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CHAPTER 14: FUNGI AS PLANT PATHOGENS
A simple classification of the major disease-causing organisms
Fig. 14.1a. Colony of Athelia rolfsii on an agar plate. The sclerotia (some indicated by arrowheads) develop near the colony margin. The white mycelium is aggregated into mycelial cords. [© Jim Deacon]
Fig. 14.1b. Young wheat plants rotted by A. rolfsii just below soil level.[© Jim Deacon]
Fig. 14.2. Experimental system to study infection of leaf petioles (living plant tissue) from sclerotia of Athelia rolfsii placed at different distances on natural soil. [© Jim Deacon]
Fig. 14.3. Crater disease of wheat caused by Rhizoctonia solani in Northern Transvaal, South Africa (Deacon & Scott, 1985). Top left: aerial view showing extensive areas of stunted plants. Top right: a disease crater, showing the sharp boundary between the healthy crop and a stunted patch. Bottom left: comparison of normal healthy plants and plants within a crater. Bottom centre: beads of Rhizoctonia on the first-formed seminal roots; these roots are important because they would normally penetrate deep into a soil profile, so that water can be tapped from a depth when the surface soil layers have dried. Bottom right: the boundary between the loose surface soil and the heavily compacted clay soil occurs at about 5-8 cm depth. [© Jim Deacon]
Fig. 14.4. Stalk rot of maize caused by Phialophora zeicola in South Africa (Deacon & Scott, 1983). Top left: premature senescence caused by drought stress, typical of thousands of hectares in the year when this photograph was taken. Bottom left: comparison of a healthy maize stalk (upper) with a shredded, dead maize stalk (lower). Right: invasion and rotting of the stalk base and roots. [© D.B. Scott & Jim Deacon]
Fig 14.5a Apple inoculated with Penicillium expansum, which causes a soft, watery, pale-coloured rot (photographed 7 days after wound-inoculation). When the apple skin was cut with a scalpel at the margin of the rot, the whole rotted area fell away, leaving uninfected tissue. This type of rot is indicative of polygalacturonase activity. [© Jim Deacon]
Fig 14.5b Apple inoculated with Sclerotinia fructigena, which produces a firm, irregular, dark brown rot that cannot be separated from the underlying healthy tissue. This might be explained by the ‘browning reaction’ in which phenolic compounds in plant tissues are oxidised when exposed to air. Oxidised phenolics are known to be enzyme inhibitors. [© Jim Deacon]
Fig 14.6a Wounded apple naturally contaminated by airborne spores of Rhizopus (mucorales), producing a fast-spreading, watery rot that collapses and liquefies the tissues. Dark, spreading hyphae and sporangiophores are seen on the fruit surface. [© Jim Deacon]
Fig 14.6b Apple infected by Sclerotinia fructigena in field conditions, showing pustules of spores on the fruit surface. The fruit progressively dries out and mummifies. [© Jim Deacon] |
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