April showers could bring fungus to fruit trees

By Denise Ruttan

As the blossoms fade in your apple and pear trees this spring, keep an eye out for a fungus that flourishes in warm, wet weather, cautions the Oregon State University Extension Service.

"The longer this spring stays wet and the warmer it gets, there are more chances that we'll see problems with apple and pear scab in our fruit-growing areas such as the Willamette Valley,

Hood River, Milton-Freewater and the Medford and southern Oregon areas," said Jay Pscheidt, a plant pathologist for the OSU Extension Service.

You'll first see smoky, green spots on the leaves, Pscheidt said. A maze of velvety spots on the leaves will enlarge and turn crusty brown until these spores eventually infect the fruit. Black spots then appear on the fruit, causing it to crack and rot, he said. Flowers and young shoots and leaves are particularly susceptible to the spores.

Two slightly different pathogens, apple scab and pear scab, attack apple and pear trees, Pscheidt said. The fungus that attacks apples can also infect crabapple trees.

The volume of rain doesn't matter; rather, it is the duration of the wetness that can rouse plant diseases such as apple and pear scab, Pscheidt said. Infection can occur after leaves stay wet for 10 to 25 hours, he added.

To control the disease, take care to rake leaves after they fall and put them in a hot compost pile or yard waste disposal bin, Pscheidt said. Prune during the dormant season in late December to March to increase airflow and dry the leaves faster, he said. You can also plant scab-resistant apple and pear trees. When flowers are blooming and buds emerge, it is time to spray fungicides, Pscheidt said. Wait, though, until the leaves have stayed wet for several hours, he advised. The Pacific Northwest Plant Disease Management Handbook recommends several fungicides for apple and pear scab.

You can also manage diseases such as scab by selectively snipping off fruit, including diseased fruit, in late spring and early summer. This thinning increases air circulation and has the added benefit of improving fruit size and quality, added Steve Renquist, a horticulturist with OSU Extension.

Thin apple and pear trees in May, about a month after full bloom, Renquist said. On apple and pear trees, clusters of blossoms emerge from spurs, or pieces of wood that grow from the branches, Renquist said. Pears and apples produce several flowers per spur each spring. When fruit develops, it can weigh down the branch attached to the spur, he said. After tiny fruit appears in the late spring to early summer, use pruning shears to snip about four or five fruits off each spur, leaving one fruit per spur, Renquist said.

For more information about fruit tree care, view the publications below:              

* OSU Extension: "Growing Tree Fruits and Nuts in the Home Orchard"

* OSU Extension: "Managing Diseases and Insects in Home Orchards"

* OSU Extension and partners: "Training and Pruning Your Home Orchard"

* The Washington State University Extension Service: "Apple Scab"

Image above: Photo by Jay PscheidDark spots on the underside of leaves can indicate the presence of pear scab, a fungus that thrives in warm, wet weather.

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