Mildew and black spot is encouraged by cool, damp weather in the spring and then dry soil in the summer. Plants that are stressed by lack of food or water are most likely to be attacked. Fertilizing with Osmocote will not kill mildew or black spot as it is not a fungicide. If your honeysuckle is suffering from lack of nutrients, using Osmocote around the roots in the spring could help it resist the fungi. Some other suggestions are to clean up the fallen leaves around the vine so they do not reinfect the vine. Then add a layer of compost mulch on top of the soil to seal in any mildew spores that have fallen on the ground. Make sure the plant's roots do not dry out in the summer. If the new spring growth is not \"clean\" or free of disease than I recommend you give up, hack down the honeysuckle and try a vine with a better attitude - trumpet vine in a hot spot or clematis if you live where summers are cool and damp. Life is too short to put up with sickly plants. Also consider annual vines that live for just one year. Black-Eyed Susan vine or Thungbergia will cover a lot of space in just one season, thrives in full sun and I have never seen this vine get mildew. Just grow for it, Marianne Binetti