Cranberry growers know the symptoms well: hard, dark-colored bumps on stems, brown leaves and dried-up fruit. The disease, cranberry stem gall, causes major damage to crops and shrugs off treatment with fungicide. However, a UW-Madison plant scientist”s latest research suggests that the key to understanding the disease may lie in the previously unexplored combination of bacteria in the soil and environmental factors.
“Growers in the past have used fungicides to treat stem gall, but we can now say confidently ”Don”t bother,”” says Patricia McManus, a plant pathologist with the College of Agricultural and Life Sciences. “The best advice is to be very careful not to injure plants while working fields-and we know that no one plans to injure their crops.” McManus” findings indicate that certain bacteria, which are commonly found in soil, infect cranberry plants through open wounds and cause rough growths that crush the water-carrying vessels in the plant”s stem.
Cranberries are the most important fruit crop in the state, according to McManus, who took her research samples from commercial farms in central Wisconsin. Last year Wisconsin growers produced more than half of the nation”s cranberries, and the industry contributes millions of dollars to the state”s economy.
Although cranberry stem gall occurs in other regions, it is most common in Wisconsin, especially in the central part of the state. Stem gall ultimately kills the shoots that bear fruit, and many farmers also experience weed problems in places where plants die out.
Growers initially thought that the stem galls were cankers caused by fungal infection or wound callus tissue, a plant”s natural response to an injury. However, McManus found that stem galls do not resemble cankers when examined under a microscope, and lack physical evidence of fungi. She also found high levels of bacteria deep within the galls.
The bacteria cause galls because they produce a growth hormone called indole acetic acid (IAA), McManus believes. These bacteria occur naturally in the soil and under normal conditions the IAA they produce may help promote root development. However, when the bacteria enter a cranberry plant-perhaps through a wound from contact with harvest machinery-they cause harmful growth, says McManus. Her research is the first to link cranberry stem gall, or any cranberry malady, to bacteria.
While bacteria cause the bumpy growth, environmental factors are responsible for the wounds that allow the bacteria to enter plants. Field equipment can cause injury, as can frequent freeze-thaw cycles. The worst recent years for stem gall were 1998 and 2002, both growing seasons that followed mild winters during which growers had trouble keeping a protective layer of ice on their plants. Excessive flooding may contribute to stem gall as well; McManus speculates that the water may wash bacteria into plant wounds.
“The best advice we can give is to be careful about injury,” says McManus. “Fungicides will not help with stem gall, even fungicides that contain bactericidal compounds like copper. Those applications will only work on the outside; they won”t get the bacteria inside the plant.”
McManus” cranberry research is supported by a Hatch grant from the United States Department of Agriculture, and the State of Wisconsin.