Cocoa trees in Ivory Coast and Ghana are under threat from a virus that causes so-called shoot swelling and leads to 15-50% yield loss. This disease is spread by mealybugs – small insects that feed on the leaves, buds and flowers of trees.
Scientists from the University of Kansas (USA), the University of South Florida (USA), the Cocoa Research Institute in Ghana, Prairie View A&M University (USA) and the University of Texas at Arlington (USA) investigated the mechanism of action of the virus and proposed strategies that will allow you to successfully protect cocoa trees from it. The corresponding scientific article was published by the journal PLOS One.
According to the study’s authors, mealybugs are not affected by pesticides. Because of this, farmers are trying to fight the virus by cutting down infected trees and growing varieties that are resistant to it. However, over the past few years, more than 254 million cocoa trees have died in Ghana. Although it is possible to graft this plant, the vaccine is expensive and the yield of grafted trees is less than it could be.
Researchers used mathematical data analysis to determine how far apart grafted and ungrafted trees should be planted to prevent the spread of the virus while controlling vaccine costs. They took into account that mealybugs can move from crown to crown, and can also be carried by ants or the wind.
The scientists used data from a pilot study previously conducted in Ghana. Then they planted cocoa in squares of 19 by 19 trees at a distance of 2.4 meters from each other (such squares were called plots). The result was 10 nested squares: the tenth, central one, had one tree, and the outer, largest one, had 72 trees. Next, some trees at each site were inoculated with the “heavy” strain of the virus, and some with the “light” strain. There were four treatments, they differed from each other: for example, during the third treatment, not a single tree was grafted with the “light” strain. When cocoa seedlings (young plants) were transplanted, they were inoculated with a “moderate” strain of the virus. After transplanting the seedlings, the trees of the third to tenth squares were observed for seven years.
Based on the data obtained using mathematical modeling, the team developed four models that will create a “protective layer” of grafted trees for ungrafted ones. Although the models are still experimental, the study’s authors noted, “This will have a positive impact on farmers’ profits as well as global chocolate supplies.”
