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Nursery & Forest
Volume 61 Number 17 Date 08/25/2016


LINDEN LEAF BLOTCH - Nursery inspectors report that this summer's very high humidity and frequent rain have intensified leaf spot disease problems. Linden leaf blotch, like other leaf spot diseases, requires either water on the leaves or high humidity for 12-24 hours to start an infection. Leaf spots often mature in 1-2 weeks, subsequently producing spores or bacteria that can spread throughout the canopy, starting a second set of leaf spots, or causing new infections on other plants. This cycle of infection and spore production repeats with each rainfall and results in severe disease when weather conditions are favorable.

Although the leaf damage can be pronounced, most leaf spot diseases do not seriously harm trees. Raking and destroying fallen leaves before the first snowfall, taking measures to avoid overcrowding plants, and pruning trees to increase light penetration and airflow are all strategies that can collectively reduce leaf spot diseases on trees in following years.

HONEY LOCUST BORER - Honey locust trees at a nursery grower in Dodge County were showing trunk wounds and exuding sap indicative of honey locust borer (HLB) infestation. Similar to other Agrilus species such as EAB, larvae of the HLB feed on the inner bark, forming serpentine tunnels packed with frass (feces). The adult beetles preferentially select trees declining due to environmental stress, cankers, or wounds, thus HLB is considered a secondary pest. Management should emphasize alleviating stress factors. Nursery stock infested with any trunk boring insect must be removed from sale and destroyed.

CEDAR-HAWTHORN/CEDAR-QUINCE RUST Humid, wet growing season weather has promoted development of this rust disease, which requires a rosaceous host and a juniper host. The characteristic pinkish aecia (tubes) are evident on the leaves of hawthorns in Dodge, Pierce and St. Croix counties. Selecting resistant hawthorn cultivars and thorough sanitation by removing infected twigs, fruit and leaves are the recommended controls. Fungicide treatments applied to new growth in spring may be needed in extreme cases.

GUIGNARDIA LEAF BLOTCH - Several horse chestnut trees in a Dodge County nursery were severely infected with this fungal leaf blotch disease. Symptoms include irregular, reddish-brown leaf lesions with yellow margins that distort affected foliage as they increase in size and severity. Disposing of fallen leaves in autumn and pruning the canopy are recommended to reduce inoculum levels.

MOUSE-EAR OF BIRCH - River birch trees grown at a nursery in Dodge County were showing signs of this growth disorder of potted nursery trees. The stunted or "mouse-eared" leaves are thought to be caused by nickel deficiency. A foliar application of nickel sulfate in mid-October or soon after budbreak can correct this condition. Lowering soil media pH to 5.0-6.0 is also recommended.

-- Konnie Jerabek, DATCP Nursery Inspector