Powdery Mildew

• Powdery mildew is a common disease affecting trees, shrubs, vines and herbaceous plants throughout the United States. According to Colorado State University, powdery mildews are adapted to certain hosts and are often unable to spread from one species to another. There are many fungal species described as powdery mildew, but they all produce similar symptoms. Colonies of these fungi are white or gray and resemble a powdery substance. Mildew mostly attacks the host’s leaves, but it can spread to limbs and stems as well as fruit, flowers and buds.
  
  

Cottony Maple Scale

• Cottony maple scales (Pulvinaria innumerabilis) are tiny insects often measuring only a few millimeters long that travel in large swarms. They are prominent maple tree pests but attack a variety of hardwood tree species. The adult insects are gray or brown. They produce large, white egg sacs that resemble balls of cotton. Due to their ordinarily large populations, hundreds of egg sacs may be visible on a single tree. The females lay their eggs during late spring and early summer, so the white sacs are only visible for a month or two. The insects also damage their host tree, causing discoloration of foliage and killing young stems. Infestations are also associated with the growth of sooty mold fungi, which causes unsightly superficial damage.
  
  

Woolly Aphid

• Woolly aphids are prolific pests that infest conifers, hardwoods and fruit trees. According to the University of Minnesota, these insects are only 2 to 4 millimeters long and are covered in white hairs. A population of aphids on a single tree can number in the hundreds or thousands, and they may completely cover the branches, leaves and stems where they feed. While they would normally be difficult to see, a large number of individuals may resemble clumps of cottony growth on the tree's limbs. Woolly aphids drain vital fluids from the tree's bark, foliage and roots, decreasing their host's growth potential and damaging its foliage.
  
  

Wood Decay

• Wood decay fungi attack decaying and living wood of trees, which makes them a significant threat to crops cultivated for lumber. Several of these fungi, like Schizophyllum commune, produce white fruiting bodies called conks on the branches or trunk. These reproductive bodies often emerge in localized clusters. Damage from an infestation may reach well beyond the conks into the living heartwood. Infections of other wood decay fungi can result in white rot of living wood, which causes infected portions of the trunk or branches to turn white and lose their rigid consistency.