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April Showers Bring May (And June) Fungus

April Showers Bring May (And June) Fungus

Posted - Mar. 28, 2001 at 6:31 p.m.



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Yes, we need the water, but even Noah got a break after 40 days. A long, wet spring will not only green up the mountainsides, but will increase the chance of plant diseases.

Most diseases require some moisture to establish and grow on their host plants, explains Jerry Goodspeed, Utah State University Extension horticulturist. Some don't care about the moisture level and create problems every year regardless, while others will thrive in this year's climate.

The most common wet spring problems affect two native trees that we commonly use as landscape plants, Goodspeed says. Quaking aspen and Gambel, or scrub, oak can have several problems. These usually go undetected on the mountainsides because the trees are found in large stands. However, when we bring just one or a few of these trees into our yard, we quickly notice when they become diseased and struggle.

Aspen leaf spot can be a problem on aspens, poplars and some willows, he says. The symptoms include small, dark brown flecks with light centers. These spots gradually grow together throughout the season, forming larger dead, black and brown spots on the leaf.

The leaves are infected in the spring just as they emerge and grow, he adds. The fungus spreads to the young leaves through wind and rain and in severe cases the leaves die and fall off the tree. Although the tree usually survives this infestation, it weakens it, and makes it more susceptible to other problems.

"Control isn't easy," Goodspeed says. "Be certain to rake up and remove all fallen leaves. Prune the trees to provide good air circulation and keep them as healthy as possible. This usually means avoiding over-watering, though there isn't much you can do about heavy rains. Keep the lawn mower and weed eater away from the trunk to reduce the chance of bark damage and watch for signs of iron deficiency."

Apply a protective fungicide spray to the newly emerging foliage, he says. Repeat the application every 10 to 14 days. When buying a spray, be certain the target tree is on the label. Daconil and fixed copper sprays such as Microcop are used to help control leaf spot.

One major problem with oaks is a disease known as oak leaf blister, Goodspeed says. It is aggravated by wet springs. The symptoms include raised or sunken lesions that look like blisters on the leaves. The blisters appear light green when they first appear and eventually turn brown. Occasionally they grow together, distorting the leaf's shape.

"This disease is only spread in the cool, wet spring, so it is important to control it now," he emphasizes. "Many of the same cultural practices used with aspens can be applied to oaks. Clean up all the fallen leaves, keep the trees healthy and be certain not to over-water. The fungicide of choice is Daconil."

Both of these diseases are usually not fatal to the trees and do not have to be treated, Goodspeed says. Aspens can survive with leaf spot for years, but are not as aesthetically appealing. This also holds true with oaks with oak leaf blister.

"Other diseases will probably affect different trees in the landscape," he adds "Most likely we will see anthracnose in sycamores, maples and oaks. This disease also loves a wet and cool spring. Its symptoms include necrosis (dead areas) that spread along the veins of the leaves, especially the mid rib. Infected leaves often drop off the tree, leaving the tree looking a little thin. Control is often not recommended because the disease normally is not fatal, just unsightly."

By Dennis Hinkamp - Utah State University Extension

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