Identifying leaf problem on mandarin trees
We recently moved from Texas to Ecuador. The home we bought has several fruit trees including two Mandarin orange trees. I raised satsumas in containers in Arlington. There, some of my trees would get something similar to what the attached photos show. I would usually spray with high pressure hose to stop infestation. My trees in Ecuador are thick in foliage and this black mold with mites are all over. What non chemical treatment would you recommend? Thanks for your time. Billy Henry
Outside United States
It looks like you may have whiteflies and scales or mealybugs from what I can see in the pictures. The overall look of the plant also appears to be too light green in color, which may be a nutrient deficiency (maybe caused by the excess rain you have in Ecuador) and is also encouraging the pest problems.
The black mold is caused by the sucking insects on the plants. It is not harmful to the plant, but is an indicator that you have a pest issue.
Non chemical control options may be to use soapy water to do an initial cleaning. Neem and/or pyrethrin sprays may also help, rotating with the soapy water. The mold will not go away unless you scrape it off or use good blasts of water. Neam, soapy water, and pyrethrin all are not a one time fix, you'll likely need to reapply every two weeks for a period of time.