Q: I have a lot of black film on mango and longan leaves; I can rub it off with my fingers.
Sprayed with copper last week–how often and how many times should I spray? I want to get rid of it before the trees blossom. Anything else I should spray them with? (asked early January; copper fungicide doesn’t help with this problem)
A: That sounds like black sooty mold. It is caused by bug poo dropped from leaves above. Aphids, scale and mealy bugs, etc. suck the sap out of the leaves, and their droppings (sometimes called honeydew because it has a high sugar content) then coat the leaves below and the mold grows on it. Look closely on the underside of the leaves above and you will probably see the bugs. A 10x loupe helps if it is something even smaller. Also look for ants. They often “farm” bugs and eat the honeydew, so even if you kill the bugs, the ants may bring in more. The ants also chase off beneficial insects (lady bugs, lace wing larvae) that would otherwise eliminate the problem.
You can use an oil or soap spray before 9am or after 5pm (so the sun doesn’t burn the leaves.) It will suffocate the live bugs, but not any eggs. Be sure to coat the leaves top and bottom. Spray again every few days for a week or two as the eggs hatch. Compost tea sprayed late afternoon adds beneficial microbes that reduce bug infestation.
Better still try this trick to keep ants out of the tree. That way the beneficial insects will have lots of food eating the aphids, and will then be able to reproduce and take care of your whole garden. You need to keep at it since ants are persistent, they may crawl over previously stuck ants or wind blown debris, the sticky stuff needs to be replaced regularly, and ants may find alternate routes if leaves touch another tree or the ground.