated grain, their reproductive system is affected by the zearalenone content (Beyer, M. and Verreet, J.A., 2005).
Disease Symptoms

The leaves of infected plants suddenly turn a dull greyish green, while the lower internodes soften and turn to dark brown. The stalks have pink to red discolouration on the internal diseased tissue. The fungus causes shredding of the pith and may produce small round black perithecia superficially on the stalks. Lesions may develop concentric rings.
Description of the Pathogen

The perithecia are bluish black and spherical. When mature, asci containing eight ascospores develop. Ascospores are 3-septate, slightly curved and tapering towards the ends. The asexual macroconidia of G. zeae (Fusarium graminearum) are 3-5 septate curved and tapering towards the ends. Microconidia are not produced, some isolates produce chlamydospores and PDA pigmentation is red with pink mycelium. The pathogen causes disease in the northern part of the country and during the wet season in the southern parts (Beyer, M. and Verreet, J.A., 2005).
 Host Range and Epidemiology

Perithecia on infected maize stalks mature under warm, wet conditions to exude mature ascospores. These are dispersed on the wind to ears or stalks where they germinate and penetrate healthy host tissue. Mycelium may develop on diseased plant parts during warm moist weather. The fungus may then over season in infected debris and seed. Hosts to this pathogen include maize, wheat, barley, and other cereals, where it also causes scab and seedling blight.

This disease occurs mostly in summer-autumn crops in the northern part of the country and the wet season in the southern parts.
Control

Select improved modern commercial hybrids and resistant varieties to grow rather than old varieties of maize. 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 post-harvest rots.


                  Effect of Aspergillus flavus on maize

Aspergillus flavus is responsible for the disease of peanuts and corn commonly known as yellow mould. This disease does not reduce the yield, but the quality of the produce is very poor. In the early 1960s, aflatoxin, a toxic metabolite of A. flavus was found in peanut meal. Feed prepared with this meal caused the death of 100 000 turkeys in Great Britain. A very small amount (10-20 ppb) can produce fatal liver cancer in young animals. Aflatoxin is a poison produced by the fungus Aspergillus flavus, this fungus resides in soil and dead/decaying matter in the field. It contaminates 25%+ of maize and groundnut crops produced in Nigeria. Aflatoxin is produced when the A. flavus attacks grains. Insect damage increases fungal growth and aflatoxin contamination.
Disease Symptoms

Symptoms first appear as spots on the cotyledons of the seedlings. Seedlings and ungerminated seeds shrivel to become a dried brown to black mass covered by yellow or green spores. Plants that survive germination and emergence appear chlorotic due to the presence of aflatoxin throughout the plant. The roots are stunted and lack a secondary root system, a condition known as aflaroot. The leaves are small and pointed with a thick and leathery texture. Infected seedlings may survive infection in optimal growing conditions. Following harvest, further infections may develop, with fungal growth covering the seed surface and invading the seed itself. A yellow to brown discolouration, and weight loss occurs as a result.
Description of the Pathogen

Aspergillus flavus produces hyphae that are colourless, septate and branched. A vesicle is borne at the end of each long conidiophore. On this vesicle, rows of sterigmata develop, that bear chains of yellow-green to blue-green conidia. Sterigmata of A. flavus are borne in single or double series on an elongate to subglobose vesicle. Yellow mould has been recorded on peanut, corn, cottonseed, coconut, pistachio nuts, and to a lesser extent on soybeans, rice, pecans, walnuts, almond and cassava.

The extent of yellow mould damage and aflatoxin production is dependent on the environmental conditions and production, harvesting and storage practices. The pathogen is seedborne and soilborne, and active in high humidity (90-98%) and low soil moisture. Temperatures conducive to growth are 17-42°C with aflatoxin production between 25-35°C.
Control

Controlling insect pests of the maize cob may reduce the infection rate due to the lack of an entry point for infection, as with minimizing damage during digging, combining and conveying. Avoiding fluctuations in seed moisture can also prevent corn damage. The use of high quality seed treated with protectant fungicides may decrease the severity of disease.

Using a safe and highly cost-effective biocontrol product developed by IITA called Aflasafe together with other good management practices. Aflasafe contains a mixture of four atoxigenic strains of A. flavus of Nigerian origin. This is broadcast on fields at 10-20kg/ha 2-3weeks before flowering of the crop. Spores of the atoxigenic strains are carried by air and insects from the soil surface to maize cobs displacing the toxin-producing strains. Atoxigenic strains in aflasafe compete with strains that produce large amounts of aflatoxin and in so doing limit the amount of aflatoxin produced.

