How to cure your tomato plant of the dreaded horn worm If you have tomato plants that are chewed up and missing pieces you probably have tomato horn worms. You can control horn worms using insecticide or you can take them off my hand. If your pecan leaves have yellow splotches and black spots on them they are probably infected with the black pecan aphid. You want to used insecticides approved for pecans and you need to be sure that the pecans have not begun to open. Lilacs that have a white substance on them most likely have powdery mildew which they are very prone to. For this problem you will use a fungicide that is approved for powdery mildew to correct this problem. Texas mountain laurels have a problem with caterpillars that feed on the leaves. Use an approved insecticide to take care of the caterpillars. If you catch it early enough you can just break the affected leaves off. So choose the right treatment to take care of your plants.