BLOOMING AND POLLINATION

Mango trees less than 10 years old may flower and fruit regularly every year. Thereafter, most mangos tend toward alternate, or biennial, bearing. A great deal of research has been done on this problem which may involve the entire tree or only a portion of the branches. Branches that fruit one year may rest the next, while branches on the other side of the tree will bear.
Blooming is strongly affected by weather, dryness stimulating flowering and rainy weather discouraging it. In most of India, flowering occurs in December and January; in northern India, in January and February or as late as March. There are some varieties called "Baramasi" that flower and fruit irregularly throughout the year. The cultivar 'Sam Ru Du' of Thailand bears 3 crops a year–in January, June and October. In the drier islands of the Lesser Antilles, there are mango trees that flower and fruit more or less continuously all year around but never heavily at any time. Some of these are cultivars introduced from Florida where they flower and fruit only once a year. In southern Florida, mango trees begin to bloom in late November and continue until February or March, inasmuch as there are early, medium, and late varieties. During exceptionally warm winters, mango trees have been known to bloom 3 times in succession, each time setting and maturing fruit.
In the Philippines, various methods are employed to promote flowering: smudging (smoking), exposing the roots, pruning, girdling, withholding nitrogen and irrigation, and even applying salt. In the West Indies, there is a common folk practice of slashing the trunk with a machete to make the tree bloom and bear in "off" years. Deblos-soming (removing half the flower clusters) in an "on" year will induce at least a small crop in the next "off" year. Almost any treatment or condition that retards vegetative growth will have this effect. Spraying with growth-retardant chemicals has been tried, with inconsistent results. Potassium nitrate has been effective in the Philippines.
In India, the cultivar 'Dasheri', which is self incompatible, tends to begin blooming very early (December and January) when no other cultivars are in flower. And the early particles show a low percentage of hermaphrodite flowers and a high incidence of floral malformation. Furthermore, early blooms are often damaged by frost. It has been found that a single mechanical deblossoming in the first bud-burst stage, induces subsequent development of particles with less malformation, more hermaphrodite flowers, and, as a result, a much higher yield of fruits.
There is one cultivar, 'Neelum', in South India that bears heavily every year, apparently because of its high rate (16%) of hermaphrodite flowers. (The average for 'Alphonso' is 10%.) However, Indian horticulturists report great tree-to-tree variation in seedlings of this cultivar; in some surveys as much as 84% of the trees were rated as poor bearers. Over 92% of 'Bangalora' seedlings have been found bearing light crops.
Mango flowers are visited by fruit bats, flies, wasps, wild bees, butterflies, moths, beetles, ants and various bugs seeking the nectar and some transfer the pollen but a certain amount of self-pollination also occurs. Honeybees do not especially favor mango flowers and it has been found that effective pollination by honeybees would require 3 to 6 colonies per acre (6-12 per ha). Many of the unpollinated flowers are shed or fail to set fruit, or the fruit is set but is shed when very young. Heavy rains wash off pollen and thus prevent fruit setting. Some cultivars tend to produce a high percentage of small fruits without a fully developed seed because of unfavorable weather during the fruit-setting period.
Shy-bearing cultivars of otherwise desirable characteristics are hybridized with heavy bearers in order to obtain better crops. For example: shy-bearing 'Himayuddin' ´ heavy-bearing 'Neelum'. Breeders usually hand-pollinate all the flowers that are open in a cluster, remove the rest, and cover the inflorescence with a plastic bag. But researchers in India have found that there is very little chance of contamination and that omitting the covering gives as much as 3.85% fruit set in place of 0.23% to 1.57% when bagged. Thus large populations of hybrids may be raised for study. One of the latest techniques involves grafting the male and female parents onto a chosen tree, then covering the panicles with a polyethylene bag, and introducing house flies as pollinators.
Indian scientists have found that pollen for crossbreeding can be stored at 32° F (0° C) for 10 hours. If not separated from the flowers, it remains viable for 50 hours in a humid atmosphere at 65° to 75° F (18.33° -23.09° C). The stigma is receptive 18 hours before full flower opening and, some say, for 72 hours after.