Why mango blossoms and young fruit drop, and how growers can prevent it

On 31 March 2020, as Bangladesh's mango orchards were setting fruit, the online news site Jagonews24 published advice on why mango blossoms and young fruit fall and what growers can do about it. Mango trees, it said, usually blossom in March; each panicle carries thousands of flowers, and the young mangoes form from them, but many drop after setting. An extension publication from the University of Hawaii adds useful context: most flowers on a mango panicle are male and never bear fruit, so the harvest depends on protecting the few that do set.

Jagonews24 named four main causes:

• heavy fog, rain, storms and hail
• a lack of moisture in the soil
• attack by mango hoppers
• anthracnose disease on the blossoms

According to the integrated pest management schedule of India's National Horticulture Mission, mango hoppers are chiefly three species: Idioscopus clypealis, Idioscopus nitidulus and Amritodus atkinsoni. Nymphs and adults suck sap from tender tissue, destroying the inflorescence and causing fruit drop. They also excrete a sweet, sticky substance on which sooty mould grows.

Anthracnose is caused by the fungus Colletotrichum gloeosporioides. A 2008 publication of the University of Hawaii's College of Tropical Agriculture and Human Resources calls it the most important and destructive mango disease worldwide. On panicles it first shows as small black or dark brown spots that enlarge, merge and kill the flowers before fruit forms. Wet, humid, warm weather favours it; its spores spread in splashing rain or irrigation water, and it survives between seasons on infected branch tips and mature leaves.

The report's remedies were:

• keep the orchard clean, weed-free and open
• prune dead branches and burn diseased branches and leaves
• gather fallen leaves from under the trees and burn them
• keep enough water at the base of the trees at this stage
• spray a sulphur-based product on the young fruit to keep fungus off
• when the mangoes reach marble size, spray 20 grams of urea per litre of water

The other sources point the same way. The University of Hawaii recommends pruning every year and clearing fallen plant debris, where the fungus survives. The National Horticulture Mission advises against dense planting and recommends thinning overcrowded branches to curb hoppers. Bangladesh's Agro-Meteorological Information Service (BAMIS) says orchards should be irrigated again by the basin method once the fruit reaches pea size.

Jagonews24 said hopper sprays are usually applied before the blossoms appear and again when the fruit is pea-sized. The BAMIS mango guide recommends a cypermethrin-type insecticide at 10 millilitres per 10 litres of water over the whole tree, first when the panicles are 3 to 4 centimetres long and again at pea size, and a mancozeb-type fungicide at 20 grams per 10 litres for anthracnose. Writing in the Agricultural Information Service's Krishi Katha in 2018, a senior scientific officer at the Regional Horticulture Research Station in Chapainawabganj advised spraying the trunk as well as the foliage to control hoppers, and using a fungicide as a precaution whenever the weather favours disease, even before symptoms appear.

The report's most important warning concerned the flowering period. Spraying is not needed once the blossoms open, because many beneficial insects then visit for pollination, and no spraying of any kind should be done when 50 per cent of the fruit has set. The National Horticulture Mission likewise says insecticide and fungicide sprays should be avoided while flowers are opening and being pollinated, and suggests keeping bee hives in orchards to raise pollinator numbers.

Photo: Kayser Ahmad / Wikimedia Commons (CC BY-SA 3.0)

Sources: Jagonews24, Agricultural Information Service (Krishi Katha), Bangladesh Agro-Meteorological Information Service (BAMIS), National Horticulture Mission (India), University of Hawaii College of Tropical Agriculture and Human Resources