I apologize that no one has responded to your question sooner. Different experts
come online at various times. I just came online and saw your question. I'm sorry to hear your plants are having a problem.
The chemical reaction between sugar and baking soda requires a catalyst. It proceeds like this:
O + CO2
OH + 3 O2
-> 2 CO2
+ 3 H2
The end result is nothing more than carbon dioxide, which is in the atmosphere all the time. Placing dry sugar and baking soda at the base would most likely do nothing at all. However, from what you have described, the black on the leaves is something called sooty mold. The mold is only a sign of an underlying problem. Sooty mold grows on a sticky substance called honeydew, which is excreted by several different kinds of tiny insects, including aphids, mealybugs, scale, and white flies. These can be difficult to see. Mealybugs and scale can produce the white webbing you see. These tiny pests can cause a lot of damage and even kill a plant if left untreated. I'll give you the method I use on my own inside trees to deal with these. Put them in the shower and give them a thorough bath. If the plants are outdoors, spray them with a hose. After that, spray it thoroughly with an insecticidal soap. These soaps are available in garden shops, nurseries, and plant departments. Then, if the plants are in pots, remove the plant from its pot. Remove as much soil as you can without damaging the roots. Some life stages of the pests live in the soil. Put the plant in a different pot with sterilized soil. If the plants are growing in the ground outdoors, soak the area underneath them with the insecticidal soap. Inspect the plants daily after that. If you see more webs or sooty mold, repeat the treatment with the insecticidal soap and also spray the soil surface. It can take several treatments. These little bugs are difficult to get rid of.
If you prefer a more potent insecticide, your garden shop will have one labeled for these aphids, mealy bugs, white flies, and scale. Another option is to use a horticultural oil labeled for use during the growing season (not a dormant oil). Regardless of the method you use, you'll probably need to treat more than once.
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