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Nursery & Forest

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Nursery & Forest
Volume 63 Number 11 Date 07/12/2018

BUR OAK TIP BLIGHT AND BRANCH DIEBACK - Widespread branch dieback on bur oak in forested areas of Chippewa, Eau Claire, Polk, Rusk, Sawyer, and Washburn counties has been observed early this summer, with symptoms also reported in eastern Minnesota. Although a definitive diagnosis is not yet available, fungal tip blight coupled with insect activity are considered potential causal factors.

Northwestern Wisconsin's cool, wet weather during the springs of 2017 and 2018 has contributed to record fungal infection outbreaks this season. One working hypothesis is that an insect similar to a gall wasp is wounding branch tips and allowing entry of Botryosphaeria shoot blight into bur oak trees. Current observations estimate twig and branch dieback affecting 20-30% of the infected trees, however new buds have been observed forming behind the dieback, so it is hoped the trees will recover over time. Botryosphaeria infection is generally cyclical and often causes problems for a year or two before subsiding. However, several concurrent years of infection can cause trees to appear stunted and tufted as the terminal buds and branches die back. A detailed report of the symptoms, diagnosis, and management recommendations for this emerging suspected insect/pathogen disorder will be published as laboratory results become available.

JUNIPER SCALE - Juniper scale was detected among 'Blue Rug' juniper plants in a Kenosha County nursery. This common yet potentially serious pest of juniper is always found attached to needles rather than the bark, particularly on the needle undersides. Juniper scale crawlers appear briefly in spring (for only ~ 24 hours) before adhering to needles to begin feeding on plant juices. Scale covering can remain attached to the needles for several years. Severely-infested plants turn yellowish-brown and appearing to be in need of fertilizer or water. Plants can decline due to scale feeding over time and may be killed by severe infestations.

As with other armored scales, treatments must be applied during the crawler stage to be effective. Applications of a high-quality 2% summer oil spray should only be made when the new needles are fully expanded to prevent injuring the needles. Using summer oils before or during needle expansion can result in yellowing and premature needle drop. Bifenthrin, cyfluthrin, or Orthene repeated at least two or three times at 10-day intervals can also provide scale control, although these aggressive treatments will also kill off natural enemies and should only be considered for extreme infestations.

ILARVIRUS IN RIVER BIRCH - Leaf mottling and discoloration symptoms caused by Ilarvirus were confirmed in young river birch at a Vernon County grower last week. The infested trees were container-grown bare-root starts from a Minnesota supplier. Mosaic viruses such as this one typically express on leaves as a distinctive, pseudo-random pattern of yellow, chlorotic tissue which can lead to slow growth, reduced vigor, and susceptibility to other plant pathogens. Ilarviruses are spread by mechanical inoculation by way of processes like poor pruning sanitation, insects, and contact with infected plants. Plants with virus symptoms should be removed from the growing or sales area and destroyed by sealing them in a plastic bag along with their container and growing media. Close inspection of plants before purchase, sterilizing pruning tools between each cutting, and industry attention to careful propagation of virus-free stock are foremost in preventing spread of this and other nursery plant viruses.

ELM LEAF MINER - Wild-grown Siberian elm trees around the perimeter of a St. Croix County nursery were found to have heavy elm leafminer damage, evident from a distance as large tan blotches on the leaves. The blotches develop when the leaf-mining larvae feed inside the leaf tissue, creating pockets between the upper and lower leaf layers. Elm leafminers remain burrowed in the ground until early spring, emerging as adults resembling small black wasps which deposit their eggs into slits on the upper surface of the leaves. After the larvae complete development in late spring or early summer, they drop to the ground. In instances of particularly severe infestations, the larvae may appear to be "raining" out of the trees, especially on windy days. One control option is to spray immediately before the adults lay eggs on the leaves, but timing can be difficult, and spraying large elms increases the potential for pesticide drift. Another option is to use systemic imidacloprid, injected into the root zone and translocated to the leaf tissue well before the larvae begin to feed.

-- Tim Boyle and Konnie Jerabek, DATCP Nursery Inspectors