Page 7 - Vol.39-No.2

CROP PROTECTION

of the chupons (vertical shoots) leaves, And there are many opportunities for spore
roots and nursery seedlings, depending on entrapment in nooks and crannies, like the
the Phytophthora species and the cocoa axils formed by attachment of peduncles
growing region. (pod stalks) to trunk or branch and chupons
In addition to spores liberated from these (vertical leafy shoots) in the upper canopy.
sources on the tree there is also an impor- Water is not the only way spores are
tant soil-inhabiting phase of Phytophthora spread inside the canopy. Insects are known
and especially with Phytophthora mega- to spread spores and particularly the small
karya in West Africa. The pathogen persists black ant (Crematogaster striatula) when
on cocoa debris in and on the soil from using dead plant tissue to construct tents
where spores are splashed up and onto around the peduncles of developing pods.
the trunk and low growing pods and this Other insects are implicated too especially
plays a key role in the epidemiology of those moving from the soil and up into the
the disease. trees like termites and the leaf cutting ants
Strength and success of Phytophthora of the New World.
as a plant pathogen lies in multiple stages Spore spread is aided by rodents includ-
including mycelium and several spore types ing rats and squirrels which inhabit cocoa
– asexual spores as sporangia and resting and feed on the pods. Pod wounding by
A mature cocoa pod with what appears chlamydospores and sexually produced rats is known to aid infection by providing
to be the beginnings of
Phtytophthora pod rod infection oospores. Main spread and dissemination an easy route of entry, as are abrasions
(Picture Dr Terry Mabbett) is via sporangia. Sporangia are typically caused by workers when using pod har-
formed and liberated during periods of vesting knives.
can result in total loss of all harvestable relative humidity of around 80 per cent Reduction of humidity levels within the
pods from the trees. Phytophthora palm- and a temperature range of 25-30 °C. canopy is the key to managing Phytoph-
ivora is less pathogenic to cocoa but has The appearance of a fine white powder thora pod rot infection. Cocoa is one of
a much wider host range encompassing on the pods some 4-6 days after infection those peculiar trees bearing flowers on
some 200 different plant species including is strongly indicative of the formation of directly on the trunk and main branches
tree crops and wild forest trees which are sporangia. (in flower cushions) so that many pods are
commonly used to shade cocoa.
situated close to the ground with some
The disease caused by the Phytophthora almost touching the soil. Plants that grow
speces in cocoa was once called ‘black pod’ under properly shaded cocoa are typically
but since other parts of the tree may also slow-growing shade plants. It is important
be attacked, and to avoid confusion with to prevent any weed growth around the
similar descriptive names of diseases such base of the tree where some pods will be
as brown pod rot caused by Botryodiplodia borne in order to minimise humidity levels
theobromae, the disease is more usually and therefore the infection of low hanging
and correctly referred to as Phytophthora pods.
pod rot.
In addition stem canker infections are
Pods may be infected on any part of their mostly found in bark and wood on this
surface and at all stages of development, Low hanging pods are especially at lowest portion of the trunk closest the
risk of infection from Phytophthora
from the tiny cherelles (new pods) borne on inoculum which can be splashed up soil. On the other hand, cover provided by
flower cushions, which are situated on the from the soil during heavy rainfall weed growth under the tree could reduce
bark, to full grown pods just weeks away (Picture Dr Terry Mabbett) amount of inoculum splashed up from the
from ripening. The classic Phytophthora Newly produced sporangia take one of soil when drips and drops of water impact
pod rot infection and symptom is a dark three paths: producing mycelium, gener- on the ground.
brown to black necrosis expanding with a ating more sporangia or liberating motile
slightly irregular front and ‘consuming’ the zoospores in the presence of free water
whole pod in just two weeks, eventually required for their movement. Optimal lib-
leaving a completely black and dried out eration of zoospores from the sporangium
mummified pod. occurs at a water film temperature of 15-
Greater crop losses occur when mature 18 ºC and ambient relative humidity within
pods are infected because ‘cherelle wilt’ 70-80 per cent range.
caused by Phytophthora is largely com- These multiple spore sources on trees
pensated for by pollination and fertilisa- and in the soil within a naturally humid co-
tion of later formed flowers. Phytophthora coa canopy, regularly drenched with intense
damage to cherelles is often confused with tropical rainfall, can create an explosive
physiological cherelle wilt which is a natural disease situation. The stage is thus set Cocoa pod initially damaged by
thinning out process. for rapid spread of spores down through rodents feeding on the sweet mucilage
Phytophthora pathogens also cause stem the canopy in drips, drops and rivulets and and beans and which subsequently
cankers on the trunk, infection and necrosis upwards from the ground in soil splash. sustained an advanced pod infection
(Picture Dr Terry Mabbett)
Circle 4 on enquiry card Vol. 39 No. 2 5