Accumulated Growing Degree Days (Base₃₀): 1065.5
This week's Indicator Plant: Common lilac (Syringa vulgaris)

• Tuliptree scale

• Boxwood psyllid

• Elm leafminer

• Phomopsis gall on forsythia

• Rhizosphaeria needle cast on spruce

As of May 12, 2011, we are at 160 base-50 growing degree days (GDD₅₀), which is 101 GDD₅₀ (17 calendar days) behind 2010, and behind the historical average (1937-2010) by 244.3 GDD₅₀ (10 calendar days). May has seen 0.19" precipitation, which brings us to 10.46" of precipitation for the year, which is 1.11" more than 2010.

Tuliptree scale (Toumeyella liriodendri) nymphs were found in Naperville on a star magnolia (Magnolia stellata). This native, soft scale preys upon tuliptree, yellow poplar, several magnolia species, and sometimes linden. During its feeding stage, it produces an abundance of honeydew (sticky sugar-laden excrement), which may eventually becomes black when sooty mold begins to grow on it. The females mature in late summer and begin baring live young in August. They only have one generation per year and overwinter as immatures. At maturity they are ¼ - ½ inch in diameter. The nymphs we're seeing now are about 1/16" long.

Management: These scales are found usually on stressed trees, so encouraging health and vigor is the best defense. If a tree is infested with tulip tree scale, using a high nitrogen fertilizer can exacerbate the problem. There are a variety of chemical treatments for this scale. Please refer to the 2010 Commercial Landscape and Turfgrass Pest Management Handbook (CLM) or 2008 Home, Yard and Garden pest Guide (HYG) for more information.

Suggested reading: http://www.entomology.umn.edu/cues/Web/208TuliptreeScale.pdf
http://www.fs.fed.us/r6/nr/fid/fidls/fidl-92.pdf

Boxwood psyllids (Cacopsylla buxi) are starting to hatch on Korean boxwood (Buxus microphylla var. koreana 'Wintergreen'). The psyllids overwinter as tiny orange eggs in the bud scales of the boxwood. As the buds open, the psyllids hatch and begin to feed. The nymphs are about 1.6 mm (1/16 in) long, yellowish, and partially covered with a white, flocculent secretion that protects them from parasitoids and chemical sprays. Their feeding causes cupping of the leaves. Winged adults appear in late May to early June. We sometimes see ladybird beetles (also known as ladybugs) feeding on the psyllids.

Management: Damage is mostly aesthetic. Shearing boxwoods reduces the population as the insect or the eggs are removed in the process. Chemical insecticides can be applied, but if using a spray, it is important to spray inside the cupped leaves.

Suggested reading:

First instar elm leaf miner (Fenusa ulmi) larvae are starting to form mines on American elm (Ulmus americana). The adults emerge in spring to lay eggs in elm leaf tissues. After about a week, the eggs hatch and young larvae begin to make mines in the leaves. The sawfly larvae are feeding on the leaf tissue between the upper and lower epidermis of the leaves.

The mines at first look like U-shaped brown spots between veins in the leaf. Eventually the insects will eat a hole through the leaf epidermis, fall to the ground, and excavate a hole in the soil to overwinter. Severe damage can result in defoliation. To test a leaf for miners, hold the leaf up to the light. If the insect is still in the leaf, you can see it. You will also be able to see frass (insect feces) which looks like pencil shavings within the mined area.  They spend most of their life cycle burrowed about an inch in the ground.  Other susceptible elms include the American elm (U. americana), English elm (U. procera), and Armenian elm (U. elliptica).

Management: We are unaware of any nonchemical control. There is only one generation per year, and the leaves that emerge later will not be infested.

Suggested reading: http://www.ext.colostate.edu/pubs/insect/05548.html

 

Damage from the European elm flea weevil (Orchestes alni) adults feeding on the undersides of newly emerging leaves of Accolade elms (Ulmus 'Accolade') has been sighted at the Arboretum. This pest first appeared in Northern Illinois in 2003 and has caused significant foliage damage to elms (particularly Siberian) during the past eight years.

