It looks like your plant is stressed due to sucking insects (like scale insect, mealybug, aphids etc.) and thus resulting in the development of sooty mold (appearance of thick, black, sticky coating on leaves and stem).
Which insecticide did you spray?

If you notice any insects again spray neem oil. To control sooty mold spray natural detergent (make sure to cover all the parts of plant, including the lower surface of leaves). Leave it for one or two days and spray the plant with a strong jet of water to wash off detergent and mold.

Also provide proper fertilizer and adequate water to plants so that it can withstand the damage and grows well.

http://ipm.ucanr.edu/PMG/PESTNOTES/pn...
http://collier.ifas.ufl.edu/CommHort/...