So in a 2-week period in February I was getting calls and emails from growers asking about the black stuff forming on the green stems of avocado trees. I thought that it was just sooty mold that was the result of insect feeding and resultant honeydew. This is a superficial fungus on stems. In some plant species, it grows on leaves and can cause defoliation. I haven't seen it on avocado leaves, just on stems.
A concerned grower brought some samples in for evaluation. When I went to rub it off, a black discoloration remained. There was sooty mold, but there was something else. So I sent it to plant pathologist, Akif Eskalen at UC Davis for PCR analysis. It came back as Capnodium sp, sooty mold. But there was more.
Sooty mold is a fungus in a saprobic (non-living tissue) association with sap-feeding insects in the Order Homoptera, which includes aphids, whiteflies, soft scale, mealy bugs, leafhoppers and psyllids. The Capnodium species of sooty molds grow on honeydew, gradually covering the surface of the plant part affected by insects, coloring it with various shades of black. These fungi do not colonize the inside of plant tissues. However, they alter the ability of the plant to perform photosynthesis and transpiration (clogged stomata that cool the plant) and can cause significant problems in some plant species. Severely affected leaves may defoliate, thereby affecting plant growth and survival.
So Akif sent back these photos of before and after rubbing the affected stems. In the first photo, you can see the sooty mold, in the lower, you can see the sooty mold rubbed off and what remains is SUN burn. No doubt, this is due to the increased heat due to the black color, but also the clogging of the stomata that cool that portion of the stem through evaporative cooling.
Blackened areas before rubbing
Still some blackened areas - sunburn
So the honeydew came from insect feeding and fungus grew on the honeydew which caused a blackening and heat damage. Quite a chain of events. But where were the insects that caused the honeydew? I didn't see them on the samples that were brought in, but there's a lot of insects on avocado trees that are normally there. They don't normally cause problems, but they are there and occasionally unleash more honeydew than normal.