Mango trees are affected by a number of fungal and bacterial diseases at various stages of their life. Anthracnose, scab, stem-end rot and bacterial spot are all recorded diseases of mango, although anthracnose is the most damaging.
Colletotrichum gloeosporioides (anthracnose) is the most important fungal pathogen in its geographic distribution, the parts of trees infected and subsequent severity of disease and damage caused. Anthracnose attacks the flowers, young fruits, leaves and twigs, and appears later as a post-harvest/storage disease of fruit following rejuvenation of latent infections occurred to young fruit on the tree. Typical symptoms are black, slightly sunken lesions of irregular shape enlarging to cause leaf spotting, blossom blight, fruit staining and fruit rot.
Leaf symptoms start as small, angular dark coloured spots and rapidly enlarge on young leaves to form extensive areas of necrotic and dead leaf tissue. Infections initiated on older leaves are restricted to spots 6mm in diameter that appear glossy dark brown and black on mature hardened leaves.
Symptoms on panicles (flower clusters) begin as small black or dark-brown spots which enlarge, coalesce and kill flowers, causing large irrecoverable losses in yield. Infections of fruit may proceed in the orchard to cause extensive decay of mangoes on the tree but are just as likely to remain dormant, ‘springing into life’ later after picking while in storage and transit. The fungus penetrates the fruit skin and remains in a ‘latent’ condition (quiescent state), showing no symptoms of disease until fruit ripening gets underway.
Physiological changes associated with ripening push the pathogen into an active state producing prominent black spots that coalesce and penetrate deep into the flesh with extensive fruit rotting. An additional aspect of fruit damage by the anthracnose is ‘tear staining’ of the skin caused by spore-laden water droplets from infected twigs and leaves spreading over the fruit to cause surface infection.
The most dangerous period for anthracnose is from the onset of flowering, until fruits are half grown, especially during prolonged rainfall, high humidity and heavy dews that cause free water to persist over the surfaces of all plant parts. Few major varieties show high resistance and many popular commercial varieties like ‘Haden’, ‘Kent’ and ‘Irwin’ are most susceptible to anthracnose.
Colletotrichum gloeosporioides is a disease responsible for anthracnose-like symptoms on a wide range of other fruit tree crops including avocado. It can survive as a saprophyte on dead and dying plant material including fallen leaves and fruits.
In its early stages, mango scab caused by the fungus Elsinöe manganifera can be easily confused with the black lesions of anthracnose but the latter does not go on to produce the scar. Mango scab only infects young tissue (young leaves and stems, flowers, fruit-stalks and young fruit) and like anthracnose is dependent on presence of free water over susceptible plant surfaces for rapid infection. Mango scab may become a problem early on in the life of the mango tree by attacking seedling trees in nurseries.
Nursery infections are often severe causing new shoots to lose all their leaves. Similarly, small young trees in moist orchard environments may develop economically damaging scab disease. Infected leaves display brown spots surrounded by haloes, lesions at leaf margins, corky lesions on the lower (abaxial) leaf surface with elongated dark coloured lesions along the raised main veins. In extreme wet conditions, many small brown lesions may form on young leaves causing severe defoliation. Additional leaf symptoms include lesions with central scabs and many minute lesions (0.1 mm diameter) along the secondary veins. Large number of lesions along the leaf margins causes leaf distortion.
Newly set fruits are highly susceptible to infections which develop into small black spots and result in fruit drop. Infected fruits remaining on the tree develop unsightly scar tissue which ruins marketability causing downgrading of fruit, and often so severe that the crop is un-saleable. Mango scab does not directly cause rotting as the fruit matures (like anthracnose), but persists as a scarring scab. Severely-scabbed fruit are more prone to development of latent anthracnose infections in the post-harvest period.
Mango scab is spread by spores and mostly by asexually produced conidia. Spread is by rain splash with prolonged periods of free surface water required for sporulation (production of spores), spore germination and infection (penetration into the leaf and spread within the plant tissue). Unlike anthracnose, the fungal pathogen (Elsinöe manganifera) that causes scab is an obligate parasite which can only survive on the living plant.
Bacterial black spot
Mango bacterial black spot (MBBS) is an ‘Old World’ disease now causing widespread economic damage in Asia and the Pacific Rim. MBBS is caused by the bacterium Xanthomonas campestris pv. manganiferaeindicae. All above ground parts of the plant are affected by infection through wounds and natural openings (e.g. stomata and lenticels). The most common symptoms occur on leaves and fruit, although cankers appear on twigs and branches during severe infections and especially on highly susceptible cultivars.
Leaf symptoms start as small water-soaked areas restricted by the veins, becoming raised, black and sometimes encircled by a halo of chlorotic (yellow) tissue. At the host cell level, bacteria cause hypertrophy (enlargement) of leaf tissue by increasing the size of air-filled intercellular spaces between the spongy parenchyma cells. This causes rupture of the epidermis with exudation and spread of bacteria across the leaf surface. Severe leaf infection may cause abscission (leaf fall) but the leaves usually remain on the tree, still carrying dried-out light brown/ash grey lesions.
