Photo 1. Black pod infection, Phytophthora palmivora, on the lowest pod on the trunk. It is likely that rain has splashed soil containing spores onto the pod where they germinate and infect.
Photo 2. The water mould, Phytophthora palmivora, infects the young leaves, especially along the veins.
Photo 3. The water mould, Phytophthora palmivora, has infected the pod and then grown from the pod into the branch. The light brown margin of the red area is where the water mould is still active. The red colour is caused by fungi.
Photo 5. Oecophylla ants tending colonies of insects feeding on cocoa pods. It is possible that ants carry spores of Phytophthora palmivora from infected to healthy pods.
Phytophthora palmivora. It is not a fungus, but an oomycete or a water mould, related to algae.
Worldwide. In the tropics. Black pod disease is recorded from American Samoa, Federated States of Micronesia, Fiji, Palau, Samoa, Solomon Islands, and Tonga, and Vanuatu.
The water mould attacks cocoa; it causes black pod and canker. Elsewhere, it causes diseases on breadfruit, coconut, papaya, and many other crops.
Phytophthora infects pods of any age, from the time they are small (when they are called 'cherelles'), to the time they are yellow and mature. Infected pods are at first brown (Photo 1), then black (Photo 2); they die but stay on the tree. Pods are destroyed in 10 days or less, depending on size
The oomycete or water mould needs water to complete its disease cycle. Spores are produced on the pods and spread in wind, heavy dew or rain to other pods nearby or to other plantations. If they land on a pod or young leaf, and it is wet, they germinate. There are two ways of germinating: (i) in warmer conditions, the spores germinate like a seed and infect the pod or leaf; or (ii) the spores produce about 20 smaller spores ('zoospores') which burst out and swim short distances over the pod or leaf surface, then they germinate and infect. In both cases, the water mould kills the cells. On pods, a brown circular spot is produced, which expands very quickly (Photo 1). On young leaves, infections often start at the leaf tip and follow the veins, which turn brown (Photo 2). Infection on the stems of water shoots causes leaves to wilt.
The water mould invades the trunk and branches causing cankers (Photo 3). This happens when the fungus in the pods or in the flowers grows back into the tree. From the outside, the branch looks healthy at first, but later cracks appear, and the infected area may be slightly sunken. When the bark is removed the red colour of the canker is seen. Trees can be killed by canker if the trunk is infected, but more often the water mould causes branch dieback.
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 1.5 m above ground, infecting pods on the trunk (Photo 1). Apart from wind and rain, spread can occur in other ways:
Black pod is a major disease of cocoa worldwide, causing annual losses of 20-30% of the pods and 10% of the trees from canker. In Pacific island countries, where rainfall is high, and where fungicides are not used, losses of at least 40% of the pods is common (Photo 6). The number destroyed depends on rainfall, variety and management of the trees.
The water mould 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 ooze at the margin of the canker, 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 where the spores form.
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, do the following:
Amelonado is more resistant than other varieties tested in Pacific island countries, such as Na32 and Trinitario. Resistant, high yielding varieties have been bred for resistance to black pod disease in Papua New Guinea.
Copper sprays are useful, if applied regularly (every 2 weeks) to the pods, especially during the main crop season. Trunk injections using phosphorous acid (as potassium phosphonate) are also effective. They are applied with a purpose-made syringe, once or twice a year, depending if there is low or high disease.
AUTHORS Helen Tsatsia & Grahame Jackson
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
The mobile application is available from the Google Play Store and Apple iTunes.