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General information on soilborne diseases
The fungi have many important functions, largely unrecognised, 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.
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.
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 soilborne 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.
Physiology and ecology
Temperature, water potential and pH have a critical influence on the growth and survival of fungi. The optimum temperatures for growth of the majority of fungi are in the range 25 to 28°C with the minimum and maximum temperatures being 5 to 10°C and 33 to 35°C respectively. Thus most fungi cannot grow at the body temperature of man, 37°C. Optimum water potentials for growth range from -0.01 to -1.0 MPa depending on the species. Growth ceases between -5.0 and -15.0 MPa. Many fungi including important plant pathogens are able to grow at potentials which cause permanent wilting of crop plants. Slightly acidic conditions (pH 6 to 6.5) are optimum for the growth of most fungi. Although fungal growth ceases at low water potentials many species are adapted to survive quite well under such conditions, either as resistant spores, sclerotia, other specialised structures or as dormant hyphae in residues. In general, cold dry conditions favour fungal survival while hot wet conditions do not.
Light and temperature commonly affect the formation of spores and fruiting structures. Temperature, and to a lesser extent light, also have a significant effect on hyphal colour and growth. Thus if colonies grow and reproduce under fluctuating conditions of temperature and light they may develop annular zonations with respect to colour, hyphal density and intensity of spore formation. The primary function of some spores is to ensure the long-term survival of the species in the absence of a suitable substrate, or during periods when other environmental conditions are unfavourable for growth. These spores are thick walled and remain where they form, germinating when contacted by a new substrate.
The primary function of the majority of the spores is, however, to enable geographic dispersal by rain splash or wind. Spores formed in a mucilaginous matrix are usually dispersed by rain splash, in small water droplets over a short distance. Wind dispersed spores are either ejected from a fruiting structure into the airstream or are formed in dry powdery masses which are swept up by air currents. Wind dispersal may carry spores over long distances but most are deposited within 100 metres of their origin. Deposition is a result of impaction, gravity or rain wash. Because a very high proportion of spores are deposited on surfaces unsuitable for fungal growth, vast numbers of spores are produced from each colony to ensure the continuity of the species when one substrate is exhausted.
Spores have an extremely important role in fungal diseases of plants. Airborne spores enable rapid spread of the fungus from plant to plant or crop to crop. Thick-walled spores such as chlamydospores are able to survive in soil for long periods and enable the pathogen to survive in the absence of a susceptible host. Some pathogens which affect crops grown over large areas, such as the stem-rust fungus of wheat, may produce vast quantities of spores which can be carried in the atmosphere as invisible spore clouds over hundreds of kilometres.
In addition to resistant spores the fungi have evolved various other modes of survival. Some species form resting-bodies called sclerotia, which consist of aggregations of hyphal cells into dense, rounded or irregular structures, generally ranging in size from 0.1 mm to 20 mm. In other species fruiting structures such as perithecia act as survival structures and release spores when conditions become favourable for spore germination and growth.
Epidemiology and Survival
Stem, collar and head rots
Seedling blights and damping-off diseases