Winter Tree Care: Identifying Common Tree Diseases in Winter, Part 9

This is the ninth part of a series on winter tree diseases and disorders. This article examines powdery mildew and rhabdocline needle cast.


During the winter months, many of the fungal pathogens that affect trees enter dormancy. The pathogens overwinter on their hosts or in the soil, awaiting spring’s arrival. Despite the frigid temperatures, trees suffering from fungal diseases may still exhibit infection symptoms in winter, especially if the disease has advanced into its later stages. The following describes some of the most common diseases to overwinter on trees, and how they may be detected.

Powdery Mildew

Powdery mildew is a fungal disease that affects a wide range of plants. It is caused by numerous fungi in the order Erysiphales, of which Podosphaera xanthii is the most frequently cited. Powdery mildew is one of the most recognizable plant diseases. It forms white, powdery patches on the leaves of infected plants. Powdery mildew seldom causes significant damage to host plants, though severe infections can reduce their ornamental value.


Powdery mildew infects thousands of plant species. The fungi that cause powdery mildew generally target specific hosts, and only infect plants from the same genus or family. Only a few species of powdery mildew fungi are capable of initiating infections in multiple plants. Many plants are susceptible to several different species of powdery mildew fungi. Trees and shrubs that are vulnerable to infection include apple, ash, azalea, barberry, birch, buckeye, catalpa, cotoneaster, crabapple, dogwood, elm, euonymus, hawthorn, ironwood, lilac, linden, magnolia, maple, oak, serviceberry, spirea, rhododendron, rose, and viburnum. Powdery mildew infections also occur on many different flowers, including African violets, begonias, English ivy, lilacs, kalanchoe, photinia, roses, snapdragons, and zinnias. Powdery mildew infections are common on many crops. Beans, cucumbers, pumpkins, tomatoes, squash, and zucchinia are especially prone to infection.

Symptoms of Infection

Powdery mildew infections are distinctive. Infected plants display white powdery patches on the leaves and stems. Lower leaves tend to be the most affected. As the disease progresses, the patches enlarge. An abundance of asexual spores are formed, and the mildew may appear on the rest of the plant. The first visible symptoms are raised blister-like areas on young leaves that cause them to curl, exposing the lower leaf surface. Infected leaves become covered with a white to gray powdery growth, usually on the upper surfaces. On plants that produce flowers, flower buds may become laden with mildew, and never blossom. The leaves of severely infected plants turn brown, and drop prematurely. Powdery mildew prefers young, succulent growth. Mature leaves are generally not affected.


  • When planting, select resistant plants, and place them in sunny locations.
  • Prune plants to promote sun exposure, and improve air circulation.
  • Remove diseased foliage from infected plants. Disinfect pruning tools between each cut. Use a solution comprised of nine parts water and one part bleach.
  • Maintain plant vigor through sound cultural practices. Ensure that plants are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of plants to improve soil quality, moderate soil temperature, and retain soil moisture. Use a slow-release, organic fertilizer to feed crops. Avoid excessive nitrogen applications.
  • Gently wash foliage occasionally to disrupt the spore-releasing cycle.
  • Neem oil and PM wash can be applied on plants that are grown indoors. These will help prevent fungal infections.
  • Chemical treatment of powdery mildew is possible. Fungicides are effective at combating the disease. Treat plants with a silicon solution. Silicon assists the plant’s cells in defending against fungal attacks by degrading fungal haustoria. Potassium bicarbonate and baking soda may also assist in treating powdery mildew infections.
  • On select plants, such as summer squash, pumpkins, grapes, and roses, milk is an effective organic treatment. Milk is often applied to infected plants by home gardeners. Milk should be diluted with water, and sprayed on susceptible plants as a preventative measure, or at the first sign of infection. Repeated weekly applications can help control or eliminate the disease.

Rhabdocline Needle Cast

Rhabdocline needle cast is one of the most serious diseases to afflict Douglas-fir. The disease is caused by the fungal pathogens Rhabdocline pseudotsugae and Rhabdocline weirii. When infected, Rhabdocline needle cast causes needles to become discolored. Eventually, the infected needles are blighted. Blighted needles are often prematurely shed. Rhabdocline needle cast outbreaks are common, and may affect Douglas-firs in landscapes, gardens, and Christmas tree nurseries. Douglas-firs in the advanced stages of infection often assume a distorted appearance. Douglas-firs that suffer from repeated defoliation tend to be infiltrated by secondary pests and other disease pathogens.


Rhabdocline needle cast infects Douglas-fir. Douglas-firs originating from the Rocky Mountain states are the most prone to infection.

Symptoms of Infection

Symptoms of Rhabdocline needle cast generally become apparent within 1 to 2 years of infection. Yellow to brown spots or lesions develop on the upper surfaces of the current year’s needles. In time, the lesions turn reddish-brown. Infected needles may feature one or multiple lesions. When trees are heavily infected, lesions can coalesce to cover entire needles. The fruiting bodies can be observed on the lower surface of infected needles. When the fruiting bodies have exhausted their spores, the previous year’s needles will typically be shed. Severely infected trees may only retain the current year’s needles. Infected needles are generally killed within 2 to 3 years.


  • When planting Douglas-firs, select cultivars that exhibit an increased resistance to Rhabdocline needle cast. Rocky Mountain Douglas-firs are more vulnerable to infection than Pacific Coast Douglas-firs. Provide trees with ample spacing to promote healthy plant growth. Plant Douglas-firs in well-drained, damp soil.
  • Inspect susceptible trees for disease symptoms in late winter or early spring. Conduct examinations of trees on overcast days, when needle discoloration is most visible.
  • Prune out heavily infected branches or shoots. Disinfect pruning equipment in between each cut with a solution composed of nine parts water and one part bleach.
  • Perform general maintenance pruning on trees to improve air circulation throughout the crown, and to encourage a rapid drying of the foliage when wet.
  • When removed, infected shoots can be placed in warm water for several minutes to determine the maturity of the fruiting bodies. If mature, the fruiting bodies will release a mass of ascospores into the water.
  • Cull severely infected trees to reduce the amount of inoculum present in an area, and to prevent the spread of the disease.
  • Maintain trees through sound cultural practices. Ensure that trees are sufficiently watered, especially during extended periods of drought.
  • Apply a layer of organic mulch around the base of vulnerable trees in spring and fall to improve the soil conditions, and to retain soil moisture. Trees can be mulched in fall once they have entered dormancy. Avoid constructing mulch piles around tree trunks. This can encourage moisture accumulation.
  • Fungicidal applications can stave off Rhabdocline needle cast infections, particularly on small to medium-sized trees. Perform initial applications in spring, when the shoots have started to expand. Administer a second application 2 to 3 weeks later. If rainfall is abundant, additional applications may be necessary to achieve control.

Photo courtesy of Jeff Kubina CC-by-3.0