Leaves inside the tunnel are healthy compared to the defoliated cherry trees in the background that were grown without protecting tunnels.
Growing fresh-market, sweet cherries under tunnels appears to have more benefits than just rain protection, says a Michigan State University scientist. Tunnels also show potential for organic production, reducing pest and disease pressure that can discourage midwestern and eastern growers from even attempting to grow organic cherries or to use integrated pest management.
Dr. Gregory Lang, MSU horticulturist, started a tunnel research trial in 2005 under the premise that a tunnel production system would keep rain off fresh-market cherries and prevent cracking, with the potential to pay for itself in one year if rain occurred. "There are much higher incentives for tunnels in the Midwest and East than the West Coast," he said, adding that growers in Michigan lose their cherry crop to weather three out of every five years. "That can mean the difference of $30,000 an acre if the grower protects his crop."
But the potential economic advantage from tunnels can turn into an expensive outlay if it doesn’t rain during harvest for several years, Lang noted, so he began investigating other potential benefits that tunnels might provide to growers, which would make them more cost effective.
The tunnel trial is set up in two locations: the Southwest Michigan Research and Extension Center in Benton Harbor, where trees were just planted under a high-density orchard system, and the Clarksville Horticultural Experiment Station in Clarksville, a trial with seven-year-old trees. The tunnels are in widths of 24 and 28 feet and are 160 and 200 feet long. Dark red and blush varieties are grown under tunnels on the dwarfing Gisela rootstocks G.5, 6, and 12. Adjacent cherry trees of the same variety and rootstock combinations are grown without tunnels for comparison.
Lang thinks the tunnels may be the answer to improved integrated pest management and organic cherry production in areas where disease and pest pressures are great. However, he has found that "the tunnels change everything and sometimes create other problems." Climate dynamics of air temperature, relative humidity, soil temperature, wind speed and gusts, leaf wetness, and solar radiation are all affected.
Cherry leaf spot, a fungus that causes premature defoliation, weakens the tree, and predisposes it to winter injury and fruit set difficulties, is a problem for growers in humid climates. In hot, humid seasons, Lang has seen significant defoliation by the end of August if trees are not protected with fungicides.
Last year, although weather conditions were conducive to leaf spot, he found no symptoms in the cherry tunnels. "The leaves were in perfect health, without any spraying of fungicides."
But unlike cherry leaf spot, brown rot did not disappear from the tunnels. Disease severity was similar on trees outside and inside the tunnels, although symptoms showed up first in the tunnels, probably due to higher humidity and temperatures.
"I anticipated that powdery mildew would be a problem inside the tunnels, but last year it wasn’t," Lang said. "Powdery mildew is more season-dependent for us in the Midwest than in the West and shows up more in dry years. It may be a disease that is problematic some years and not others."
Preliminary data also revealed that bacterial canker was significantly reduced in the trees under the tunnels, with fewer trunk cankers. "Bacterial canker wipes our orchards out little by little," he said. "It is a true cancer for midwestern and eastern growers."
In the coming season, Lang plans to evaluate organic products to control brown rot and powdery mildew inside the tunnel system.
The absence or reduction of pests like plum curculio, cherry fruit fly, and Japanese beetles inside the tunnels are what really piqued Lang’s interest in using the tunnels for improved integrated pest management and organic production of cherries. Trees outside the tunnel were infested and sprayed conventionally, but there was no movement of the pests into the tunnels beyond the border trees (the first few within the tunnel). In the second year of the trial, no pesticides were used inside the Clarksville tunnels.
"The tunnels were a real plus when it came to Japanese beetle. We’ve had a huge problem with the beetle skeletonizing all of the foliage."
In 2006, damage from Japanese beetle was the worst Lang has ever seen, with the pest moving into orchards in mid-July and remaining until October. Surprisingly, the beetles did not move into the tunnels, which he attributes to the screening out of ultraviolet light by the plastic used in the tunnels.
"I think that Japanese beetles are like honeybees and need polarized light or UV light to navigate," he said.
Bird damage was reduced in the tunnels, but Lang thinks that may be because birds had a choice between trees in tunnels or not. He thinks that birds would likely learn to fly inside the tunnels if they were the only cherry trees around.
"We learned that some pests don’t like to go into the tunnels, but some do," he said, adding that they observed more aphids and spider mites inside the tunnels than out in one of the two years.
The Michigan State researchers also want to learn if the tunnels—with their protection from rain and UV light—will improve the efficacy of insecticides and fungicides. There is the possibility that spray applications inside tunnels can be minimized because the active ingredient doesn’t degrade as it would when exposed to rain and sunlight.
Additional research is needed to learn whether integrated pest management strategies can approach organic certification. However, Lang stated that the dramatic results of the preliminary studies have revealed that the tunnels have potential advantages for Great Lakes growers interested in targeting dwarf cherry production for high-value fresh markets.