THE present mango disease that has devastated many mango trees first in the Punjab and recently in Sindh, is visually identifiable by slowly yellowing and wilting leaves in scattered areas. And, as the disease catches hold, the main branches begin to die, the leaves turn brown but most remain attached to the trunk as if the tree had been burnt by a trash fire.

The wood of the affected branches under the bark invariably show a brown discolouration and the severely affected plant gradually dies. This disease, V Verticillium Wilt, exists the world over but in its present epidemic form as in Pakistan, there are no reports from any other country to the best of the author’s knowledge.

An attempt to restore the vigour of an affected tree is difficult. However, some young nursery plants grown on land previous sown with tomato in Florida were at times saved by soil injection of seqestrene at the rate of 12 grams per one litre of water per tree, sprayed on the ground under tree canopy, as reported in Florida State Horticulture Society in 1976.

It is suspected that this disease came from the previous tomato crop in the field and the rotting tomatoes left on the soil surface. The fungus first attacked that and entered the soil which caused the disease. A similar disease is also spread by potatoes fields as per reports. This disease has been observed in subtropical and temperate zone fruits, such as cherry, nectarine, pome fruits, citrus, avocado, mango, persimmon, walnut, pistachio, olive, apricot and almond and also in raspberry, strawberry and melons. Among field crops and vegetables, it is found in cotton, alfalfa and egg fruit or brinjal.

The disease makes its appearance through the sudden wilting of foliage on one or more branches in the summer. The primary effect of the disease on trees is low productivity and poor growth. When the bark of the affected branches is removed, a portion of the underlying sap wood colour is found to be brown to black. This symptom has also lead to its name, black heart.

Different names are given to pathogen like verticillium alboatrum (1879) and Verticillium dahliae (1969). New studies shows that the two are caused by different fungi, and albo aturm is more pathogenic to tomato, potato and cucumber than V. daliae. The latter can grow even at 30°C, whereas the former can not, ideal temperatures for which are 2024°C.

The two fungi live from one year to the next in the soil, to a depth of 3036 inches and some times to 48 inches with maximum concentration in the upper 6 to 12 inches. They can survive at least one year and in some cases longer, for a few years. If the soil is flooded, the microsclerolia population reduces rapidly but at favourable soil moisture levels, the population again rises to the surface for several months.

Since soil temperatures at two to four feet depth are about the annual average temperatures (27.5°C for Hyderabad, 26°C for Larkana and 27°C for Jacobabad), it is certain that the fungi will survive at 36 to 48 inches depth from one year to the next. Infection is less in dry than in moist soils.

Microclerotia remain dormant until stimulated to germinate by root exudes. Roots grow during the dormancy of the tree and after the harvest of the fruit. In these two seasons, different tree roots support the nutrient requirements of different branches and it is easy to know which roots are damaged leading to the wilting of certain branches.

The fungus spreads to other lands through tilling tools like ploughs or harrows or manual soil working tools like spades or grafter and budders knives. Root stocks used for grafting but produced in infected soils can propagate the disease. Studies also show the incidence to be corelated with potassium deficiency in the leaves, with poor tree growth and yield. The disease exists throughout the world, but until now an understanding of the disease has been incomplete.

One method of control is to avoid inter-cropping of orchards with susceptible plants such as cotton, tomato and potato, but this is difficult as orchards designed in the 19th century have wide spacing for intercropping. The returns were from the intercrop for about two decades rather than the fruit crops. This design was continued with in the twentieth century and even now. There are some weeds susceptible to fungus in Sindh but these have not been studied or segregated. Nitrogen and potassium fertilisers applied through the soil reduce the disease in susceptible plants but do not eliminate it.

Blood meal, fish meal, cotton cake and ammonium sulphate applied to the soil can also reduce the inoculation potential of the fungus. Chloropicrin applied to the soil at the rate of 2 to 2.5. ml per square foot at a depth of 12 inches is effective in destroying the fungus. This fumigant moves in the soil vertically down, horizontally and also upwards. Clean cultivation promotes the problems, whereas maintenance of sod strip between tree rows offers some control. Trunk injection by fungucides have not helped and application of sequestrine (chelated iron) has given mixed results.

The disease is prevalent in the whole of Sindh now but in the Punjab the disease arrived some years earlier and at some places tree loss has been over 15 per cent. Hitherto, on our farm, only two mango trees have died from the disease but these were isolated trees in the area of other tree crops. Proper care of them was also not taken in terms of application of macro and micro nutrients, disease control and eradication of weeds by mulching for some years.

Studies have shown us that symptoms started with the rotting of roots, colour change under the bark, drying of wood, separation of bark followed by attack of beetles and borers in search of fungus for their food and the oozing out of the dark brown liquid at the crotch.

The typical symptom is yellowing of the leaves first, followed by their gradual decay to brown colour but they still stick to the branches even when the whole tree dies. On cutting down the infected branches, it was seen that the trunk had been infected below the crotch. By cutting down the trunk a couple of feet above the ground and attempting to barkgraft the tree, the operation was a total failure as the main trunk had been infected and was dying. It was interesting to observe that part of trunk was almost bone dry and part was healthy, but on the way to its death. The root system had also decayed, which was the beginning of the infection.