August 2000. Why do mango trees often flower, but bear no fruits, asked Samoa?
The answer may be fungus, insects or both.
In countries with high rainfall, flowers are commonly infected by the anthracnose fungus, Colletotrichum gloeosporiodes (sexual stage, Glomerella cingulata). It causes a blossom blight. Symptoms begin as small black spots on flower buds, peduncles, pedicels and the rachis of the inflorescence. Necrotic flowers abscise leaving the persistent peduncles. Lesions may enlarge and coalesce to form large patches of necrotic, brown tissue.
In Australia, Johnson and Muirhead (1988) recommended a spray with mancozeb (800 g/kg) at the rate of 2 g/l weekly during blossoming and then monthly until harvest.
As for non-chemical methods: prune out diseased twigs and clean up fallen infected trash under the trees (and also make sure there are no mangoes sitting in the trees as they are likely to be a source of inoculum). Sounds good, but almost impossible to do on large trees!
The other cause of the problem is blossom moth, the larvae of which eat the flowers.
The easiest way to recognise blossom moth damage is to look for clumps of flower debris held together by webbing. If the remains of the flowers are pulled apart, it is possible to find the small caterpillars hiding inside.
Other insects are a possibility: beetles, large grasshoppers or chafer beetles, which would only be obvious by night time observation. Put a sheet or umbrella beneath the trees to catch any insects that may be present, then tap the flowers.