June 20, 2007

Christmas Tree Scouting Report #13 - June 20, 2007

Weekly newsletter compiled by Sandy Gardosik, PA Department of Agriculture, Harrisburg, PA.

Growers are asking if a fourth spray for Rhabdocline needle cast is needed. When I'm not sure about this myself, I conduct a test by taking some infected needles and placing them in warm water, after which I wait about 5 or 10 minutes.  I then look at the bottom surface of the infected area to see if it has swollen, and if the epidermis (needle surface) has split and the area is still an orange color - all of these results indicate that the infection is alive and can still be transmitted.  I performed this test with needles from Dauphin County infected with Rhabdocline, and some of those lesions were still expressing symptoms of active infection. You may want to consider a fourth spray if it has been two or three weeks since you applied your third spray, depending on factors such as (1) the severity of the needle cast the previous year, (2) how close to market sale the trees are, and (3) whether or not a low infection level is acceptable.  For next year's scouting purposes, you may want to tie a marker ribbon this fall to a tree infected with Rhabdocline, so that you can monitor that known, infected tree during the next year's inoculation period.

Growers may have noticed a flush of new growth on Douglas fir and maybe even spruce. This is known as "lammas growth" or a second flush of growth. Many temperate zone perennial plants, especially conifers, have this growth habit. Lammas shoots occur only when the drought period in summer is broken by sufficient rain to relieve the moisture stress in the tree, and only then when the tree has been dormant for a relatively short time. If the rains occur in late August or September, the annual growth cycle of the tree will have proceeded far enough that a growth response will not likely occur as a consequence of a favorable moisture supply.

Cinara aphids were found feeding on the new shoots of Colorado blue spruce and Norway spruce in York County. These aphids were light brown, and feed in colonies. My attention was drawn to the aphid colonies by the presence of ants that were "tending" the aphid colonies on the new shoots. Aphids consume large volumes of sap, with the excess being secreted as sticky "honeydew" . Some ant species tend aphids for this honeydew, and "return the favor"  by protecting the colony from predators and parasites. If aphid populations get sufficiently large, sooty mold can grow on the honeydew, making the needles and stems black and unsightly.  Aphid feeding can cause needles to brown and drop off.  You should apply controls against aphids if 30 percent of the shoots have aphid colonies. However, if aphid predators such as lady beetles are present, you should limit the use of broad-spectrum insecticides, which often kill beneficial insects.

First generation larvae of the introduced pine sawfly were found feeding on white pine in York County.  Other pine species are attacked by this insect, but white pine seems to be its preferred host. Larvae hatch from eggs laid in needles in early spring. While newly hatched larvae feed in colonies and only partially eat the needles, larger larvae feed singly and devour entire needles.  Larvae are about an inch long when mature, and are black with two dark stripes down the back and rows of yellow dots on the sides. First generation larvae feed on the old needles; those of the second generation appear in August and feed on both old and new growth.  If populations become heavy enough, larvae can defoliate a tree; however, population levels usually do not get sufficiently large to cause this kind of injury.

The galls caused by the Douglas fir needle midge are becoming noticeable on Douglas fir in Schuylkill and York counties. As the larvae develop, the needles swell and exhibit a gall. The needle will be kinked at the gall location, and be light in color. If the needle is dissected, a tiny white maggot can be found. As these maggots continue to develop inside the needle, the gall will harden and darken. By late November, larvae will be mature and chew out the backside of the needle and fall to the ground. The midge larvae overwinter in the soil and emerge as adults in early spring of the following year. Controls should be applied just before- or during the time the adults emerge, but before eggs are laid.

Cryptomeria scale and elongate hemlock scale crawlers were both observed on Fraser fir in Schuylkill County this week. This is the stage in the life cycle that is most susceptible to chemical controls.