                 









                 Effect of Pythium on maize

There are many Pythium species with varying degrees of host specificity and pathogenicity. Diseases can be expressed as seed decay, pre- or post-emergence damping off and infection of the roots or stems of young plants. The pathogen is inclined to moist soils typical of those subjected to excess irrigation, poor drainage and very high humidity. Little research has been done on this disease in Vietnam, however there is a high potential for this pathogen to cause economic losses here.
Symptoms

Damping off of seedlings is expressed as cotyledon and leaf chlorosis, then a watery rot appears in the taproot and hypocotyl at or near the soil line. When the roots decompose, the stele is left intact to leave only a white strand, which is followed by seedling death. On soybean seedlings P. ultimum (Trow) causes a wet rot, where P. debaryanum (Hesse) causes small, black dry lesions on the cotyledons. Plants with damaged root systems may continue to grow, but can appear stunted to varying degrees.

When onion plants are infected after the seedling stage they are stunted and the leaves yellow from tip to base.

In root rot of mature plants, feeder roots die, then lesions up to 2 cm long develop on the lateral roots. The size of the lesions increase, the plant shows aboveground symptoms of wilt, chlorosis and necrosis and the disease then spreads along the runners. In cucurbit root rots, fruit is then exposed to sunburn and the quality is reduced.

P. aphanidermatum (Edson) Fitzp. causes a rot of maize stalks at the internode just above the soil line at the time of tasselling. The diseased area is brown, soft, wet and collapsed. The stalks may be twisted. Plants may remain green for a few weeks after being infected, as the vascular tissue is not affected. A watery wound rot of potatoes occurs upon invasion of wound sites by P. ultimum and P. debaryanum. Infected tissue is spongy, wet, discoloured and may have cavities.
The Pathogen

Pythium belongs to the order Peronosporales within the class Oomycetes. Pythium produces a white, fast-growing mycelium, which produces sporangia. The sporangia can germinate directly by producing one to several germ tubes, or hyphae with vesicles at the end form. From the vesicles, 100 or more zoospores are released, they form cysts and then germination occurs. The germ tubes that are produced upon germination can penetrate host tissue to initiate infection or produce another vesicle to continue the zoospore life cycle. Club-shaped antheridia produced in the mycelium develop germ tubes which enter the spherical oogonia and fertilization occurs. The wall of the oogonium thickens to form an oospore. From the oospores come sporangia and the cycle repeats.

The Pythium species that infect these plants are ubiquitous in soil. They will cause a disease because environmental conditions are favourable, not as a result of the spread of the pathogen into a new area. Water movement, through irrigation or rain splash can however disperse active zoospores. Plants are generally most susceptible to Pythium when the conditions are unfavourable for plant growth such as unfavourable temperature, excessive moisture, low light or poor nutrient availability.
Control

Soil with large populations of fungi and other organisms suppress both the saprophytic and pathogenic activity of Pythium species. The addition of organic matter may contribute to soil microbial populations.

Minimizing periods of excessive soil moisture are essential for controlling Pythium diseases. Improve the drainage of the seed raising beds by planting on raised beds and if symptoms are present only irrigate for short periods on alternate plant beds to maintain plant growth.









                   Effect of Puccinia sorghi on maize

Puccinia sorghi is a fungus that causes the disease of maize known as common rust. It can usually be found in corn fields sometime during the growing season but generally not appear before tasseling. It causes yield losses in field and can be easily recognized by the development of dark, reddish brown pustules scattered over the surface of the corn leaves ending with severe chlorosis and causes death of the leaf.

Symptoms: Reddish or golden-brown pustules erupting from the leaf. Extremely common and can be found in almost every field. Some inbreds have a resistant flecking response that looks similar to eyespot or genetic spots.

          



Disease Management

Using of fungicides.

Growing of resistant hybrids.

                                 



      Effect of Collelotrichum graminicola on maize

Collelotrichum graminicola is a fungus that causes disease of maize known as Anthracnose. It incubate in moist chamber - Look for black setae in a mass (acervulus) of pinkish-colored spores.

Symptoms: Irregular-shaped lesions first visible on the bottom leaves. Dark brown centers with chlorotic margins. Lesions may coalesce to kill larger portions of the leaves.
      Effect of Sclerophthora macrospora on maize

This fungus causes crazy top disease of maize. It causes infection when corn plants are subjected to saturated soil conditions for 24-28 hours from planting to about the five-leaf stage of growth. It causes deformation of plant tissues including excessive twisting of leaves and also cause proliferation of the tassel. Infected plants are frequently stunted.

 Disease management

Provide good soil drainage

Do not grow in low, wet fields.





N.B: ALL THESE EFFECTS OF SOILBORNE FUNGI ON MAIZE MAY CAUSE LIVER CANCER IN HUMAN IF THE MAIZE IS BEEN EATEN BY MAN












1 comment:

  1. Thank you Mr Azeez, your article is a bunch of knowledge and understanding. You said in your NB. that all these effects of soil-borne fungi on maize may cause liver cancer in human if the maize is been eaten by man. Sir i want to know if these has any effect in other animals if they feed on it?

    ReplyDelete

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