Ornamentals Hotline - Issue 1, March 27, 2009

INSECTS
Brian Kunkel
Ornamental IPM Specialist

EUROPEAN PINE SHOOT MOTH larvae are active between 0 1889 [223 peak] GDD . This moth, Rhyacionia buoliana, prefers to feed 50 on Austrian, Eastern white, mugo and Scotch pines. Young trees between 6 8 m in height are very susceptible.

Overwintering larvae emerge and resume feeding on new shoots or expanding buds after about 20 200 GDD have accumulated 50 and this causes the greatest damage. Larvae are yellowish brown to brown with black head capsules and are about 13 mm when mature. Trees with feeding damage may have dead or stunted shoots appearing as if infected with Sphaeropsis (Diplodia) shoot blight. Shoots damaged but not killed continue to grow in an “S” shape called a posthorn. Spring feeding causes trees to produce numerous adventitious buds resulting in a witches-broom effect. The larvae pupate around mid-May and after about three weeks the orange-red adults emerge and lay eggs on needles, needle sheaths, or buds of new growth. First instars construct silken webs on needles of the current year's growth before they bore into needles to feed. Older larvae hollow out newly formed buds to overwinter.

NANTUCKET PINE TIP MOTH adults are active 100 1001 GDD , 50 when Syringa vulgaris is in bloom or Cornus mas is in full bloom. This moth, Rhyacionia frustrana, is a close relative of the previous moth and attacks most two- and three-needle pines shorter than 15 feet tall found in open locations. The adults are about ½ inch long. The forewings have reddish brown patches with silver-gray bands. Eggs laid by adults take between five and ten days to hatch unless cold weather follows oviposition, then it may take 30 days. The damage is often observed as delicate webbing found in the axil formed between the developing needle and stem. Shoots of infested terminals turn brown and are easily observed from a distance.

Predatory insects, parasitoids, and birds all attack the larvae of these moths. Chemical control includes: azadirachtin (Azatin), spinosad (Conserve), tebufenozide (Mimic), diflubenzuron (Dimilin), acephate, cyfluthrin (Tempo), or dimethoate.

DISEASES
Bob Mulrooney
Extension Plant Pathologist

WINTER DAMAGE. Temperatures have not fluctuated as drastically as in past years but we had cold temperatures this winter and there is some obvious winter damage on Japanese holly, yew and boxwood. These plants have dead branch tips scattered on the sun exposed sides. There has been minimal marginal leaf browning on broadleaved evergreens, but Mahonia species in the UDBG seem to have the most damage. Pruning out the dead branches and picking off the worst leaves is suggested now. It may be too early to assess other possible winter damage until new growth begins.

ROSES. Now is a good time to get out and prune roses to remove winter damaged and diseased canes. Cane cankers caused by black spot and powdery mildewinfected canes should be removed to reduce the incidence of these two important rose diseases, especially on hybrid tea roses. This is a good time to apply limesulfur to burn out any infections left after pruning.

KABATINA BLIGHT is present on susceptible upright and prostrate junipers. Look for scattered dead branch tips on infected plants. Often there will be a gray canker on the twig separating the dead tip from the healthy part of the branch. Infection takes place during the late summer and fall especially on stressed plants and the symptoms appear in the spring on the old growth. (As a reminder, Phomopsis tip blight it a different disease and only infects new growth.) Removing the dead tips is probably the best control. The infected tips will be shed anyway. Rake up infected tips as they fall to the ground and dispose of them to reduce the fungal spores at the site and limit future infection.

WEEDS
Gordon Johnson
Agricultural Agent, Kent County

CRABGRASS GERMINATION is closely correlated to soil temperatures and also to degree day accumulations based on maximum and minimum air temperatures. In work done in the 1990's at the University of Maryland, they found the following: The minimum soil temperature for initial crabgrass appearance is 54 F. The minimum soil temperature for major emergence of crabgrass is between 60-70 F. Soil temperature measured at the 1 inch depth should be used. Measuring soil temperatures first thing every morning is the best way to get this number. A rule of thumb is that crabgrass will start germinating with 3 consecutive night soil temperatures above 50 F. Preemergence herbicides should be applied prior to this time (when you observe the first nighttime soil temperature above 50 F germination will soon follow). Degree days can also be used. Degree days for crabgrass germination using a base of 53.6 follows: initial emergence 76-140 DD, 25% emergence 558 DD, 50% emergence 801 DD. As of March 23 in Dover we have accumulated 20.6 degree days based on 53.6 F. You can do your own degree day calculations using weather information from the DEOS system available at http://www.deos.udel.edu/.