ISSUE 1 May 4, 2006
COTTONY ASH PSYLLID
The cottony ash psyllid was recently discovered in North Dakota during 2005. Nymphs of the cottony ash psyllid have been observed feeding on black ash or manchurian ash trees in Fargo (pers. contact, J. Knott) and Bismarck (pers. contact, D. Nelson). They do NOT feed on green ash. Early stage nymphs are very difficult to see due to their small size and white to pale orange color. Nymphs feed on the underside of leaves, and inject a toxin into tree leaves causing curling ("cauliflowered") leaves and premature leaf drop in summer. Nymphs secrete a cottony material when enclosed within the leaf, which can make insecticide control difficult. Proper timing and complete coverage of insecticides is crucial for effective control. Insecticides should be applied during the early nymph stage. Once the psylllid adult stage is reached, it is too late and control measure are largely ineffective. Some of the insecticides available to the homeowner include: Orthene or Isotox (acephate), sevin (carbaryl), Bayer Advanced Garden, Tree and Shrub (imidacloprid), Astro (permethrin), malathion, horticultural oils, to name a few. Field reports indicate that Orthene (acephate) does a very good job of eliminating psyllids with no damaging side effect on the trees. Although dormant oils are more environmental friendly, oils did not provide good control of pysllids.
photo courtesy J. Knott
SPRING EVERGREENS SHOW WINTER INJURY
Historically – and this year is no different – evergreen samples and turfgrass samples begin to arrive at the lab in late April and early May. Winter injury was observed on evergreen samples submitted to the lab in late April. Different types of winter injury include desiccation or drying, early fall or late spring freeze, winter sunscald (also known as winter burn), bud or shoot death, and frost cracks, among others. The type observed in the lab was most likely winter desiccation.
On evergreens, winter desiccation, winter sunscald, and freeze injury are more common than other types of winter injury. Symptoms of reddish or purplish brown needles appear when temperatures warm up. Sometimes, with these types of winter injury, healthy green limbs are apparent at the lower part of the tree or shrub, under what was once the protective snow layer. These types of winter injury are not typically lethal on evergreens, since limbs and buds are usually not killed. However, death of the tree can occur if the injury is severe. Monitor the buds and new growth to determine the extent of damage. Have patience – as long as buds remain viable and new growth occurs, recovery is possible. Surviving plants should look nice again after a year or two, although they might be a little unsightly for the summer.
To help protect evergreens from winter desiccation and winter burn, provide adequate moisture during dry times, particularly late summer and early fall. Avoid injuring root systems. Cultivation, such as around larger or established trees, or deeper than normal cultivation, can cause root injury that limits adequate uptake of moisture by the plant. Avoiding fall fertilization can also help prepare trees for winter. A spring-applied fertilizer can be helpful in stimulating growth and improving the vigor of an affected plant.
For more information on the different types of winter injury, how to distinguish among them, and how to help manage the problems, refer to the following NDSU publications available on-line and in print:
Diseases of Evergreens and Related Problems http://www.ext.nodak.edu/extpubs/plantsci/trees/pp789-1.htm
Insect and Disease Management Guide for Woody Plants in North Dakota
Salt injury was also a probable cause of symptoms observed in a spruce sample submitted to the lab. Winter sunscald injury can resemble salt injury. To help differentiate the two, consider the location of the affected plant. If it is near a highway where salts are used, the culprit may be salt injury.
SOME HOME LAWNS SLOW TO GREEN
Earlier in April, as the snow melted, lawn samples submitted to the lab (including samples that were submitted as digital images only) appeared to be affected by snow molds. Two types of snow molds commonly occur in North Dakota. For homeowners, the treatment for both types is similar and focuses on cultural management strategies rather than use of fungicides. Snow molds seldom cause plant mortality, but if the lawn is thinned, over-seeding with a disease tolerant variety is recommended. Raking up the matted areas in spring helps the lawn recover. Other cultural practices that help minimize future impacts of snow molds include avoiding excessive fall fertilizer. The fall fertilizer application should be applied after the grass has become dormant, to prevent excessively lush growth of the grass as it prepares to overwinter. According to NDSU Extension Horticulturist, Dr. Ronald Smith, the final mowing in fall should be about an inch shorter than normal. Although grass clippings are beneficial to the lawn when not removed (in the absence of foliar diseases), if snow mold is a chronic problem, clippings from the final fall mowing should be collected. Bagging grass clippings from the final mowing and raking up fallen leaves reduces the organic matter on which snow molds flourish. Dr. Smith also recommends core aeration if soils are compacted. Core aeration offers many benefits to a lawn, but with respect to managing snow molds, this procedure helps limit establishment of the snow mold pathogens by improving soil drainage.
NDSU Plant Diagnostic Lab