In this Issue:
- Anthracnose: Infections on sycamore, maple, and oak may occur under rainy conditions
- Lilac-ash borer: Treat susceptible tree trunks when lilacs are in full bloom, through early July
- Elm insects: Elm leafminer and elm flea weevil active in northern Utah soon
- Powdery mildew: Monitor plants for white mycelium after leaf-out
- Sequoia pitch moth: Apply trunk spray in mid-May (northern Utah)
- European pine sawfly: Larvae visible in mid to late May, feeding on needles
Anthracnose of Sycamore, Maple, and Oak
Plan to treat foliage before or between rains
Anthracnose is a fungal-caused disease that thrives in cool, wet weather. It causes blotch and blight symptoms on leaf shoots. Leaves that are infected in early spring will drop, and new infections may occur with additional cool, wet weather.
When temperatures increase and rains stop, anthracnose will become inactive, and infections will stop for the season. In addition, the tree will produce a new flush of healthy foliage.
Maple and oak anthracnose overwinter in fallen leaves, so these diseases are the most severe in natural or wooded areas where the fallen leaves collect from year to year.
Sycamore anthracnose is more harmful because the fungus invades the wood of twigs, causing small cankers. Each spring, these cankers produce spores which infect foliage.
- Maple and oak anthracnose are treated by applying a preventive fungicide as leaves expand. For commercial applicators, there is a long list of fungicides (Abound, Heritage, Luna Sensation, Reliant, Topguard). For backyard applicators, options include Spectracide Immunox, Monterey Fungi-Max, or Natural Guard Copper Soap (organic). Raking leaves in spring and fall will also help.
- For sycamore anthracnose, the same fungicides listed above can be used. For specimen trees, a professional applicator can apply a trunk injection (Arborfos, Arbotect) in spring or fall that will provide up to 2 years of protection.
Treat susceptible trunks monthly, starting lilac full bloom, until mid-July
The emergence of lilac-ash borer adults coincides with full bloom of lilacs, indicating the time to protect tree trunks against egg-laying.
Green and white ash (Fraxinus) are the most susceptible. Sometimes, mountain-ash (Sorbus) and privet are attacked.
Lilac-ash borer does not directly kill trees, but repeated infestations can cause branch dieback and can leave trees susceptible to breakage in storms. Infested trees will have round exit holes on the bark, sawdust-like frass near the holes or at the base of the tree, and rough, swollen, cracked bark, mostly near branch crotches.
Healthy plants are able to withstand minor infestations, while stressed plants are more susceptible to attack and failure, so give trees optimal water and fertilizer, and prune properly.
Insecticides target the adults. Small trees can be treated by the home gardener, but in order to get thorough coverage on large trees, treatments should be made by a licensed pesticide applicator.
Applications should protect the bark for 6 weeks (usually until mid-July), so repeat as needed.
- Residential options: Hi-Yield Permethrin, Spectracide Triazicide (lambda-cyhalothrin); spray trunks a total of twice, spaced 3 weeks apart. An organic option is any product containing spinosad or azadirachtin (Safer BioNeem), but applications must be made weekly.
- Commercial options: Acelepryn (chlorantraniliprole), permethrin (Astro, Covert, Prelude), or bifenthrin (Onyx)
Elm Insects: Flea Weevil and Leafminer
If these are issues in the past, treat trees in the next few weeks
Siberian elm is not our favorite tree, but still, it is good to know about two of the (many) pests that are active soon.
The elm flea weevil is a tiny insect that often goes unnoticed. It overwinters as an adult, and starts feeding on newly emerging foliage. It hides on the undersides of leaves, chewing tiny holes. Females lay eggs along the leaf veins, and the larvae mine the inside of the leaves for several weeks. The adults are around through mid to late summer.
Elm leafminer is a sawfly whose feeding will be noticed soon. The larvae also feed within the leaves, between the upper and lower layers. If several larvae are mining one leaf, their mines will coalesce, leaving the entire leaf brown and hollowed-out. The full-grown larvae emerge from the leaf and drop to the ground where they remain until the following spring, when they then pupate to an adult.
Insecticides are rarely needed, but if applied, should target the adults of both species.
Powdery Mildew of Ornamentals
Treat foliage as necessary
As the temperatures warm, powdery mildew will start to show up on many ornamental plants. Be sure to scout early and often to catch this disease before coverage is over 10% of the plant.
- Residential: Avoid overhead irrigation, improve air circulation between plants, and rake fallen leaves in the fall. Fungicides include horticultural oil (0.5%), potassium bicarbonate (Bicarb), or Bayer Advanced Natria applied once per week as organic options. Conventional options include Spectracide Immunox, or captan that are applied once every 2 weeks.
- Commercial: There are many fungicides, including sulfur products, Banner Maxx, Bayleton, Eagle, Heritage, PropiMax, and more.
Sequoia Pitch Moth
Treat trunks of pines in mid-May
The Sequoia pitch moth (Synanthedon sequoiae) is a clearwing moth whose larvae feed on the outer and inner bark of pine trees. Their feeding stimulates the tree to produce copious sap that forms large, drippy masses on the trunk.
From mid-May through August, adult moths lay eggs on bark, often preferring pruning or other wounds, branch collars, or existing pitch masses. When the eggs hatch, the larvae bore through the bark and create meandering tunnels in the cambium, or may bore into the sapwood. The feeding is usually localized around the pitch mass, and sometimes, the larvae will exit the wood and feed on the pitch.
Smaller trees are at greatest risk of being affected by pitch mass borers. Larger, healthy trees can withstand moderate attack.
Keep in mind that after the moths exit the trees, the pitch mass will remain on the trunk, possibly for several years, unless they are removed by hand.
Management should focus on keeping trees healthy, with the following advice:
- Avoid pruning pine trees from early spring to early fall and only prune from October to February.
- Avoid any other type of wounding such as from bad pruning cuts, mowers, etc.
- The most susceptible pines are scotch pine, Alleppo pine, Austrian pine, lodgepole pine, Japanese black pine, ponderosa pine, and Monterey pine. If you intend to re-plant a pine, consider a 5-needled pine.
- Trees under drought stress are much more susceptible, so apply adequate irrigation.
- Manually remove existing pitch masses with a strong putty knife to prevent further egg-laying. Make sure the larva inside the mass (or against the bark) is killed (one larva per pitch mass). This can reduce the local population and prevent additional egg-laying onto old masses. Abandoned masses are dry, gray, and cracked while newer masses are lighter in color and shiny.
- If a spray is warranted on larger trees, it should be applied by a professional, and options include permethrin or bifenthrin (trunk sprays). Start application in early to mid May, so that trunks are protected through early August.
European Pine Sawfly
Larvae of the European pine sawfly will soon be feeding on last year’s needles of mugo, Scotch, Austrian, and ponderosa pines. In large infestations, they will leave behind bare branches but thankfully, they do not feed on the current season’s buds. As a result, the new needles will often hide the damage.
Sawflies are not caterpillars. They are related to ants, bees, and wasps. They overwinter as eggs laid in slits along the length of needles. The larvae feed for about 4 to 6 weeks. They then pupate, and adults emerge in the fall to mate and lay eggs.
Initial damage looks like brown wilted foliage because the larvae are only feeding on the margins of the needles. As larvae mature, they group together and gorge on entire needles, sometimes causing complete defoliation. When attacked or harassed, the larvae rear back their heads in defense, and are able to eject a repellent substance from a sac off the foregut.
- Treatment is rarely needed, as feeding is localized and usually not severe enough to harm the plant. Because they feed together, removing the branch removes most of the larvae; hand-picking is another option.
- If necessary, spray options include acetamiprid (Assail, Tristar) azadirachtin (Safer BioNeem, Safer Grub Killer), spinosad (Conserve, Fertilome / Monterey), horticultural oil, or insecticidal soap. Bacillus thuringiensis (Bt) does not work on sawfly larvae.