It is caused by a bacterium (Candidatus Liberibacter africanus). The disease is transmitted by citrus psyllids (Trioza erytreae) and through use of budwood obtained from diseased trees. The disease is not soil-borne. It is not seed-borne and it is not mechanically transmitted.
In Kenya, the disease is not found below 800 m above sea level because both the bacterium causing the disease and citrus psyllids are sensitive to high temperatures. Optimum temperature for symptom expression is 21 to 24 degC; symptoms are masked above 26 degC. The disease is especially destructive to sweet oranges and mandarins. It is less severe on lemon, grapefruit, citron and West Indian lime.
Symptoms on the leaves show mottling, yellowing of veins or zinc defiency (i.e. small leaves, interveinal chlorosis and brush-like growth). Zinc deficiency induced by greening is confined to one or several branches within a tree (sectoral infection).
Trees infected by greening are distributed within the orchard randomly. Affected branches bear few fruits and in some cases do not fruit. The affected fruits are usually under-developed, reduced in size, lopsided, start to colour from the stem end instead of the stylar end as in the case with healthy fruits. Affected fruits drop prematurely. In seedy citrus varieties, seed abortion occurs. Severely diseased trees exhibit open growth, sparse chlorotic foliage, dieback of branches and severe fruit drop.
Rootstocks have no effect on greening disease.
- Use disease-free budwood
- Strict control of citrus psyllids.
- Very severely infected trees not producing economical yield should be up-rooted. If only a few branches are affected, they can be pruned out.
- Diseased young citrus trees should be replaced, as they will never bear fruit.