Mar 29, 2021Researchers ponder presence of cranberry false blossom
Cranberry growers are seeing a reemergence of a disease that was a serious problem in the early 1900s but rarely observed after about 1940. Cranberry false blossom nearly decimated cranberry production in some U.S. cranberry growing areas until the introduction of resistant cranberry varieties and broad-spectrum insecticides curtailed the incidence of the disease.
New occurrences of cranberry false blossom were first reported in New Jersey in the late 1990s, followed by Massachusetts in 2017 and Wisconsin in 2018, according to University of Wisconsin-Extension fruit pathologist Leslie Holland.
Holland discussed the reemerging disease during the 2021 Wisconsin State Cranberry Growers Association annual winter meeting. Due to COVID-19, this year’s meeting and trade show were held virtually Jan. 27-28.
Cranberry false blossom disease is caused by a phytoplasma, a single-celled microorganism that lives in the phloem or sugar-conducting tissue of plants.
The disease results in yield loss by causing flowers to abort in the year that symptoms appear and by reducing or eliminating bloom in subsequent years.
“Infected vines do not recover. They do not bear fruit. This is a very devastating disease,” Holland said.
Cranberry false blossom was largely eliminated after cranberry growers adopted the use of insecticides that controlled the blunt-nosed leafhopper, which spreads the disease, and the extensive planting of resistant varieties like Stevens.
The blunt-nosed leafhopper has several host plants besides cranberry, all in the family Ericaceae, including fetterbush, leatherleaf, dwarf huckleberry and wild cranberries.
Even though the disease had seemingly been controlled in commercial cranberry plantings for decades, Holland said the phytoplasma has likely persisted at undetectable levels.
One theory for the reemergence of the disease is the adoption of more targeted insecticides which has given the blunt-nosed leafhopper the chance to multiply again, Holland said. Climate change may also be a factor in the increased activity of leafhoppers, she said.
The blunt-nosed leafhopper shows a preference for some cranberry varieties like Howes and Centennial, while other varieties like Stevens and McFarlin seem to be less attractive to the insect. Little is known about the leafhopper resistance of the newer cranberry varieties growers are now planting, Holland said.
The only known vector of the disease is the blunt-nosed leafhopper. The leafhopper picks up the false blossom pathogen from an infected plant and spreads the disease when it feeds on other plants. It can take years after infection before the vines exhibit symptoms.
“This really exacerbates this disease issue because you’re now dealing with something that could have been infected a year ago or more. It makes it a little bit difficult to get an accurate diagnosis of this disease in a timely fashion,” Holland said.
Infected plants may show a variety of symptoms, but not necessarily each one of the characteristic indicators. Instead of their normal light pink color, cranberry blossoms of infected plants often are dark pink or streaked with red. The pedicels or flower stalks tend to be erect rather than arched. The stamens, or male flower parts, and the pistils, or female flower parts, are abnormal as well, Holland said.
Diseased vines tend to extend above the plant canopy and turn red prematurely in the fall. Infected vines may have smaller leaves folded close to the stems. The stems also are often closely spaced creating a “witch’s broom” appearance (pictured at top), Holland said.
Because cranberry false blossom persists in plants from year to year, the disease can be introduced into new locations via vine cuttings or plugs used for propagating new cranberry plantings.
There is no control for cranberry false blossom once vines are infected. Typically the infection occurs in scattered areas of a cranberry bed. Holland’s advice to cranberry growers is to dig out the infected vines, including the roots.
Pamela Verhulst, co-owner of Lady Bug IPM, a cranberry crop consulting service based in Pittsville, Wisconsin, found cranberry false blossom in a few isolated locations this past season. At this point, Verhulst said neither the disease nor the blunt-nosed leafhopper appears to be widespread in Wisconsin’s central cranberry growing region.
“But growers and crop consultants are adding blunt-nosed leafhoppers to the list of important pest insects. We’ll be scouting for blunt-nosed leafhopper nymphs from May through June and be on the lookout for false blossom during bloom and the second week in September, when the disease is easiest to observe,” Verhulst said.
Although there are no established economic thresholds for blunt-nosed leafhopper, Verhulst said leafhoppers feeding on vines can cause twisted and deformed leaves. In high enough numbers, the damage leads to aborted flowers, pinheads and fruit, she said.
Not all blunt-nosed leafhoppers carry the disease. Working with Ocean Spray agriculture scientist Dave Jones, Verhulst submitted a sample of leafhoppers from a cranberry marsh that has cranberry false blossom present, but the insects tested negative for the disease.
“It is possible that only a small fraction of the 100 plus leafhoppers we provided Ocean Spray actually carried the phytoplasma and during the random selection for the test, those that were selected for testing did not carry the disease,” she said.
When false blossom is found, its spread can be minimized by controlling the blunt-nosed leafhoppers. Verhulst said blunt-nosed leafhoppers seem to be attracted to diseased plants, feeding on them and spreading the pathogen to healthy vines.
Holland said phytoplasmas require living tissue to survive and multiply and will not persist in dead plants, soil or water. The disease is not spread by irrigation or flooding cranberry beds, she said.
Holland and UW-Extension fruit crop entomologist Christelle Guedot hope to do research this coming growing season to determine the prevalence of cranberry false blossom in Wisconsin and what steps growers can take to minimize the incidence of the disease.
“I don’t think there’s a large presence of cranberry false blossom in Wisconsin. So far, we’ve confirmed through the diagnostic lab that it is present on a few marshes. But that’s part of the research that myself and Christelle are proposing we do to investigate how widespread false blossom is here in Wisconsin,” Holland said.
— Lorry Erickson, FGN correspondent