Good Fruit Grower

February 15

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Diseases & Disorderseases Fireblight, antibiotics, and the NOSB Resistant rootstocks won���t be widely available for several years, and they do not confer resistance to the top of the tree, where infections occur. by David Granatstein ntibiotics have long been key disease control materials for fireblight, one of the few uses of antibiotics in plant agriculture. These materials are natural compounds produced by naturally occurring soil microorganisms. For the past several decades, the most common control for fireblight has been the use of antibiotic sprays based on need using a predictive model of disease development. The two antibiotic materials used are oxytetracycline (from the tetracycline group of antibiotics) and streptomycin. The former is primarily used in the Pacific Northwest, while Midwest and eastern growers rely on the latter. Streptomycin is now generally ineffective in the Pacific Northwest due to resistant disease strains, while oxytetracycline can provide 85 to 95 percent control if application is timed correctly. These materials have shown the best and most consistent control compared to other options, which include plant genetics, sanitation, removal of diseased limbs, nitrogen and water management, copper, and biocontrol products. Biological controls have been researched and developed continuously since the 1980s, but to date, no single control alternative has proven equal to antibiotics. For example, Dr. George Sundin and colleagues at Michigan State University field tested several biocontrols over seven years in Michigan, New York, and Virginia, and reported in 2009 that ���the prospects for biological control of fireblight in the eastern United States are currently not high due to the variability in efficacy of existing biological control options.��� In the Pacific Northwest, oxytetracycline is applied when needed during the bloom period of apples and pears, and there is little evidence of residues on the fruit at harvest. When the National Organic Standards were being drafted, the two antibiotics, both considered synthetic products because of their manufacturing process, were ultimately put on the National List of Allowed Synthetics with the specific annotation for use only for fireblight control in organic apples and pears. Materials on the list are reviewed every five years as part of the sunset process to determine whether their continued use is justified or whether a suitable organic-compliant alternative has become available. Given that these are the only allowed uses of antibiotics on organic foods, there has been pressure to phase them out. Various interests have put forward justification over the years, while growers have cited the lack of effective alternatives. In 2006, the National Organic Standards Board decided by a vote of 7 to 4 to renew the two antibiotics until the next sunset review. Then, the NOSB received a petition in October 2007, to FIREBLIGHT is native to North America photo courtesy of tim smith, wsu F 12 FEBRUARY 15, 2013 GOOD FRUIT GROWER ireblight is a plant disease of apples and pears caused by the bacterium Erwinia amylovora. It is native to North America where it resides in wild hosts such as crab apple and hawthorn. Over the decades, it has spread to other pome fruit producing regions in Europe, the Middle East, and New Zealand. But it is currently absent from South America, South Africa, East Asia, and Australia. Apple and pear orchards in the Pacific Northwest are at risk from this disease, which is even more challenging in other parts of the country. A serious infection does not occur every year in every orchard, but constant vigilance is required. Because the most common infection route under Northwest conditions is through the blossom, monitoring and control is generally most critical at bloom. The disease can be vectored in many ways, including honeybees, other insects, birds, rain, wind, and hail. Once a tree is infected, the bacteria travel downward in the tree���s vascular system and can kill the entire tree and can rapidly spread to infect an entire block. Thus, the economic consequences can be devastating.

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