Good Fruit Grower

February 15

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6 FEBRUARY 15, 2016 Good Fruit Grower H istorically, the Konnowac Pass area near Wapato, Washington, has been known for Bartlett pear production for the canning industry. Of course, pears are particularly susceptible to fire blight, and that history — combined with prime weather conditions last year — are triggering a test bed for the disease at a rootstock trial site owned by the Washington Tree Fruit Research Commission near the USDA entomology laboratory in Wapato. A Gala replacement tree rootstock trial is underway there to better understand the replant disease tolerance in orchards with occasional tree losses. It turns out, this block of Gala also has acquired significant fire blight pressure due to the unique weather of 2015. Unfortunately, the block is mostly on Malling 9 rootstock, which is highly susceptible to fire blight. Nearly 10 percent of the M.9 trees planted in 2010 have had the rootstock die, as of September 2015. The circumstances are giving researchers there an unplanned oppor- tunity to learn more about fire blight, and better understanding the dis- ease is crucial to the industry. "Growers are spending $50,000 an acre to plant a new orchard. We can't have any tree losses due to fire blight after planting new trees," said Tom Auvil, horticulturist with the Washington Tree Fruit Research Commission. Ground zero for fire blight Fire blight is caused by the bacterium Erwinia amylovora. The bacteria overwinter in cankers and can float in the air before coming to rest on flowers in the spring, colonizing the stigma prior to being washed into the base (floral cup or nectary) of the flower by dew or rain. The flower cluster becomes blighted, and bacteria enter the vascular system of the tree. Early season dieback of shoots starts from flower infections. The bacteria move with water, wind and insects, and any event that wounds tissue — such as hail or wind — provides infection opportuni- ties. The bacteria begin to multiply once temperatures are greater than 65°F. Growers should turn to fire blight models to determine their risk level once temperatures begin to get into the 70s. The research plot near Wapato sits in a bowl of flat ground in an extended area where temperatures can warm quickly and with high potential for dew to appear well up in the tree. The area includes roughly 30 acres of pears, as well as 20 acres of apples, including a 2010 planting of 15 acres of Gala. The first thing researchers noted last year: Roughly one in 10, or 10 percent, of the trees planted in 2010 defoliated, and on each of those trees, the rootstock died before dormancy. An earlier planting in 2006 didn't have anywhere near the amount of fire blight, Auvil said. Gala has a unique feature where some spurs will bloom a few days after the one-year wood — a kind of double bloom that is similar to Barlett pears, he said, which opens the susceptibility window for fire blight. Add in the high potential for dew and the inoculate load, and the area was ripe for bacteria to multiply rapidly. Despite the region's historical susceptibility to fire blight, this is the first time the research block has really been clipped by it, Auvil said. "Whatever conditions occurred in 2015, we just had a little uptick in that delayed bloom, plus some bloom in August/September, then the weather triggered the blight infections." Learning opportunities In some seasons, trees bloom in August and September, dramati- cally increasing the risk of tree infection through flowers. M.9 and M.26 Fire Blight CENTRAL A research plot near Wapato, Washington, has become a test bed for fire blight. by Shannon Dininny Risk factors D r. Ken Johnson, plant pathologist at Oregon State University, reminded growers at the Washington State Tree Fruit Association Annual Meeting about risk factors for fire blight, including the vigor of the tree (a vigorous tree can spread the disease more quickly) and among some of the newer cultivars, such as Jazz and Cripps Pink. But the biggest risk factor for fire blight is a tree's age — and Washington growers are planting a lot of young apple blocks. "It's just about impossible to keep it out once it starts cooking in a young block of trees," he said. "Programs need to be a little bit more intensive in young blocks. There's a lot invested there, and if you mess it up, you mess it up big." A few things to remember: Model thresholds for action should reflect the orchard's risk — whether there was fire blight in the orchard or in the neighborhood the previous year, the age of the block and the cultivar, among other things. Johnson recommends that growers lower their risk thresholds on the models if they have younger trees that are more susceptible, even if there was no fire blight in the area last year. A fire blight infection can come late in the bloom period. While it may be difficult to find the pathogen at full bloom, it becomes more abundant as bloom progresses, and by petal fall, can be found easily. Any interpretation of moisture depends on the orchard location and should be considered when determining risk level. Likelihood of dew and rain are both considerations, as are irrigation, both in the grower's orchard or from a neighbor's field. —S. Dininny Tim Smith cuts into a young Gala tree looking for fire blight damage during a Washington Tree Fruit Research Commission rootstock field day in Wapato, Washington. Diseases

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