clays of short-life soils. This has been linked to a
higher population of antagonistic bacteria, because
montmorillonite clays hold more nutrients and create
a more favorable pH for bacteria.
At one stage the progressive decline of banana
plantations in Central America threatened the entire
future of banana production in that region, which
was based on a single cultivar, “Gros Michel,” that is
highly susceptible to Panama disease. The threat
was averted by the chance discovery of a more wilt-
resistant cultivar of the “Cavendish” type. Owing to
its value as a disease-resistant cultivar, “Cavendish”
bananas were only planted on the more favorable sites
in Central America, and they have remained disease-
free for more than 50 years. However, the situation
is different in the subtropics (South Africa, Taiwan,
Canary Islands, and Northeast Queensland) where
the Cavendish cultivars have been introduced. Many
of these plants have now died from Panama disease,
coinciding with the emergence of a new pathotype
(race 4) of F. oxysporumf. sp. cubense. The conditions
in these countries are marginal for banana production
because the winter temperatures drop to 12°C or
lower. Bananas stop growing at this temperature
(evidenced by their failure to make any further leaf
growth), but F. oxysporumf. sp. cubense can still grow
at 12°C in vitro. This could have facilitated the emer-
gence of the new pathotype.FUNGI AS PLANT PATHOGENS 295
Fig. 14.15Panama disease of banana plants, caused by Fusarium oxysporumforma specialis(“special form”) cubense.
(a) A young banana plant in an advanced stage of disease; most of the leaves have died and collapsed as a “skirt”
around the stem base, and the erect leaves show progressive yellowing and necrosis. (b) Older, fruit-bearing banana
plants infected by F. oxysporum, showing extensive collapse of the leaves.
(a) (b)