Agriculture Reference
In-Depth Information
sufficient to break the bark and become visible on the
surface. The bark in the region from where the gum
exudes is typically damp and soft, with the wood
underneath decaying and discoloured olive to brown or
black, affecting even the cambium. With time, the
affected bark cracks open and the wood beneath dries
and hardens. Callus tissue often surrounds the affected
area. Typically, the pathogen's progress slows or stops
once weather conditions become less conducive to the
disease.
Source of infection and spread
The
Phytophthora
species causing these diseases are
common soil-inhabiting fungus-like organisms that
require water for spore production and infection.
Infection usually occurs from swimming spores
(zoospores) that enter the host under wet conditions. In
the case of root and collar rot, the zoospores enter the
host via wounds or directly through the elongation zone
Fig 6.33 Collar rot symptoms. The bark has been removed to show
the brown discolouration of the wood. A distinct margin between
diseased and healthy tissue forms. The white growth of saprophytic
Fusarium
fungi commonly occurs on affected trunks.
Fig 6.34 Phytophthora brown rot on fruit. White fungal growth
develops on the surface of rotted areas during wet weather.
Fig 6.35 Gummosis. Note the large amounts of gum oozing down the
trunk.
