Diseases and pests
Roughly a third of the world's potential cocoa crop is lost to disease and pests every year. Which ones, and where, is not incidental to the market: the two largest producers are fighting different enemies, and the weather that helps one hurts the other.
Black pod
Phytophthora rot — P. palmivora everywhere cocoa grows, the more aggressive P. megakarya across West Africa — turns pods brown then black within days and spreads by rain splash. It is the disease of a wet year. Losses run from ten to thirty percent in an ordinary season and much higher when the rains are heavy and the canopy is dense. Control is fungicide, pruning for airflow, and removing infected pods promptly, all of which cost labour and cash. When the rainfall anomaly on the market page reads "too wet", black pod is what it means.
Swollen shoot
Cocoa swollen shoot virus (CSSVD) is spread by mealybugs, causes shoots to swell and leaves to redden, and kills the tree in two to four years. There is no cure. The only control is cutting out infected trees and their neighbours and replanting, which is why swollen shoot is the single biggest reason Ghana's capacity has fallen: hundreds of thousands of hectares in the Western and Western North regions have been lost, and rehabilitation has lagged. It is a structural, not a seasonal, driver, and it does not respond to a good rainy season.
Capsids and mirids
Sucking insects that feed on shoots and young pods, killing the tissue and opening wounds for fungal infection. They are the reason for the insecticide in the input bundle. Damage is worst in dry conditions with an open canopy.
The diseases West Africa does not have
Witches' broom (Moniliophthora perniciosa) destroyed Bahia's cocoa industry in the 1990s and turned Brazil from an exporter into an importer; Brazil is still rebuilding. Frosty pod (M. roreri) has spread through Central and South America and reached Bolivia and Jamaica. Neither is present in Africa or Asia, and phytosanitary controls exist to keep it that way. A confirmed outbreak in West Africa would be the largest supply shock in the history of the market.
Galamsey
Not a disease, but in Ghana it belongs on this list. Illegal small-scale gold mining destroys cocoa farms outright and poisons the water for those around them; the land does not come back. Estimates of the area lost are contested, and none is published as an official series. It is part of why Ghana's crop has drifted below where its tree stock says it should be.
Reading the weather through the diseases
Drought shrinks the crop directly. Excess rain shrinks it through black pod, with a lag. A hot, dry harmattan suppresses disease but also flowering. This is why a rainfall anomaly is not a single good-or-bad reading and why the market page shows the direction of the anomaly and the stage of the crop together: the same millimetre of rain that helps a filling pod in July destroys a ripening one in November.