I have a grapefruit tree that has a green film growing all over the trunk and branches. The branches have lost their strength and the fruit is small. The leaves have fallen off some of the branches and the ones that remain are curled. On some of the fruit a black mold developed but was easy to wash off. They were still tasty but not so pink. What do you suggest I do?
Certified GKH Gardening Expert
The green film is likely algae or mold but the black mold is sooty mold and is common on citrus trees. While unsightly, it's usually harmless. The mold grows on the sweet honeydew (which could also be attributing to the greenish mold) from sap-sucking insects like aphids, scale bugs, etc. You'll first need to treat the offending insect pest (hosing off plant and using garlic spray, insecticidal soap, or horticultural oil). We prefer neem oil, which is safe and effective. As a bonus, neem oil also make a great fungicide, so it will usually take care of both issues. Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm, https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm