UNIVERSITY PARK, Pa. -- The fifth-wettest summer in more than 100 years followed by the remnants of four hurricanes dumping historic amounts of rain on Pennsylvania this fall likely will result in the state's autumn foliage display being less spectacular than usual, according to forest experts in Penn State's College of Agricultural Sciences.

"From what we are seeing, we can speculate that the foliage display won't be as brilliant as it is many years," says Jim Finley, professor of forest resources. "Some species appear to be experiencing early leaf drop. In addition to all the rain, we've had significant wind loads from the recent storms across the state. So there are places you normally might expect to see bright colors where trees are partially bare."

Don Davis, professor of plant pathology, confirms that the extremely wet weather has caused trees to be stressed by fungal diseases. "It's a rule of thumb that in wet years, insect problems are reduced because the bugs just don't tolerate moisture well," he says. "But in wet years, fungal diseases are a problem for trees. This year, fungal diseases such as anthracnose on maples and oak leaf blister are causing the leaves on many trees to turn brown and even fall off twigs early.

"As a result," he adds, "I'm afraid the foliage display in some regions won't be as spectacular as it usually is."

Finley notes that the wet weather has enhanced the growing season, and in well-drained areas where trees have escaped disease problems associated with excessive moisture, tree canopies are dense. "So we expect it to be a regional phenomenon," he says. "The foliage will be bright as usual in some places, but where you probably will notice a lack of color will be in low-lying areas that have been exposed to wind."

Penn State forest experts agree that for foliage to be bright and beautiful, it must stop raining in the next few weeks. Some weather-related things must happen in coming days and nights for the foliage display to reach its potential, points out Marc Abrams, professor of forest ecology and physiology. "It must cool off," he says. "Night temperatures dropping into the 30s and 40s are good for the foliage.

"The worst thing that could happen for the foliage would be if it stayed warm and rainy," he adds. "We want to see bright, sunny but cool weather."

For nearly two decades, Abrams has studied how seasonal precipitation and temperature influence timing and intensity of fall colors in central Pennsylvania. "We believe that clear, bright days, low -- but not freezing -- temperatures and dry conditions promote the best fall colors," he says.

"Cooler temperatures signal deciduous trees to stop producing chlorophyll, the green pigment responsible for photosynthesis," he explains. The chlorophyll breaks down and disappears, unmasking other leaf pigments. It's these other pigments -- called xanthophylls and carotenes -- that create the yellows and oranges seen in the leaves of yellow poplar, hickory, sycamore, honey locust, birch, beech and certain maples.

After chlorophyll production stops, trees also produce another pigment in their leaves called anthocyanin, Abrams says. The anthocyanins create the brilliant reds and purples seen in maple, sassafras, sumac, black gum and oak.

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