Plant diseases can be controlled by manipulation of indigenous microbes or by introducing antagonists to reduce the disease-producing pro-pagules (Linderman, 1992). AM fungi and their associated interactions with plants reduce the damage caused by plant pathogens (Siddiqui and Mahmood, 1995; Siddiqui et al., 1999; Harrier and Watson, 2004). With the increasing cost of pesticides and the environmental and public health hazards associated with pesticides and pathogens resistant to chemical pesticides, AM fungi may provide a more suitable and environmentally acceptable alternative for sustainable agriculture and forestry. The interactions between different AM fungi and plant pathogens vary with the host plant and the cultural system. Moreover, the protective effect of AM inoculation may be both systemic and localized.
Plant parasitic nematodes occur in agricultural soils worldwide, and most crops are susceptible to damage by these parasites. Nematode parasitism on host plants may cause up to 50% yield losses, and these losses may be aggravated when the plant is predisposed to other pathogens. Diseases caused by fungal pathogens persist in the soil matrix and in residues on the soil surface. Damage to root and crown tissue is often concealed in the soil; thus, diseases may not be noticed until the above-ground parts of the plant are severely affected.
Colonization of the root by AM fungi generally reduces the severity of diseases caused by plant pathogens. Reduced damage in mycorrhizal plants may be due to changes in root growth and morphology; histopatho-logical changes in the host root; physiological and biochemical changes within the plant; changes in host nutrition; mycorrhizosphere effects which modify microbial populations; competition for colonization sites and photo-synthates; activation of defense mechanisms; and nematode parasitism by AM fungi (Siddiqui and Mahmood, 1995). Of the various mechanisms proposed for biocontrol of plant diseases, effective bioprotection is a cumulative result of all mechanisms working either separately or together. The challenges to achieving biocontrol via use of AM fungi include the obligate nature of AM fungi, limited understanding of the mechanisms involved, and the role of environmental factors in these interactions.
AM fungi are rarely found in commercial nurseries due to the use of composted soil-free media, high rates of fertilizer application and regular application of fungicide drenches. The potential advantages of AM fungi in horticulture, agriculture, and forestry are not perceived by these industries as significant. This perception may be due in part to inadequate methods for large-scale inoculum production.
Cropping sequences, fertilization, and plant pathogen management practices affect both AM fungal propagules in soil and their effects on plants (Bethlenfalvay and Linderman, 1992). In order to apply AM fungi in sustainable agriculture, knowledge of factors such as fertilizer inputs, pesticide use, and soil management practices which influence AM fungi is essential (Allen, 1992; Bethlenfalvay and Linderman, 1992). In addition, efficient inoculants should be identified and employed as biofertilizers, bioprotectants, and biostimulants for sustainable agriculture and forestry.
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