Fruit symptoms start as tiny water-soaked spots at the lenticels (breathing pores), later becoming star-shaped and erumpent with exudation of bacteria-laced gummy ooze.
Severe infection leads to fruit drop but many fruit remain on the tree with reduced quality and more prone to invasion by secondary pathogens.
There are distinct differences in disease susceptibility between mango varieties (cultivars) and in relation to different stages of plant development. For instance, young leaves are resistant because they lack functional stomata. Leaves show increased susceptibility as they expand with lesions appearing immediately after leaf hardening and maturity, but decreasing in susceptibility thereafter. Fruit susceptibility increases with development and peaks during the month before picking when the lenticels are most vulnerable to infection.
Disease spread between orchards occurs through infected plant material. However, within orchards, periods of high humidity, surface wetness and wind-blown mists and rain cause most rapid and maximum dissemination of bacteria and disease. Haden, Irwin, Fascell, Keitt, Kent, Tommy Atkins and Zill cultivars all show varying degrees of susceptibility to MBBS.
Stem-end rot and alga spot
A wide range of tropical fruits including avocado and banana are plagued by fungal stem-end rots and mango is no exception. Infections occur on the tree at weak spots around the fruit stalk attachment where moisture accumulates and persists, but the wet rot symptoms generally appear during the post-harvest period. Stem-end rots are caused by a huge range of weak parasitic fungi in different parts of the world.
They include Diplodia natalensis, Dothoirella sp, Lasiodiplodia theobromae, Botryodiploda sp, Botryosphaeria sp and Phomopsis sp as well as Colletotrichum gloeosporioides, the causal fungus of anthracnose. In general, stem-end rots of affected mango fruit appear as dark brown and water-soaked areas which may extend internally as rots into the flesh. Some of these fungal pathogens are additionally implicated with nutrient deficiencies in a broad range of chronic disease and disorders of the mango tree loosely termed ‘mango decline’.
Mango is unusual in suffering from a potentially damaging disease called red rust, green scurf or alga spot caused by a parasitic alga called Cephaloeuros virescens. Leaf spots begin as round grey-green areas turning rusty red as the parasite produces a mass of rust-coloured ‘spores’ on the leaf surface. More serious stem tissue infections may also occur with cankers developing in the bark causing stem thickening.
Severely diseased branches must be cut off and burnt. Algal spot is not a common problem but can become serious where growers become over-dependent on non-copper fungicides for control of anthracnose and other fungal diseases of foliage and fruits.
Control with copper fungicides
Copper fungicides provide long-term protection of the foliage (leaves, flowers and fruits). Key to their continuing success, even after a hundred years of usage, is a broad spectrum of actions against a wide range of pathogens, including fungi, bacteria and algae, and high tenacity of deposits to the intense weathering pressures typically experienced in the main mango growing areas of the world.
Anthracnose poses year round threats to virtually all above ground plant parts and therefore can generate a huge mass of spores especially during favourable conditions of high humidity and surface wetness.
Twin aims of fungicide spray programmes for mango are reduction in overall load of conidia produced by Colletotrichum gloeosporioides and to specifically protect flowers and developing fruit from anthracnose disease. This means spray programmes must start just before flowering and continue on a regular basis until the fruit is at least half grown. But care must be taken not to stop too early because a late burst of fruit enlargement can dilute fungicide deposits on the surface of the fruit to sub-active levels. In many tropical climates, mango trees typically produce new leaves in short bursts called ‘flushes’ in response to changing wet and dry seasons.
An abundance of new foliage vulnerable to diseases and especially scab at times of the year when spray programmes are not usually in operation may require extra sprays to protect the new leaf growth.
Like anthracnose, other economically important diseases of mango, including scab, MBBS and stem rot, develop and spread most quickly in conditions of high humidity and prolonged surface wetness. Fortunately, they can all be controlled by copper containing fungicides and are generally controlled as part and parcel of the anthracnose control programme. Routine sprays of copper fungicide are applied in nurseries to manage scab disease. Cuprous oxide which has the highest proportion of active copper has consistently shown the highest activity against a broad range of pathogens on a gram for gram basis.
Trials conducted in the Philippines with cuprous oxide (Nordox 50WP) showed significantly superior control of blossom blight and fruit anthracnose (assessed 7 and 14 days after harvest), compared with a proprietary 50 per cent WP formulation of cupric hydroxide.
Nordox 50WP was superior when applied at one-third of the rate of cupric hydroxide (150 g/100 litre (L) compared with 450 g/100 L). In a separate Philippines study covering mango scab and diplodia stem-end rot, as well as anthracnose, cuprous oxide (Nordox 50WP) at 100 g/100 L outperformed cupric hydroxide (350 g/100 L). Fifteen-year-old trees of cultivar ‘Carabao’ were treated to good spray coverage using spray volumes 60-80 L/tree by power sprayer. Nordox 50WP achieved significantly less blossom blight and fruit anthracnose, a lower incidence of scab on mango fruits and reduced levels of diplodia mango stem-end rot compared with all other fungicide treatments.
Mean number of fruits set per panicle and weight of sound fruit harvested were highest in the Nordox 50WP treatment.
By Dr Terry Mabbett