6[7] Phytophthora Palmivora [ EDFS

This leaflet is produced by the Ministry of Agriculture & Livestock, Solomon Islands, with support
from IPPSI: Improved Plant Protection in Solomon Islands, a project financed by ACIAR, the
Australian Centre for International Agricultural Research, Canberra. Authors: Helen Tsatsia, MAL &
Grahame Jackson, TerraCircle Inc.

Extension Fact Sheet 6:

Cocoa Black Pod & Canker

Common name: Black pod

Scientific name: Phytophthora palmivora

Host: The fungus attacks cocoa; it causes Black pod and
canker. Elsewhere, the fungus causes diseases on
breadfruit, coconut, papaya, and many other crops.

Damage

The fungus infects pods of any age, from the time they are small, and called cherelles,
to the time they are yellow and mature. The pods are at first brown (photo, left), then
black; they dry but stay on the tree (photo, centre). Pods are destroyed in 10 days or
less, depending on size. In Solomon Islands, the fungus destroys up to 40% of the
pods. The number destroyed depends on rainfall, variety and management of the trees.

The fungus invades the trunk and branches causing cankers (photo, right). This
happens when the fungus in the pods or in the flowers grows back into the tree. From
outside, the branch looks healthy. It is only when the bark is removed that the red
colour of the canker is seen. Trees can be killed by canker, but more often the fungus
causes branch dieback.

The fungus also infects young leaves (photo, lower right), especially the leaves of
water shoots or chupons. In this case, the damage is usually minor.

Biology and Life Cycle

The fungus needs rain to complete its life cycle. Spores are produced on the pods and
spread in wind and rain to other pods nearby or to new plantations. If they land on a
pod or young leaf, and it is wet, they germinate. There are two ways: a) the spores
germinate like a seed and infect the pod or leaf; or b) the spores produce smaller
spores which burst out and swim short distances over the pod or leaf surface, then
they germinate and infect. In both cases, the fungus kills the cells. On pods, a brown
circular spot is produced, which expands very quickly. On young leaves, infections

often start at the leaf tip and follow the veins, which turn brown. Infection on the
stems of water shoots causes leaves to wilt.

White areas containing the spores form on the pods. Spores are also produced on the
leaves, but they are less obvious. Spores on pods and leaves are splashed by rain to
those all around; they also fall to the ground and remain in the soil or on leaf litter.
Heavy rains splash the spores from the soil up to 1 m above ground, infecting pods on
the trunk. Apart from wind and rain, spread can occur in other ways:
•  Fungus in the canker grows back into the pod, via the stalk;
•  Ants build ‘tents’ of soil containing spores over mealybug colonies on the pods;
•  Flying beetles that breed in the pods move spores to healthy pods;
•  Rats moving through the plantations transfer the fungus by chewing first on

infected and then healthy pods. Possibly, bats do the same;

• On pruning tools used to cut off pods and chupons.

Signs and Symptoms

The fungus is obvious on pods as brown rapidly growing spots. Cankers are less
obvious; sometimes, they dry out, become sunken and there are splits between dead
and healthy tissue. There may be a red liquid, but this is rare on Amelonado.

If it is not certain that Phytophthora is the cause of a black pod, take a piece of the
pod, cut a hole in a healthy pod and place it in the hole. Put the pod in a plastic bag.
Look for a rapidly growing brown rot, and a white area at the margin.

Management

Cultural control:
Good circulation of air in the plantation is important to dry pods and leaves quickly
after rain, reducing the time when infection can occur. Therefore:
•  Use a light shade (about 600 trees per hectare if they are planted);
•  Plant cocoa not less than 3 m apart;
•  Prune to create an open canopy: cut out branches close to the fork (jorquette),

especially those forming a second storey; cut out branches near or touching the
ground; remove water shoots.

Remove diseased pods as often as possible, at least every month, preferably more
often during the main crop season. Take the pods from the plantation and bury them.

Resistant varieties:
Amelonado is more resistant than other varieties tested in Solomon Islands, such as
Na32 and Trinitario. Resistant, high yielding varieties have been bred for resistance to
Black pod disease in Papua New Guinea.

Chemical control:
Copper sprays are useful, if applied regularly (every 2 weeks) to the pods, especially
during the main crop season. Trunk injections using phosphorous acid are also
effective. They are applied with a purpose-made syringe, once or twice a year,
depending if there is low or high disease. See MAL extension staff for advice.