Plant Diseases/black powdery residue on leaves
My gardenia bush has/had scale and teeeeeny white bugs. The leaves (except new growth) were covered with black stuff. I sprayed it thoroughly with Ortho Volk Oil spray. We have not had rain since I sprayed. The black stuff on the leaves is now drying up and is a flaky, powdery residue, which wipes off pretty easily. Is there a better way to remove this black nastiness? That's a lotta leaves to wipe individually. Can I just turn the hose on it?
The white insects are aphids. They suck the plant juices and secrete a substance called honey dew which is high in sugar and sticky. A black sooty mold will grow on the honey dew. This is the black on the leaves and stem. If you control the aphids then the black sooty mold will go away. You can use the hose to wash it off or wait until the rain washes it off. Either way the sooty mold will not harm the plant. If the oil did not control the aphids then I would try an insecticide called orthene. This is systemic and will kill the aphids easily.