Have You Seen Black Spots on Avocado Stems?

Dr. Ben Faber recently shared a blog post noting California avocado growers were reporting “black stuff” on the green stems of their avocado trees. Sooty mold such as this is a result of insect feeding and is considered to be a superficial fungus. However, after reviewing a few samples, Dr. Faber noted that when he rubbed off the black sooty mold, some black discoloration remained. He sent the samples to Dr. Akif Eskalen for further analysis.

Dr. Eskalen’s PCR analysis confirmed the sample had Capnodium sp, sooty mold, which is associated with sap-feeding insects. While the fungi do not colonize plant tissues, they can impede a plant’s ability to effectively photosynthesize and transpire (cool the plant). In severe cases, this could lead to defoliation.

Dr. Eskalen also determined that the black discoloration left behind on the stem after rubbing off the sooty mold was sunburn. The researchers determined the sunburn was a result of the black coloration of the mold increasing the heat of the stem and the fungi clogging the stomata whose job it is to cool the stem. While incidents like this don’t normally lead to problems for the tree, it is something for our growers to be aware of as they walk their groves.