It sounds like the name of an exotic new drink, but tropical soda apple has been more aptly described as the “plant from hell” say University of Florida researchers who have developed a natural way to control the rapidly spreading weed.
“The highly invasive plant, which forms a dense and thorny thicket that is impenetrable to animals and people, has been classified by the federal government as one of the nation's most noxious weeds,” said Raghavan Charudattan, professor of plant pathology with UF's Institute of Food and Agricultural Sciences. “In Florida and seven southeastern states, it's literally taking over, displacing native plant species in infested areas.”
He said the weed, native to South America, is a serious environmental threat to natural areas, and it has become a major problem for the beef and dairy cattle industries.
Sharp thorns make the plant's foliage unpalatable, but livestock, wild animals and birds that eat the fruit help spread the seeds. Mature plants can produce 50,000 seeds that germinate under a wide range of conditions. Seeds also can be spread by compost, sod and moving water, Charudattan said.
Another concern, he said, is that cattle shipped out of Florida may harbor plant seeds in their digestive tracts and spread the weed to neighboring states. To stop the spread of seeds, Georgia, South Carolina and other Southeastern states may require Florida cattle to be held on weed-free pastures for 10 days before being shipped to nearby states.
“Pastures infested with the weed have less area available for cattle grazing, which means the stocking rates — the number of animals per acre — must be reduced,” Charudattan said.
Wade Grigsby, vice president of the cattle division for Alico Inc. in LaBelle, said the weed is an economic and environmental headache for the livestock industry.
“It may be impossible to eradicate tropical soda apple, but Charudattan's new bio-control is the best option we have for bringing the weed under control,” Grigsby said.
Until now, the only way to control the weed was with repeated mowing and chemical herbicides. But, Charudattan said, applying herbicides is a problem for the cattle industry because of possible chemical residues in milk and meat.
Charudattan's research has shown that a common plant virus can be used to kill tropical soda apple, and he is seeking commercial partners to produce and market the virus as a natural bio-control or bio-herbicide.
“During a routine examination of several plant pathogens for their ability to cause disease on tropical soda apple, we discovered that tobacco mild green mosaic virus (TMGMV) kills the weed,” he said. “Tests in two pastures demonstrated the virus kills up to 97 percent of the weed.”
To determine which plants may be vulnerable to the virus, Charudattan is testing the virus on some 200 different plant species, including other weeds and cultivated plants. The virus does not affect people or animals, he said.
“We know that some varieties of tobacco and peppers are susceptible, but the virus can be used safely in areas where tobacco and peppers are not grown,” he said.
The virus, which can be applied easily and inexpensively with a portable back-pack sprayer, is effective against tropical soda apple under a wide range of temperatures and year-round growing conditions.
Charudattan said the bio-herbicide would be easy to produce. “High concentrations of the inoculum can be produced inexpensively in tobacco plants and stockpiled for use. It remains effective for decades,” he said.
Susceptible tobacco plants could be used to mass-produce a commercial bio-herbicide. As they mature, infected leaves are harvested, freeze-dried and ground into a fine powder for storage at room temperature, he said.
“To demonstrate how easily the bio-herbicide could be produced, we are establishing a pilot production facility at UF,” Charudattan said. “The prototype production system could be established as a self-sustaining service from UF or licensed to a commercial company.”
Other UF plant pathologists working with Charudattan on the tropical soda apple product include professors Bill Zettler and Ernest Hiebert and graduate student Matt Petterson.