The mango is known to suffer from a number of diseases caused by different organisms, which affects different parts of plants, at all stages of growth and development. Powdery mildew (Oidium mangiferae), sooty mould (Capnodium romasum or Tripospermum acorium), fruit rot (Aspergillus niger), leaf blight (Pestaloptiopsis mangiferae), anthracnose (Glomerella cingulata-Colletotrichum gloeosporioides), stem blight or die back (Diplodia spp.), root rot (Rhizoctinia and Fusarium species) and tip die back (Fusarium equiseta, Alternaria alternate, Aspergillus niger and Rhizopus nigricans) are recorded as fungal diseases; bacterial leaf spot (Erwinia mangifera); and malformation of mango inflorescence (the actual cause is still not confirmed) are reported from Pakistan. No information regarding the extent of damage to the plant is available, but some of these are certainly responsible for causing considerable damage and become a limiting factor in many mango-growing areas.
It is also pertinent to mention here that almost all commercial mango varieties are old, released from 1949 to 1967 and are under cultivation. Therefore, it is a question that how these varieties, after long period, could be resistant to the diseases? More or less, all varieties are reported to be susceptible, to most of the above diseases. Mango growers also neglect their orchards and show interest only at the time to decide contract rate and period. Contractors pay their attention and or interest and try to recover their money spent on contract and obtain high profit at low cost of production, but do not think about health and life of mango trees. Even growers avoid irrigating their orchards at requirement; interculturing and manure applications are also not carried on timely. Now it is up to the growers, to whom they may call responsible for present severe and serious situation of diseases in mango orchards. What the researchers would like to do for identification of actual problem. How the existing disease or complicated disease's case occurred in mango orchards could be controlled?
Anthracnose (Fig 1) is prevalent in almost all parts of the country. The leaves, petioles, twigs and fruits are attacked. Appearances of oval and irregular blister like brown spots develop on the tips, margins and centre of leaves and twigs. Blackening of petioles, elongated necrotic streaks on young twigs and branches are observed, and leaves drop down from top to bottom. Drying and rupturing of affected tissues, drying and falling of leaves, brightening of blossoms, staining and rotting of fruits are also main symptoms of anthracnose.
Tip die back disease occurs on the branches/ trunk of infested trees that start drying slowly at first and suddenly branches become completely dried / killed resulting gummy substance oozes out or remains hanging on the tree
Stem blight or die back become evident by discoloration and darkening of the bark as well as exudation of gum from infected portion (Fig 2). The withering of leaf tips and shedding of leaves (Fig 3), the twigs dying back from top to bottom is also one of the major evident of this disease. Brown streaks are observed on vascular tissues by length wise splitting of diseased twigs (Fig 3). Almost all mango varieties are reported as susceptible to this disease.
Root rot is also prevalent in almost all orchards; manifest itself as withering and drying of the plant from top to bottom and whole plant die up (Fig 4). Initially rootlets are affected and are rotten, later on the smaller, tertiary roots and ultimately the bigger, secondary and primary- main roots are infected which result in gradual decline of the plant and the plant die (Fig 4). The uptake of nutrients and water is blocked due to the rotting of the roots, which results in drying of the plants.
Leaf blight; initially appear as minute yellowish spots on the upper surface of the mature leaves. The spots become more in number, bigger in size and often coalesce, forming big irregular patches. The lower size of the spot is brown in colour, while upper portion becomes white in centre with a brownish purple margin that separate diseased and healthy portion. Some of the symptoms are present in infected mango trees.
Bacterial leaf spot is noticed on the leaves as angular water soaked spots or lesions, surrounded by clear holes. These become necrotic and dark brown and viscous bacterial exudates deposit on these necrotic portions that become corky and hard after drying. Sometimes, longitudinal cracks also develop on the petioles. Some of the similar signs are present in suffering mango orchards (Fig 5).
Fruit rot starts usually in the ripe and over ripe fruits as spots of different colours, which soon increase in number and size. Affected fruits may become soft, pulpy and unfit for consumption. The present situation is so much away from this problem (Fig 6).
Powdery mildew (Fig 7) is one of the worst diseases affecting almost every variety and is common in all over Pakistan. It appears from December to March, as superficial whitish or grey powdery growth on the flowers and flower buds (inflorescence), tender leaves, thin stem (shoots and trunk) and spreads to fruits. Infected flowers fail to open and sometimes shed before being fertilised and results in a substantial reduction of fruit set. If the fruit is already set, it may drop off prematurely. The axis may begin to dry, showing characteristic dieback symptoms, but dying of mango trees is not due to this disease, but is another unknown havoc disease.
