Issue link: http://read.dmtmag.com/i/54939
Building on BIOCONTROLS Several strategies are being explored for controlling fireblight without conventional antibiotics. by Geraldine Warner otics for controlling fireblight, biological controls have the advantage of multiplying and spreading after they've been applied—something that's not possible with a non- living control agent, says Pusey, plant pathologist with the U.S. Department of Agriculture in Wenatchee, Washing- ton. Several biological controls—beneficial bacteria and yeasts selected to outcompete the disease organism Erwinia amylovora—have been commercialized. Because the fireblight disease generally begins in the D flowers, Pusey's research has focused on the biology of flowers and the microorganisms that are found on them. During warm weather, fireblight bacteria increase to high levels on the flower stigma, the part of the flower that receives the pollen. The stigma exudes nutrients and moisture that not only promote pollen tube development but encourage growth of the bacteria. Then, in moist con- ditions caused by rain or dew, the bacteria move down to the hypanthium (flower cup) and infect flower tissues through the nectary openings. Growth of the fireblight bacteria on flowers, and on the stigmas in particular, is dependent on temperature, as is the growth of beneficial microorganisms, Pusey said. The age of the flower also plays a role. Generally, stigmas are most conducive to bacterial growth during the first four days after petal expansion. After that, the cells collapse, and the secretions that support microbial growth stop. High moisture levels also promote growth of micro - organisms in the hypanthium. In dry conditions, nectar 20 FEBRUARY 15, 2012 GOOD FRUIT GROWER r. Larry Pusey is collaborating with scientists around the world to optimize biological con- trol of fireblight and integrate it with other control strategies. As well as providing an alternative to antibi- sugars in the hypanthium are usually high enough to sup- press many microorganisms. However, as the moisture level increases, the nectar sugar concentration decreases, allowing bacteria to grow more rapidly and increasing the risk of infection through nectary openings. It's also been shown that the water potential in the inner tissues of the flower is closely related to disease incidence, suggesting that in some situations, it might be wise to limit early irrigation to reduce the fireblight risk, Pusey said. Microorganisms During the 1990s, Pusey and his colleagues collected large numbers of microorganisms from apple blossoms and evaluated more than 1,000 strains as potential bio- logical controls by assessing their ability to suppress the fireblight organism on flower stigmas. Although strains of the yeast Aureobasidium pullans were the most common on flowers, the bacterium Pantoea agglomerans strain E325 (now available as Bloomtime) was the most effec- tive against the fireblight organism. Other effective strains were identified as species of Pseudomonas, the same tax- onomic group to which the BlightBan bacterium belongs, Pusey said. Two strains of A. pullans had been commercialized in Germany as biological control agents for postharvest fruit diseases. They were later tested for fireblight control and are the active ingredients in BlossomProtect. Pusey's studies have shown that bacteria and yeasts naturally colonize both stigmas and hypanthia but tend to dominate at different times in the life of the flower. Bac- terial control agents are more likely to colonize the stigma and young flowers, whereas yeasts prefer the hypanthium and dominate in old flowers. The implication of this is that bacterial biocontrol agents should be applied in early bloom, but applications of yeast-based products could be delayed. FUTURE STRATEGIES Pusey has developed a five-year research plan covering bacterial biological control, yeast biocontrol, and integrated management. Bacterial biocontrol Pusey doubts that further bacterial strains will be dis- covered that are more effective than existing ones, so ways need to be found to improve them. This might be achieved by a higher cell concentration in commercial products or higher application rates in the orchard. Improvements in formulations to increase the survival of the bacteria and their spread by pollinating insects are being researched. Pusey has looked at increasing the tol- erance of bacteria to dry conditions by subjecting them to high salt concentrations in growth media. So far, this has increased the stability of freeze-dried formulations before treating flowers but has not significantly improved the survival of bacteria on flowers. Pusey is collaborating with nanoengineers Drs. Kevin Kim and Hyungsoo Choi at the University of Illinois who specialize in fabricating controlled-release micro - capsules. They are microencapsulating cells of Pantoea agglomerans E325 and hope to reduce the size of the microcapsules to the size of pollen grains, which might lead to better dispersal of the organism in the orchard. The microcapsules might also improve the survival of the microorganism and protect it from materials applied in the orchard, such as lime sulfur. www.goodfruit.com