Root-infecting fungi

Fusarium monoliforme, F. graminearum, F. avenaceum and F. culmorum are pathogens of the Gramineae whilst F. solani has been isolated from a wide range of plants in which it causes root rots, particularly of various members of the Solanaceae and beans. F. oxysporum var. cubense produces a devastating wilt of bananas. 

The pigments of the Fusaria have been described in Chapter 7. Many of the characteristic phytotoxic metabolites are the sesquiterpenoid trichothecenes (8.38) (see Chapter 5). The more highly hydroxylated members have considerable mammalian toxicity as well as phytotoxicity. Trichothecenes have been identified as metabolites of species from ten of the twelve sections of the genus Fusarium as classified by Booth. The trichothecenes occur with various combinations of oxygen substituents at positions 3, 4, 7, 8 and 15. Several trichothecenes contain macrocyclic esters linking C-4 and C-15. These are known as the roridins and verrucarins. 

tricinctum, which was isolated from turf that had undergone a yellow wilt, produced a good yield of T-2 toxin in the laboratory. Although these compounds are phytotoxic, they are better known as mycotoxins and are discussed in Chapter 9. 

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The early literature on naturally occurring plant growth inhibitors and the influence one plant might exert on another by chemical means (allelopathy) is considered comprehensively in various reviews (12, 15, 36, 37, 48, 49, 61, 67, 94, 121, 162). Reviews of studies concerned with the influence of plant exudates on root-infecting fungi (130) and the effects of phytotoxins which arise as decomposition products (113) have appeared recently. Because of the excellent coverage of the topics by others, no attempt is made here to review the early literature exhaustively. Instead, consideration is restricted essentially to specific compounds and to some of the more recent literature. 

M. N. Schroth and D. C, Hildebrand, Influence of plant exudates on root-infecting fungi. Annual Review of Phytopathology 2 101 (1964). 

G. C. Papavizas and P. B. Adams, Survival of root-infecting fungi in soil XII. 

Larvae also feed on roots of young plants in spring. Plants turn yellow, wilt, and may die. Symptoms can be confused with cutworm damage and also root-infecting fungi confirm the presence of leatherjackets before taking action. 

Garrett, S.D., 1960. Biology of Root-infecting Fungi. Cambridge University Press, London, XIX and 293 pp. 

D. Morandi, J. A. Bailey, and V. Gianinazzi-Pearson, Isoflavonoid accumulation in soybean root infected with vesicular-arbuscular mycorrhizal fungi. Physiol. Plant Pathol. 24 357 (1984). 

Poisoning does not destroy root disease fungi infecting roots of treated trees. It does hasten death of stumps and somewhat reduces chances of killed root infection. However, if stumps are left in the field and new trees are planted around them, root diseases will usually be increased about 50% in the new plantings over what would be ordinarily expected. Where root disease incidence is likely to be high, it is better to fell the trees before or ofter trees are poisoned, and remove the stumps from the field with as much dispatch as practical considerations allow. 

Upper soil levels. They also show that secondary forests may re-establish deep root systems (6 m deep) within l6 years. They further show that abundant fine roots infected with mycorrhizal fungi characterize the deep roots of secondary forest. These circumstantial data suggest that secondary forests are mining nutrient reserves in deeper soils, but conclusive data are still unavailable. 

Endotrophic mycorrhizae are more common than are the ectotrophic but are not so well understood. This is partly because of the difficulty in growing them and also because they live inside the plant tissues. They are easily confused with nonmycorrhizal or root-rot fungi that may also be present. Endotrophic fungi are usually beneficial but not always, probably because they may under some conditions weaken the host plant and thereby increase the chances of infection by pathogenic organisms. 

Nicole M, Geiger J P, Nandris D 1986 Penetration and degradation of suberized cells of Hevea brasiliensis infected with root rot fungi. Physiol Mol Plant Pathol 28 181-185... 

Chapters 7 and 9 discuss specific exchange of molecular signals (the so-called molecular cross talk ) between beneficial microorganisms, such as rhizo-bia and mycorrhizas, and their host plants. Molecular cross talk seems to be a prerequisite mechanism for most of the plant infection by soil microorganisms (14). Only for a few microbial infections, however, the sequence and type of molecular signals involved have been characterized. Thus, there is the need for further studies to elucidate the unknown molecular cross talk between the most common rhizobacteria and fungi and the plant roots it is also needed to better understand how molecular cross talk responds to the changing environmental conditions. The potential applications of these studies are important because the... 

J. C. Dodd, C. C. Burton, R. G. Bums, and P. Jeffries, Phosphata.se activity associated with the roots and rhizosphere of plants infected with vesicular arbuscural mycorrhizal fungi. New Phytol. 707 163 (1987). 

The effect of flavonoids on spore germination and hyphal growth of ecto-mycorrhizal fungi is poorly known. However, several metabolites relea.sed by the plant roots trigger events leading to their infection (44,55). In the saprotrophic phase, spores of several ectomycorrhizal fungi respond to stimulation by abietic acid, the diterpene resin acid, in root exudates (56). 

The growth of ectomycorrhizal trees is frequently improved by their increased phosphorus (P) accumulation (3), and this, in turn, is related to the intensity of the mycorrhizal infection. Ectomycorrhizal fungi solubilize insoluble forms of A1 and Ca phosphates as well as inositol hexaphosphates, though a wide interstrain variability has been recorded (112). These complex P forms are digested by the secretion of extracellular acid and alkaline phosphomono- and phosphodi-ester-ases. Pi in soil solutions is easily taken up by ectomycorrhizal hyphae and then translocated to the host roots. Its absorption and efflux are probably regulated... 

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