Mango killer disease in Sindh

Mango, Mangifera indica L. is one of the most important foreign currency earning fruit crops of Pakistan and offers a wide choice of near about 200 mango varieties. Some of the most famous varieties are Sindhri, Dasheri, Siroli, Summer Behist Chaunsa, Baganpali, Swarnarika, Anwar Ratole and Langra. It is known to suffer from a number of insect pests and diseases caused by different organisms, which affects different parts of the plant, at all stages of growth and development.
No information regarding the extent of the 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.
The present situation of mango orchards, with special reference to the province of Sindh, needs special attention, where almost all mango orchards are suffering due to an unknown diseases, causing the decline or sudden death of the 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 opinions by the plant-disease specialists, but the preliminary studies carried out on the problem reveal that the ASIAN AMBROSIA BEETLE is a vector of a Killer Mango Disease in Sindh. Beetle and disease both could be controlled through IPM.
Actually the Asian ambrosia beetle (AAB), Xylosandrus crassiusculus, has been identified from the various samples taken from the mango orchards of Sindh. This beetle has a stout reddish brown body and is reported as a pest of woody ornamentals, fruit and nut trees. It spreads through the movement of infested wood logs/pieces and by its own movement.
These beetles are so tiny; initially, the attacked trees do not show any symptoms. The strands can identify infestation as they excavate galleries into twigs, branches or small trunks due to the female beetle activity, leaving only pencil-lead sized holes in the bark.
The Ambrosia beetles are associated with symbiotic fungi, which females carry to the tree. Both the adults and the grubs of the beetle feed on the growing fungi and on the wood. The tunnelling of the AAB in trees generally is not as serious as the effect of the fungi on the plant.
The fungi usually leaves dark stains on the wood of the ambrosia-beetle infested tree and may be largely responsible for the plant's death. Fusarium species are found predominantly associated but Fusarium and Rhizoctinia spp. (rotting roots), Diplodia spp (causing gummosis or stem blight or die back.), Glomerella cingulata-Colletotrichum gloeosporioides (cause of anthracnose) and Pestaloptiopsis mangiferae (causing leaf -blight) are also found present in the infested samples.
It may be considered that the beetle, along with these all diseases, is responsible for the decline (dying of trees) and the condition may be termed as 'Disease complex' (presence of more than one disease-causing agent).
It may also be proved through the presence of the symptoms of most of these diseases in a single plant at the same time. Isolation of near about all the fungi (causing these diseases) from the diseased specimens collected from the same single diseased plant; as well as the nature and behaviour of these disease causing organisms is another evidence.
Moderate temperature, high relative humidity and weak plants are the 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 the dying of trees or decline is generally observed more in the neglected orchards, which is a common factor. The shortage of irrigation water may also be another main cause, which helps disease causing organisms to grow, develop and spread more.
However, the development of small saprophytic white mushrooms on the dried trunk (stem) of diseased trees, the rotting of roots/trunks having very bad smell and small holes on trunk due to beetles are the most commonly occurring symptoms and are present in almost all mango orchards.
The growers cut-off/remove the diseased trees from their orchards because they find there is no alternative except for the destruction of the diseased mango trees; whereas, researchers have conducted various experiments on the control of the beetle and diseases of the mango plants and fruits. Preliminary results showed that prevention is the best cure. They have evaluated various ways by which the diseases can be controlled.
Therefore, it is suggested that integrated pest and disease management practices must be applied by using different insecticides in combination with suitable fungicides; whereas different cultural practices may also help to check the problem. It must be kept in mind that the use of insecticides as well as fungicides is not lone source.
On the other hand, it is also pertinent to mention here that insecticides and fungicides increase the cost of production. Their use is risky for the health of the worker as well as the mango consumers of each age.
These decrease fruit quality because of their toxic substance residue that may remain for long in the tree and persist into the fruit. Therefore, the following management principles should be practiced on a top priority basis to cover the risk of environmental pollution and the toxic effect on human beings.
1. To keep trees healthy and avoid any unnecessary stress (drought, injury, nutrition, etc).
2. As the infested tree plants survive and often can recover with age, therefore larger spacing in nurseries and orchards help slow the spread from plant to plant.
3. Only healthy seeds, seedlings and plants should be used for planting and budding or grafting.
4. 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 the orchards.
5. Irrigation must be applied as per requirement, but not subject to availability.
6. Avoid high doses of nitrogen fertilisers, but apply proper and balanced fertilisers, on the basis of soil type, age and need of the tree.
7. 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 beetle and the fungi-causing disease complex situation.
Therefore heavily infested plants or plant parts should be removed and destroyed. Once attacked, the tree becomes more attractive to female beetles, therefore infested trees must be removed and burned before completing the life cycle of beetles that requires 55 days hence it be ensured that the attacked trees should not be a source of beetles.
8. Easily available broad-spectrum insecticides, in combination with fungicides should only be used when all other ways fail. Proper application methods with optimum dose must be confirmed. However, encouraging results could be obtained by consulting with an entomologist and pathologist.