There are increasing reports of leaves being darkened by a sooty substance that looks and feels like coal dust. There is no coal dust transportation through North Goa and the emission from cars and buses is alarming but not enough to cause such a deposit. So, what can it be? This is agitating many and my friend and mentor, Ashok Dande was the first to red flag this problem. It is not soot but a fungus that looks like it and is called ‘sooty mould’.
The sooty mould thrives on a sugary substance known as ‘honeydew’ excreted by a variety of insects from mango hoppers to mealy bugs and even ash white flies. While the fungus can be peeled off by spraying laundry starch or rice ‘canji water’, letting it dry as a film embedding the soot and then washing it off or letting the wind blow it off, the problem will recur as long as the original cause is not dealt with: the insects that produce honeydew. There are some non-toxic solutions to the problem.
There are many species of mango hoppers, flying insects between the size of a mosquito and a house fly, present in the cracks of the bark around the year but multiply rapidly when mango trees begin to flower and fruit in November-April period. This will lead to a reduction of mango yield from 20 to 100 per cent. The Idioscopus clypealis is the most common species of mango hopper in the Konkan, including Goa.
Nymphs and adults of Idioscopus species suck the sap from the flowers and leaves. The affected flowers turn brown and dry up. The farmers blame the dew or ‘murem’, which actually creates favourable conditions for the hoppers and the fungus to multiply. These hoppers excrete a sugary substance called honeydew which promotes the fungi like Meliola mangiferae causing black, sooty mould on the leaves. The excess drips off as an oily substance on the ground below. Look out and take corrective action.
The peak population is when the mango fruit is from pea to marble sized resulting in dropping of immature fruits. At the post-bloom stage a population of two adults per panicle is sufficient to cause yield reduction. Each female lays about 150 eggs which hatch in about one week. There can be four generations of mango hoppers within the season with a force multiplier of 150 for every female alive and mating.
Mealy bugs abound on mango stem and roots, hibiscus stems, sour-sop fruits and a host of other plants. Big black ants move them to younger shoots as cowherds move cattle to greener pastures. Honeydew is like milk to the ants. What is left over is immediately used up by the sooty mould.
A spray of a fungus formulation, either liquid as in “Bio Power” of T-Stanes or “Baba” of Multiplex or talcum-based as in “Daman” or Bio Power on the tree trunk and leaves before flowering is best to control these insects. Once the plant is flowering, it could interfere with the bees that pollinate the flowers. Spray now or limit to spraying on the tree trunk later.