If your apple trees are diseased, and the principal symptom is an appearance of "white fur" around the branches, it's likely that you have a case of woolly aphids. These aphids form colonies that resemble soft, white fur or thick cobwebs in a pure white colour. While the insects target a few other trees, including elm and pine, the apple tree is among their favourites.
The full-grown woolly aphid has a purple colour. However, it's unlikely you'll notice the insect's purple carapace, as it's generally covered by a thick mass that resembles cotton or fur. The insect secretes this material around itself as protection from predators. In fact, the material even serves as protection from insect sprays, rendering them less effective against the insect. As the woolly aphid usually lives in colonies of many insects, you can usually spot a large cloud of the white substance, shrouding the entire colony in long strands. Woolly aphids may or may not have wings; an aphid born in the summer is wingless, whereas the generation born in the autumn has wings, useful for spreading the population to additional trees. Wingless aphids may remain on the same tree for their entire lives, sometimes going below ground during the winters.
The aphid's life cycle is tied to surviving the winter. Aphids born in the late summer are born as males and females. During the rest of the year, almost all of the aphids are born female. The generation of aphids produced from these late-summer aphids are egg-producing aphids, which lay their eggs in sheltered nooks and crannies in the tree's bark or around buds to survive the winter and hatch in the spring. The rest of the year, the female aphid population reproduces asexually, through parthenogenesis. A newborn aphid can reach maturity in 10 to 14 days. An adult aphid produces up to five offspring daily throughout her one-month lifetime.
To monitor for woolly aphids, check the tree's scars from pruning for any signs of forming colonies. Gently blowing on the white "fur" will remove it and reveal the live aphids below. Some insecticides intended for removing the aphids include diazinon, thiodan and dimethoate. However, these will only control above-ground populations, while more aphids may continue to live underground, around the roots. You can also smother the insects with horticultural oils, best applied during the dormant winter season. Among the aphid's natural predators are ladybirds, syrphid fly larvae and lacewing larvae.