Monday, April 21, 2014

Pest News for Week of April 21st


From: Steve Frank, Extension Entomologist

Oak Eriococcin Scale Active

Oak eriococcin, Acanthococcus quercus, is not very well known. Not much research has gone into understanding its biology or control. It is in the family Eriococcidae which includes several felt scales including azalea bark scale. This scale is quite common around Raleigh and is very apparent this time of year. As the name implies its primary hosts are oak trees. I find it primarily on willow oaks along streets. The oaks on Hillsborough Street by North Carolina State University campus are literally covered head to toe. The scale produces cottony white egg masses that are often in the crotches of twigs. Very little efficacy data is available, but there are reports that imidacloprid and other treatments for soft scale work for these as well. Even horticultural oil may be an effective treatment this time of year right after egg hatch. Visit website:

Boxwood Leaf Miners Emerging

Boxwood leafminer adults emerged this week. Look for small orange flies hovering around boxwoods and for pupal casings sticking out of leaves. This indicates adults recently emerged. The maggots pupate in the leaf blister. As the adult emerges the pupal case gets caught on the leaf. This holds the leaf in place so the adult can wriggle out. Boxwoods can be treated with a pyrethroid to prevent flies from landing on the bush to lay eggs, but watch out for mite outbreaks. Imidacloprid will kill maggots within the leaves, but it is best to apply after flowering to protect pollinators. 

From: Mike Munster, Ornamental Pathologist, Plant Disease and Insect Clinic

Pseudomonas syringae Blight and Dieback

As March was going out like a lamb, a nursery submitted four container-grown shrubs to the Plant Disease and Insect Clinic: three rose cultivars and a lilac. Very young shoots on these plants were withering and dying. At least in the case of the lilac – and possibly with the roses, too – the new flush of growth had been hit by freezes. While you’d expect the tender shoots to be blasted by the cold, in this case the woody stems were also dying. Bacterial streaming was seen in much of the stem tissue. We did not see fire blight on rose or lilac, so what was happening? It turned out to be the bacterium Pseudomonas syringae. One of the roses also had Botrytis canker, common canker, and downy mildew.

Although Pseudomonas syringae is named after lilac (Syringa), it is capable of causing cankers and dieback in a wide variety of plants. Besides lilac, we have found it on the following woody ornamentals: cherry-laurel, flowering quince, Indian hawthorn, Yoshino cherry and multiple varieties of roses. In addition, we have recovered it from leaf spots of hydrangea and Japanese holly. Bacterial canker caused by Ps. syringae can be a serious problem in peach orchards and with woody ornamentals, we almost always sees it in nursery situations. One exception came in last year on the twig of a weeping willow from a home landscape. As the weather warms up and cankers become inactive, this disease becomes more difficult to detect. According to the PDIC's records, almost every case of Pseudomonas bacterial canker on woody ornamentals since 2008 was diagnosed between February and May. The bacterium is still present on and within plants during the summer, but the disease process temporarily shuts down.

Note: We occasionally find Ps. syringae causing leaf spots on ornamentals in the greenhouse, and there are variants – called pathovars – that cause certain very specific problems such as bacterial speck of tomato and angular leaf spot of cucurbits.

Like many bacteria, Pseudomonas syringae is able to live and multiply on plant surfaces. This is known as its epiphytic (“on the plant”) phase. In the recent case, the bacteria were almost surely present before the spring flush occurred, and so were able to strike quickly. These bacteria enter plants following injury, in particular frost damage. What’s more, the bacterial cells actually promote freeze damage through a process known as ice nucleation. As if this trick were not enough, Pseudomonas syringae even produces a toxin that damages plant cells.

The most important way to minimize damage to woody plants from Pseudomonas syringae is to limit the stressors that predispose plants to infection. Stress factors include pruning injury and frost injury. Bacterial canker of stone fruits caused by Pseudomonas syringae can be reduced by pruning in the early summer, instead of the fall or winter. Sanitize shears or knives frequently and avoid working the plants when wet. Keep tabs on substrate pH. Do not over fertilize plants, especially when they need to harden off for the winter. Protect plants during cold snaps. Don't allow plants to undergo stress from too much or too little water. Keep foliage and stems as dry as possible by changing irrigation methods or reducing overhead irrigation, which favors and spreads the bacteria. If you have already had this problem, Ps. syringae is probably present as epiphytic populations on the surfaces of much of your nursery stock and even the surrounding weeds. There are few chemical options that hold any promise, at least not enough to make a recommendation.

Recommendations for the use of chemicals are included in this publication as a convenience to the reader. The use of brand names and any mention or listing of commercial products or services in this publication does not imply endorsement by North Carolina State University, North Carolina A&T State University or North Carolina Cooperative Extension nor discrimination against similar products or services not mentioned. Individuals who use chemicals are responsible for ensuring that the intended use complies with current regulations and conforms to the product label. Be sure to obtain current information about usage regulations and examine a current product label before applying any chemical. For assistance, contact an agent of North Carolina Cooperative Extension.
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