Crabapple at the Minnesota Landscape Arboretum.

Don Breneman, Minn. Extension Service

How to care for a crabapple

  • Article by: Nancy Rose
  • April 27, 2004 - 11:00 PM

QWe have a 25-year-old flowering crabapple that drops its leaves all summer. We were told to pick up all the leaves, but this is difficult. We very much want to keep the tree. What can we spray it with, and is there a danger to the person spraying?

A Your flowering crabapple is susceptible to apple scab, a common fungal disease that causes leaf spots of yellow-green or brown. Severely infected leaves drop off, which can result in bare and forlorn-looking trees. Apple scab also damages the fruit of flowering crabapples and edible apples.

The fungus needs certain moisture levels to develop, and then it spreads when raindrops (or sprinkler water) knock the spores to new leaves. Picking up diseased leaves and fruit when they drop can greatly reduce the number of fungal spores in the vicinity. But you're right; it can be very difficult and time-consuming to try to pick up every leaf. And it can be a daunting task because it takes only a few overwintering spores to get the disease cycle going again the next year.

As with the green ash mentioned in the next question, the best solution to recurrent severe disease problems is often the removal of the plant. There are many excellent disease-resistant flowering crabapples to choose as a replacement. But, as in your case, there are times when a plant has such high aesthetic or sentimental value that removal is the last resort.

Apple scab can be controlled with well-timed fungicide sprays. Commercial orchardists must do this to produce the blemish-free apples that most consumers expect at the market. For ornamentals such as flowering crabapple, fungicide sprays should be considered only in special situations.

Fungicides work to prevent disease infection, not to cure existing problems. This means that proper timing is essential. Sprays must be applied before the disease cycle starts, which means starting before flowering and continuing through much of the summer in the case of apple scab. Thorough coverage of all foliage surfaces is also essential.

Homeowners can use approved fungicides such as thiophanate-methyl. Be sure to read and follow the label precisely -- it will tell you what personal protective equipment you need, as well as rates and application methods. However, the spraying equipment available to homeowners (such as inexpensive hose-end sprayers) are often wasteful and inefficient. Consider hiring a professional arborist to do the spraying.

Q I have two 15-year-old green ash trees that did not leaf out well last summer. Are these trees likely to grow out of this, and is there anything I should be doing to treat them?

A Green ash (Fraxinus pennsylvanica) has been popular for its fast growth, cold hardiness (through Zone 3) and tolerance to drought and poor soils. This shade tree has been used extensively in residential and commercial landscaping throughout the Midwest.

Unfortunately, many green ash trees have started to decline and die in the past 10 years or so. Several specific diseases and insect pests may be the culprits, but what often does in an ash tree is an unspecified combination of environmental stresses (such as drought and severe winters), diseases and insect attacks.

The term "ash decline" often is used to describe the effect caused by this combination of stresses. The usual symptoms include sparse foliage and dead branches throughout the tree's crown. The specific stress factors that seem to cause ash decline vary from region to region, which makes it even more difficult to come up with a precise diagnosis when ash trees fail.

One disease that often recurs in our area is ash anthracnose. This fungal disease shows up during cool, wet weather in spring or summer. In early spring, ash anthracnose can infect newly emerging foliage, causing the young leaves to fall off the branches. If the disease appears later, when foliage has matured, large brown spots may appear on the leaflets and midveins of the leaves.

Healthy green ash trees are capable of sending out new foliage even if the first batch of leaves gets infected and drops off. However, if the tree already is stressed or anthracnose has hit repeatedly, the disease can more seriously weaken the tree.

Often the best solution to repeated ash anthracnose (or unspecified ash decline) is removal of the trees. If you don't want to remove and replace the trees, there are a few things you can do to help.

First, do everything you can to reduce environmental and mechanical stresses on the trees. This means watering the trees during long dry spells, fertilizing once a year, applying a wide swath of organic mulch (2 to 3 inches deep) in a circle around the tree, raking up and disposing of diseased leaves and twigs, and avoiding trunk damage from mowers and string trimmers.

If you consider your trees to be especially valuable and they have clearly had ash anthracnose in three out of the five previous years, consider applying an approved fungicide such as chlorothalonil. Spraying must start as the buds break. Because large trees can be difficult to spray effectively with typical gardening equipment, consider hiring a professional arborist.

Nancy Rose is a horticulturist, writer and photographer. To ask her a gardening question, call 612-673-9073 and leave a message. She will answer questions only in this column.

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