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Disease destroying mango
trees
By M.H. Panhwar
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.
Courtesy : The
DAWN
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