Interactions with Arbuscular Mycorrhizae

In a recent review of the role of arbuscular mycorrhizae in plant disease prevention, Borowicz (2001) came to the conclusion that of the published studies on this subject between 1970 and early 1998, her meta-analysis revealed that most studies were performed on economically important plant species (agricultural crop plants) in low phosphate soil conditions in greenhouse or microplot experiments. She concludes that a much wider set of studies must be carried out before we can extrapolate the information gained to agricultural ecosystems in general, let alone to natural ecosystems. From her analyses, we can see that in general, 50% of the studies showed that arbuscular mycorrhizae afforded some degree of protection to their host plant against plant pathogenic fungi and nematodes. The effect of the pathogenic fungi was usually to reduce the growth of the arbuscular mycorrhizal fungus, but this effect was not seen as frequently for nematodes. The interaction between the two fungal functional

groups resulted in a reduction of growth of both competing fungi in only 16% of the reported cases.

The effect of arbuscular mycorrhizae is probably to improve the nutrition of the host plant or alter its physiology in such a way that the plant is better able to defend itself against the pathogen (Dehne, 1982; G. S. Smith, 1988; Volpin et al., 1994) rather than by direct competition between the mycorrhizal fungal and the pathogenic fungus or nematode. The effect of arbuscual mycorrhizae on plant parasitic nematodes includes reduction in fertility and egg production of the nematode, reduced penetrability of the root, and enhanced phosphorus content of the plant, affording improved growth (Roncadori, 1997).

Abdalla and Abdel-Fattah (2000) showed that there was a significant protective effect of arbuscular mycorrhizal fungus Glomus mosseae against the two pod rot pathogens Fusarium solani and Rhizoctonia solani (Table 3.23). The mycorrhizal effect increased peanut plant growth and yield. At plant maturity, inoculation with F. solani and/or R. solani significantly reduced the shoot and root dry weights, pod number, and seed weight of peanut plants, but growth and biomass of peanut plants inoculated with G. mosseae was significantly higher than that of nonmycorrhizal plants, both in the presence and absence of the pathogens. Although the presence of the pathogens reduced root colonization by the mycorrhizal fungus, propagule numbers of each pathogen isolated from a variety of plant parts were significantly lower in mycorrhizal compared to nonmycor-rhizal plants, thus not only did G. mosseae protect peanut plants from infection by pod rot fungal pathogens, it reduced the fecundity of the fungal pathogen.

In culture conditions, Elsen et al. (2001) demonstrated that the presence of the arbuscular mycorrhizal fungus Glomus intraradices significantly reduced the reproductive capacity of the burrowing nematode Radopholus similis on carrot

Table 3.23 Interactions Between Two Root Pathogenic Fungi (Fusarium solani 1, Rhizoctonia solani 2) and an Arbuscular Mycorrhiza (Glomus mosseae) on Plant Growth and Fecundity of Peanut Plants, Showing Alleviation of the Detrimental Effects of the Pathogen by the Mycorrhizae

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