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Mancana ash disease tops reader concerns

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Problems with Mancana (Manchurian) ash have dominated reader tree and shrub concerns this year, and I've even seen recently purchased nursery trees that show all the early symptoms of a potentially fatal disease believed to be related to a rare type of bacteria called phytoplasma.

An early sign of the disease is flattening of the current-growth twigs, which also have a tendency to curve and slightly twist. Those twigs may grow multiple buds that are side-by-side when there should only be one bud. Last year's branches often produce multiple twigs from a common location on the twig -- there should only be two twigs.

Most noticeably, branches die sporadically in the tree, and twigs often display unusual deformity in their growth.

Unfortunately, there is no known cure or method of controlling these bacterial infections. The infected tree will likely die and should be removed. Never grow another Mancana ash in that location, as the roots will be infected as well.

QUESTION: The leaves and twigs of my ninebark shrubs have started to grow a white-grey mildew-like powder on the leaves. Later, the twigs start getting these problems. How do I control this?

ANSWER: Ninebarks everywhere are being attacked by a powdery mildew, although it's more common in shrubs that have a dense mass of woody stems. Poor ventilation in the shrub and high humidity encourages the growth of mildew. This can also be a problem on nannyberry viburnums.

Plant mildew can be controlled with garden fungicides, but it's important that the stems of the plant be thinned out by as much as 50 per cent in the fall after the leaves have dropped. Collect all diseased material and place them in the garbage or if permitted by your local bylaws, burn it before winter arrives.

QUESTION: My willow hedge is turning yellowy green, and I see more and more twigs and branches dying each year. Can you help me with this problem?

ANSWER: I've examined several problematic willow hedges in rural areas this year and I often see a portion growing on low-lying ground. With heavy rains, these low spots accumulate water and keep the ground saturated. Most willow varieties prefer drier conditions.

As well, I often see twigs in the hedges that are partly black with orange-brown patches on their stems, a sign of willow blight disease. Dead and dying portions of the shrubs can be pruned out, but it's absolutely essential that you sterilize your pruning tool with bleach and water, methyl hydrate or denatured alcohol after every cut. It's easy to transmit the disease to other willow branches through disease contaminated tools.

Heavily infected plants are unlikely to recover and should be removed. All dead and diseased twigs and branches should either be covered with clay in a shallow pit or burnt. Never leave them outside in an uncovered pile -- the disease spores will be carried by winds to a nearby willow and infect it.

Michael Allen is a consulting urban forester and certified arborist and owner of Viburnum Tree Experts. He can be reached at 204-831-6503 or His web site is

Republished from the Winnipeg Free Press print edition July 13, 2013 F10

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