Blight hitting apple trees hard

By MARGIE O’LOUGHLIN

Have you been wondering what’s wrong with the apple and crab apple trees around town lately?

Our cool, wet spring contributed mightily to a disease called apple scab. The symptoms of apple scab are as dismal as the name sounds. Susceptible tree crowns thin, with withering leaves dropping rapidly to the ground.

Unlike the fungi involved in most other leaf spot diseases, the apply scab fungus remains active throughout  the season. As long as there is sufficient moisture, new infections multiply quickly, creating an epidemic among susceptible hosts. Susceptible tree species include apple, crabapple, hawthorn and mountain ash.

The apple scab fungus over-winters in leaves that have fallen to the ground. So, to stop the spread of apple scab, it’s important to rake up fallen leaves. Don’t postpone - the time is now! Spores develop during the winter and mature in the spring. Spores are released into the air when it rains, with peak spore release occurring during bloom time. Winds carry the spores to new buds, where a film of water is necessary for successful spore germination. Infection occurs most rapidly when fruits and leaves remain wet for a minimum of 9 hours, and temperatures are between 55° and 75°F.

If spring weather is dry, apple scab will not likely be a problem. During years with especially wet springs, such as the one we’ve just had, entire trees can be defoliated by the end of June.

For home owners, it’s important to note that apple scab doesn’t usually affect the long-term health of trees. So don’t cut the trees down! The disease can cause severe defoliation and a loss of fruit crop. It also can lead to reduced growth, susceptibility to secondary pest problems and sensitivity to winter injury. However, the trees are not likely to die and will recover during years with less rain. Karen Zumach, community forester with the organization Tree Trust says, “With luck and a dryer season next year, the affected trees will come around.”

How to avoid apple scab in future plantings? Apple and crab apple trees are available in a wide range of sizes and colors, so gardeners can choose those that work best for their yard. Be sure to check for susceptibility to fire blight and Japanese beetle, as well as resistance to apple scab, before making a choice. Reputable nursery staff can tell you which varieties are going to best meet these criteria.

If you have a susceptible or infected tree, the best strategy is good, thorough raking. It’s important to rake fallen leaves before they have a chance to dry and break up into bits too small to gather.

Prune or thin your trees during the winter months to maintain open canopies. This will improve air circulation and shorten the time necessary to dry leaf surfaces. Use watering practices that saturate the roots, not the leaves, to promote optimum tree health.

What to do with all those infected leaves you’ll be raking up? According to Audrey Matson, co-owner of Egg Plant Urban Farm Supply (1771 Selby Ave.), home compost piles are the best solution. “Just make sure your compost pile is good and hot, so the spores won’t survive the winter and contaminate your compost,” she says.