Diseases and Pests Oyster shell scale and Maple leaf blister

Pest of the Month: Oystershell Scale Disease of the Month: Mapleleaf Blister

by Heather Prince

Pest of the Month: Oystershell Scale

Oystershell scale (Lepidoasaphes ulmi) is a common armored scale that can develop heavy infestations that Oystershell scale thickly coat twigs and branches on several woody plant species, including birch, crabapple, dogwood, elm, maple, poplar, walnut and willow. They often cluster together, heavily covering individual branches rather than evenly distributing themselves among all of the twigs. Spoon-shaped and resembling oysters, adults are about two to three millimeters long and can be gray or brown. Eggs overwinter under the protective covering or “scale” after being laid by females. The young crawlers hatch, emerge from the protective scale, and become active from May through June. Crawlers will choose a location to settle, then pierce the plant, and suck its fluids. They remain in that location throughout adulthood. Their feeding can result in yellow leaves, leaf or twig stunting, twig dieback, and occasionally plant death. Heavy infestations can also stress the plant, making it more susceptible to additional issues.

Treatment:

Cultural practices like irrigation, fertilization, and mulching can help reduce plant stress and allow plants to better withstand infestations. If chemical treatment is warranted, treatments should be applied when crawlers are present. Adults are not killed off by pesticides because the products do not penetrate their protective scales. Oystershell scale crawlers have two distinct biotypes which differ in color and timing of activity. Brown oystershell scale crawlers are present when 275 to 500 degree-days have accumulated (base 50) or when VanHoutte spirea (Spirea x vanhouttei) is in late bloom. Gray crawlers are present when 400 to 600 degree-days have accumulated (base 50) or when VanHoutte spirea has finished blooming. Scout carefully as crawlers are tiny and hard to find. Use a hand lens or place two-sided tape around a branch and inspecting it for crawlers. Insecticides recommended for controlling oystershell scale crawlers include acephate, carbaryl, malathion, insecticidal soap and horticultural oil. For a more sustainable approach, oystershell scales can be mechanically removed by gently scraped away from the bark using a mesh dish scrubber. Heavily encrusted twigs or branches can be pruned out.

Disease of the Month: Mapleleaf Blister

As usual, a cool, wet spring brings a variety of fungal problems, including maple leaf blister. A type of Taphrina, it commonly infects silver and red maples and their hybrids. This group of pathogens infects leaves early in their development, frequently at bud-break. As the leaves mature, they become resistant to the fungus, so there is effectively one infection cycle per year. The pathogen resembles green blisters that do not cross leaf veins or infect petioles. The blisters quickly transition a Mapleleaf blister brownish-black color.

Treatment:

Maple leaf blister damage is cosmetic, although may cause client alarm. It may cause some defoliation, but a new flush of leaves quickly emerges. Several fungicides are labeled to control Taphrina diseases, but their use is generally limited to fruit orchards. These fungicides are preventive and won’t have any effect on current infections.

Additional resources:

University of Illinois Extension Service https://web.extension.illinois.edu/state/ horticulture/index.php 217-333-0519 The Morton Arboretum http://www.mortonarb.org/Plant Clinic: http://www.mortonarb.org/trees-plants/ tree-and-plant-advice/ 630-719-2424 Chicago Botanic Garden Plant Information Service: https://www.chicagobotanic.org/ plantinfoservice 847-835-0972