Answer: Bacterial leaf spots are caused by lots of different kinds of bacteria and the best course of action is to reduce or eliminate the cultural conditions associated with the development of the disease. In order for a disease pathogen to get a foothold, it needs a susceptible host and the right environmental conditions such as a film of water on the leaf surface, coupled with poor air circulation. Plants under stress tend to attract disease and insect problems, so your hibiscus may be struggling with over or under exposure to sunshine, poorly draining soil, or competition for moisture and nutrients with the roots of other plants.
Try removing the affected leaves (or even an entire branch) as soon as you notice black spots developing. This will remove the pathogens from the area and eliminate the possiblity of them splashing onto healthy plant parts. Removal of leaves and/or branches will also open your plant up to better air circulation, which will allow the foliage to dry quickly after rain (or water from the sprinkler). Finally, try not to get water on the foliage. If the leaves are dry from late afternoon on, they won't be so prone to developing disease problems. Hope this information is helpful!
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