dinsdag 25 oktober 2011

Cacao Diseases in Central America 4

The Tropical Agricultural Research and Higher Education Center (CATIE) is a regional center dedicated to research and graduate education in agriculture and the management, conservation and sustainable use of natural resources.
Its members include the Inter-American Institute for Cooperation on Agriculture (IICA), Belize, Bolivia, Colombia, Costa Rica, the Dominican Republic, El Salvador, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Venezuela and Spain.
Source:  Tropical Agricultural Research and Higher Education Center, CATIE, 2009.

Diseases that mainly attack other parts of the plant
Phytophthora disease and trunk canker: (Phytophthora palmivora or P. capcisi)
diseases are caused by the same organisms that cause black pod

How do we recognize the disease?
Infected suckers (chupons), descending dieback in nursery plants.

How does phytophthora affect the plant?
a) It produces dieback (death) from the top to the bottom of the young shoots (suckers or chupons) of adult plants and nursery seedlings.
b) It causes canker on the trunk of adult trees, characterized by the appearance of circular lesions that are red in color when the bark is removed, and can eventually cause the death of the tree. Reddish-brown lesions appear on the roots and water and nutrient absorption are disrupted, which can also kill the tree.
How does it spread and what factors favor the disease?
The factors that favor the spread of the foliage damages are the same as those described for black fruit rot in cacao. Trunk canker usually occurs in waterlogged areas or during prolonged flooding.

How do we combat the disease?
In the nursery: reducing the level of moisture in the nursery and building raised beds covered by a layer of sand helps to mitigate the effects of the disease. During cool weather the seedlings can be protected by applying a copper-based fungicide weekly during periods of high humidity. Dead seedlings should be carefully eliminated.

In the field: suckers (chupons) should be eliminated periodically to prevent them from becoming infected and thereby becoming a source of contagion for other organs.
Proper construction and maintenance of the drainage ditches on the plantation helps prevent the appearance of trunk canker. When damage occurs, cut off all affected tissues and apply a tree-wound dressing to the cuts. 
Cacao tree with healthy pods on the left, and pods with black pod disease on the right.

Ceratocystis wilt (machete disease) Caused by the fungus Ceratocystis cacaofunesta  http://www.public.iastate.edu/~tcharrin/Cacao.html

How do we recognize the disease?
Sudden death of the tree with the leaves drooping down.
Dried leaves remain for a long period hanging on the tree.
Reddish lesions visible on the stem.
How does the disease affect cacao trees?
The fungus grows in the internal conducting tissues of the trunk and branches, blocking the flow of water and nutrients. The tree wilts and dies as a result. The disease occurs in a sporadic and dispersed manner on the plantation, but it can become a very serious problem when the planted materials are genetically uniform or when they are grafted onto rootstocks that are not resistant to soil-borne diseases.
How does it spread and what factors favor the disease?
The fungus produces most of its spores within the tree, especially in the galleries or tunnels made by barely visible Xyleborus beetles. The spores are disseminated by these beetles when they move from one tree to another, or by the wind, along with the wood dust and the feces of these and other perforating insects.

For an infection to occur, there must be wounds in the trunk and/or on the branches that are caused naturally or by the action of insects or tools such as machetes, shovels, pruning shears, etc.
Ceratocystis wilt of cacao, caused by a host-specialized form of the fungus, has been locally important in Latin America, where it is believed native and called mal de machete. Its importance in Brazil has been recognised increasingly since 1998 and has been associated elsewhere with drought, with South Bahia experiencing reduced rainfall in recent years. Xyleborus beetles are attracted to the diseased trees and bore into the branches. The frass from beetles is pushed to the outside of stems as a light, powder and contains viable inoculum of the fungus, which may be spread by wind or rainsplash.
How do we combat the disease?
A number of useful measures for preventing the appearance and spread of this disease include: disinfecting all work tools with a suitable product before using them; applying tree-wound dressing to the cuts and wounds on pruned trees; and avoid planting uniform material and grafting onto susceptible rootstocks.
Sick trees should be eliminated by burning or burying the residues. If a patch of affected trees is detected, follow the procedure indicated for controlling rosellinia (black root rot).

AnthracnoseColletotrichum gloeosporioides
Caused by the fungus
How do we recognize the disease?
Dry, brown spots with yellow edges that spread over the leaves and dry them.
Diseased fruits show signs of the fungus.
Seedling blight of cacao caused by P. megakarya
A dry broom formed from infection of an apical  vegetative bud by C. perniciosa
How does anthracnose affect the cacao tree?
It attacks the young shoots, leaves and stems that are most exposed to the sun, particularly those in the crown of the tree, which limits the development and production of the plants.
It causes dry lesions with yellow edges that normally spread from the edge to the center of the leaves, eventually damaging them completely, and then the leaves fall off, leaving the branches bare.
This stimulates the growth of new branches that are also infected, which finally take on the appearance of small brooms.
In the nursery it causes similar lesions and defoliation, as well as deep lesions on the stem.
The damage to the fruits is not economically significant and can be identified by the appearance of deep brown lesions on fruits of a certain age. White mycelia appear on the lesions, which turn pink when the fungus produces spores. The diseased fruits turn black and die.

How does it spread and what factors favor the disease?
The spores are produced on the stem and fruit lesions when conditions are humid.
They are disseminated by the wind, rain water or irrigation, insects and tools.
Infection of the foliage occurs during the rainy season and is propitiated by wounds caused by insects.

How do we combat the disease?
Adequate amounts of shade in the field (30%-50%) and shade in the nursery (50%-70%)prevents damage by anthracnose. Infections in the nursery can be reduced by building raised beds covered with a thick layer of sand or mulch* to avoid the splashing of rain water.
Diseased seedlings should be carefully eliminated and a copper-containing fungicide should be applied to the rest of the seedlings at the recommended dose and frequency. In adult plants diseased tissues should be pruned 10 cm below the affected area, applying tree-wound dressing to the thick stems and disinfecting the tools properly.
* Layer of decomposing vegetal residues

next and last episode: Other cacao diseases