Hello! First of all, it is worth mentioning that black flies can be fungus gnats, which infest mainly the soil, but also the substrate in pots, containers, etc., feeding mainly on decaying organic matter. As their development is also favored by high, persistent moisture in the substrate, a first measure that can be applied is to pay attention to the frequency of watering, letting the substrate dry slightly before watering again, while complying with the moisture requirements of each species. You may consider replacing the first few centimeters of substrate (they can usually be found there) with a new, fresh one (). Additionally, for monitoring and catching flies, it is recommended to use an adhesive trap such as . The pests are attracted by the color of the trap and stick to the special adhesive.
Secondly, the symptoms in the attached photos may be caused by several abiotic or biotic factors such as: light, atmospheric humidity, watering, temperature, possibly a constant high humidity in the substrate or a disease, etc. In this case, it is recommended to regularly remove the affected leaves with sharp and disinfected tools. You may also consider a fungicide-bactericide treatment such as , 20 g/10 L water, applied by spraying.
7 days after the foliar treatment, you may consider using an organic fertilizer spray containing beneficial micro-organisms that increases the plants’ resistance to stress factors, such as .
Next, it is recommended to take proper care of the plants as described in our articles: Hibiscus syriacus, planting guide and care work; Dieffenbachia, plant care and growing guide.