Adult feeding results in tiny shot holes in the leaves, and heavy feeding can cause newly expanding leaves to wither and turn brown. After feeding, the female weevil cuts a cavity into the leaf mid-vein and inserts an egg. The hatching larvae create blotch mines at the leaf tips. Larvae feed for about 2-3 weeks, and then pupate within the mined leaf. The significant feeding can reduce photosynthetic capacity of the tree, thereby impacting overall tree vitality.

 

Management: Chemical controls are difficult to apply on a large tree, but when practical, control the flea weevil adults in early May or late June with a spray of acephate, imidacloprid, bifenthrin, or carbaryl. The acephate or imidacloprid will also prevent the larval mines from appearing later.

A systemic soil drench of imidacloprid (Bayer Advanced Tree and Shrub Insect Control, available to homeowners) applied in autumn after leaf drop will control early spring feeding of adult elm flea weevils.

A systemic soil drench of imidacloprid applied in early spring will help with the leafminer stage that shows up in late spring and the adults feeding later in the season. This drench does not help with the over-wintering adult feeding early in the season as it takes about two months for the insecticide to be taken up throughout the tree.

Suggested reading: http://www.mortonarb.org/component/content/article/193-insects-diseases/757-european-elm-flea-weevil-orchestes-alni.html

http://www.mortonarb.org/component/content/article/193-insects-diseases/16896.html

A sample of Phomopsis stem gall on forsythia was been brought into the Plant Clinic last week. The galls, caused by the fungus Phomopsis sp., are light brown and irregularly shaped with a bumpy, rough texture. The galls ranged in size from 0.65 – 2.54 cm (0.25 – 1 in) in diameter. Phomopsis galls can occur on many tree and shrub species, including viburnum, privet, American elm, hickory, maple, and oak. Gall size varies with the host species and time. Infection symptoms include twig dieback if the galls cause girdling. The disease is frequently mistaken for crown gall, which is a bacterial disease that usually attacks plants near the soil. Phomopsis galls are located higher on the stems, not near the soil line.

Management: There is little known about the disease cycle of this fungus. The only suggested control measure is to prune out the galls. It is imperative to sterilize pruning tools between cuts by dipping them in 70% isopropyl alcohol.

The weeping blue spruce (Picea pungens 'Glauca Pendula') arch in the Children's Garden is showing symptoms of Rhizosphaera needlecast, a disease caused by the fungus Rhizosphaera kalkhoffii.

This fungal disease overwinters on spruce needles. Infection occurs in spring on needles of the lower branches first and gradually progresses up the tree. Symptoms become apparent in late summer as infected needles turn a mottled yellow. By late winter and early spring, the needles turn a brown to purplish-brown and fall off the tree the following summer and fall. Small dot-like fruiting bodies (pycnidia) can be seen (with a hand lens) in rows. The fruiting bodies are in rows because they erupt through the stomata (which are small pores on the needles). Although trees are not usually killed by this pathogen, branches that lose needles for three to four consecutive years may die. Colorado blue and Engelmann spruces (Picea engelmannii) are highly susceptible to Rhizosphaera needle cast. White spruce is moderately susceptible and Norway spruce is relatively resistant. Hosts in other genera include true firs, Douglas fir, and pines.

Management: Rake and dispose of infected needles to reduce the source of inoculum. Prune off lower branches, provide adequate spacing between trees, control weeds, and remove unwanted shrubs to improve air movement. Chemical controls are most effective if the disease is detected early. Fungicides should be applied when needles are half-grown (as soon as bud caps fall off) and again when fully elongated. Two years of applications are usually required. For further information on chemical controls, refer to the CPM or HYG. Rhizosphaera is a disease common in plants grown outside their native ranges; it is inconsequential in natural forests. The best control for the future is to plant resistant native species.

Suggested reading: http://ohioline.osu.edu/hyg-fact/3000/3059.html
http://www.extension.umn.edu/yardandgarden/ygbriefs/p435rhizosphaera.html

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