Powdery mildew affects many plants, from roses to green beans. Fruit growers have a particular interest in keeping it off their trees, as the fungus can damage fruit and leaf growth, especially on new growth. The fungus that causes powdery mildew is called Sphaerotheca pannosa, which thrives in cool, wet and humid conditions. Prevention is key to controlling powdery mildew, but there are also organic measures you can take to stop infestations on your apple or other fruit trees.
Prevent powdery mildew by watering your trees correctly. Water the soil at the base of the tree; don't spray or splash water onto leaves and branches, as water on the leaves encourages mildew growth. Avoid overwatering so that the humidity surrounding the tree doesn't get too high. Only water when necessary.
Keep trees clean and uncrowded. Prune dead, dying or broken branches, twigs and leaves regularly to provide fewer opportunities for mildew to set in. If your apple trees are growing in a thick clump where their branches touch one another, train or prune them apart. In extreme cases, you may want to thin out the trees, transplanting some to a different location.
Avoid overfertilizing. Nitrogen helps plant growth, but it also boosts fungus growth like powdery mildew. In organic growing, this means easing off of high-nitrogen fertilizers like chicken manure or compost. Fertilize just once in the spring.
Spray your trees with liquid seaweed. This is a treatment safe for organic growing that works especially well on fruit trees. It will help encourage plant health and repel the mildew at the same time. Pour liquid seaweed at the bases of the trees if you don't have a sprayer.
Spray the trees with a mixture of skim milk and water. Mix the milk and water at a ratio of about one part milk to nine parts water, and keep the mixture around room temperature so as not to shock the trees with cold liquid. The solution kills mildew, and the fat-free milk will not attract other fungi.