Agriculture Reference
In-Depth Information
Fig 16.15 Phytophthora heart rot. Rotted leaf bases from a terminal
whorl of leaves.
Phytophthora cinnamomi
infects plants most commonly
through the root tips, grows up the root through the
older part of the stem and eventually to the growing
point of the stem resulting in heart rot.
Phytophthora
cinnamomi
can also infect through the leaf bases to
produce heart rot.
Serious losses from heart rot caused by
P. cinnamomi
generally follow periods of heavy autumn and winter rains.
Although disease can develop between 15-30
°
C, the
optimum range is 19-25
C. It is relatively inactive in soils
below pH 4. Pineapple crowns and slips are easily invaded
by the fungi, particularly when planted in autumn. Suckers
are less susceptible. The disease generally occurs first in
low-lying or poorly drained fields.
°
Fig 16.14 Phytophthora heart rot showing rotting in the growing point.
base of the leaves is water-soaked and rotten with a foul
smell due to the invasion of secondary organisms. The
growing point of the stem becomes yellowish-brown with
a dark line between healthy and diseased areas.
Source of infection and spread
Chlamydospores of the two species are the primary
inoculum, and they can survive in the soil or in infected
plant debris for several years. They can germinate
directly to produce hyphae that are able to infect roots
and young leaf and stem tissue, or indirectly to produce
sporangia. As free water is required for producing
sporangia and releasing motile zoospores, infection and
disease development is exacerbated in soils with
restricted drainage, where soils remain saturated for
long periods. Phytophthora pathogens are soil
inhabitants and require water for spore production and
infection.
Phytophthora nicotianae
is less dependent on
free water than
P. cinnamomi
.
Phytophthora cinnamomi
prefers cool conditions, and is active in soils with a
wider pH range.
Fig 16.16 Phytophthora heart rot showing infection of the basal white
tissue in leaves.