Malformation (Fig 7) is very common and widely distributed in orchards (to a lesser or greater extent) in all over country. Compact leaves formed at the apex of shoot or in the leaf axil, to form the bunchy top seedlings and are usually shallow with few tertiary roots. The tap root may be twisted and may show necrosis. Sometimes, small leaves appear as shootlets, growth of which is checked and several similar shootlets arise from the axil of the scaly leaves and form bunches, which are thicker than the main stem. Floral aggregation (malformation) may also appear on shortened primary axil of the inflorescence, which is further branched to secondary and tertiary branches, on which flowers are borne in clusters and no fruit or very poor fruit setting is observed. Infected inflorescence remains green for long time; no malformed heads dry up in black masses but persist on trees. The symptoms are persistent in almost all those gardens in which pruning have not yet been done.
Sooty mould (Fig 8) appears as black velvety growth on the leaf surface. The entire leaf surface or portion of the leaf may be covered with fungal growth and in severe cases the whole plants are affected. The thin layer formed on the leaf surface can be rubbed off easily. Under the dry conditions this may be blown off as small fragments by the wind. The disease causing fungi in true sense are non pathogenic, however, photosynthetic activity of the plant is impaired due to covering of the leaves. The symptoms occurring in diseased orchards are different to that of this disease.
Sudden death of mango trees (Occurrence of new symptoms, especially in Sindh)
The present situation of mango orchards, with special reference to the province of Sindh, needs special attention, where about all mango orchards are suffering due to an unknown disease problem, causing sudden death of mango trees. At present mostly 3 to 5 and most likely 60 to 85 percent of trees are suffering from this type disease, in various mango orchards of Sindh. There are different opinion of plant disease specialists, but I think it is complicated case emerged mostly due to combine attack of anthracnose, stem blight or die back, root rot, tip die back, leaf blight, bacterial leaf spot and malformation diseases, which are responsible for the decline (dying of trees). That may also be proved through presence of the symptoms of most of these diseases in a single plant at same time; isolation of near about all the fungi causing these diseases from diseased specimens collected from same single diseased plant; as well as nature and behaviour of these diseases causing organisms. High temperature, high relative humidity and weak plants are main contributing factors in the spread of anthracnose and die back, while root rot and tip die back caused by Fusarium species develop more on weak plants, in high soil temperature and drought conditions. It is also true that dying of trees or decline is generally observed more in the neglected orchards, which is common factor. The shortage of irrigation water may also be another main cause, which helps to the disease causing organisms to grow, develop and spread more.
However, development of small saprophytic white mushrooms on dried trunk (stem) of diseased trees (Fig 9), rotting of roots / trunks having very bad smell and small holes on trunk (Fig 10) due to inset pests (perhaps because of bark beetles) are the most common but newly occurring symptoms, which are also present in almost all mango orchards. The growers are going to cut-off / remove diseased trees from their orchards because they feel no way except destruction of diseased mango trees (Fig 11).
Possible control measures:
The researchers had conducted various experiments on the diseases of mango plants and fruits and evaluated various ways by which the diseases can be controlled. Therefore, it is suggested that integrated disease management practices must be applied by using different fungicides in combination with suitable insecticides; whereas different cultural practices may also help to check the diseases. It must be kept in mind that use of fungicides is not alone source. On the other hand, it is also pertinent to mention here that the fungicides increase the cost of production. Their use is risky for the health of worker. The fungicides decrease the fruit quality because of their toxic substance residues that may remain present so much time in the fruit. Therefore, following integrated disease management principles should be applied.
1. Only healthy seeds, seedlings and plants should be used for planting and budding or grafting.
2. Inter cropping, mix cropping, cover cropping and or multiple cropping in or in the surrounding of mango orchards must be avoided so that the similar possible pathogens of the sown crop and mango (both) may not be multiplied and spread in orchards.
3. Irrigation must be applied as per requirement, but not subject to the availability.
4. Avoid high doses of nitrogen fertilisers, but apply proper and balanced fertilizers, on the basis of soil type, age and need of the tree.
5. Proper sanitation of orchards and pruning of trees (eradication of disease causing organisms through removing and burning of diseased plants or plant parts) helps to reduce the infection and growth, development and further spread of the disease causing fungus or so.
6. Easily available broad spectrum fungicides should be used, in combination with insecticides at proper dose. However, encouraging results could be obtained with consulting pathologist and entomologist.