Fig. 14.4Stalk rot of maize caused by Phialophora zeicolain South Africa (Deacon & Scott 1983). (a) Premature senes-
cence caused by drought stress, typical of thousands of hectares in the year when this photograph was taken. (b) Comparison
of a healthy maize stalk (upper) with a shredded, dead maize stalk (lower). (c) Invasion and rotting of the stalk base
and roots.
(a)(b) (c)Fig. 14.5(a) Apple inoculated with Penicillium expansum, which causes a soft, watery, pale-colored 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. (b) 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 oxidized when exposed to air. Oxidized phenolics are known to be enzyme inhibitors.
(a) (b)