Published by Utah State University Extension and Utah Plant Pest Diagnostic Laboratory PLP-013-PR June 2021 Fire Blight in Utah: An Annual Management Guide Marion Murray, IPM Project Leader Fire blight is a serious disease of apple and pear trees. In the last decade in Utah, there were several seasons where this disease caused extensive losses. The prime conditions for infection require the combination of open blossoms, warm temperatures, and wet weather. Because this disease spreads rapidly and symptoms are not visible until two weeks after infection, it can catch growers off-guard. Having a well-executed and integrated year-round management program can reduce losses from fire blight. Why Is Fire Blight an Issue? • Apple varieties grown by commercial producers are driven by consumer demand and include Fuji, Gingergold, Gala, and Honeycrisp. Unfortunately, these varieties are more susceptible to fire blight, especially when compared to the traditional varieties previously grown (McIntosh, Red Delicious, Cortland). • Most large, commercial apple orchards are planted in a high density. This provides greater yields, but also allows fire blight bacteria to spread rapidly from flower to flower. • Trees planted in high-density orchards are often on dwarfing rootstocks such as M.9 or M.26 that are Fig. 1. Cankers often appear water-soaked or sunken (top). susceptible to fire blight. Scraping the bark away reveals the canker margin (bottom). FIRE BLIGHT BIOLOGY Fire blight is caused by the bacterium Erwinia amylovora. Bacteria survive the winter in cankers (areas of dead bark and phloem killed by fire blight the previous year). Cankers can be small lesions associated with wilted terminals, while others are large areas on scaffold limbs (Fig. 1). In spring, bacteria multiply and start oozing out of infected plant tissue, colonizing the bark surface. Bacteria become slowly active at 50°F and multiply more rapidly as temperatures increase (Johnson, 2015). Fig. 2. Bees can spread Erwinia bacteria to other flowers. In Utah, the most important infection entry point is the visible symptomatic tissue. Pruning crews working through blossoms. Bacteria from the oozing cankers are in large orchards may need to take multiple passes in spread within trees and from flower to flower by wind- the orchard, but the results will pay off. For dormant driven or splashing moisture and by pollinating bees pruning, tools do not need to be sterilized between cuts or other insects (Fig. 2). When Erwinia bacteria land on (Lecomte, 1990). the flower stigma, they multiply rapidly as temperatures warm. Infection occurs when large amounts of bacteria Other winter practices include: on the stigma are washed into the floral cup by rain, • Prune out any 4-inch stubs that were left from pruning irrigation water, or heavy dew. Just two hours of the prior summer (see “Summer” section and Fig. 4). moisture is needed for infection, and heavy dew plays a • Remove all dead trees and trees so severely infested big role in Utah in the infection process. Infected flowers where it is impossible to remove all the blighted tissue wilt and turn brown or black, and bacteria travel within with pruning. plant tissue to leaves, up succulent shoots, and beyond. • Remove all root suckers and rootstock sprouts. If they become infected, they may put the entire tree Shoot infections can sometimes occur later in spring or at risk depending on the rootstock shown on Table 1 early summer when moisture carries bacteria to foliage (Crassweller & Schupp, 2018). injured by hail or other trauma. These infections are not associated with blossom blight or existing cankers but Winter debris does not need to be burned or chipped are considered new infections. Once plant tissue hardens and can be left on the orchard floor and mulched. A cut off, the risk of shoot blight declines (Johnson, 2015). dormant branch on the ground will not be able to spread inoculum the following spring (Johnson, 2015). As tree growth slows in hot weather, bacterial progress within the tree also slows. In some trees, damage may Table 1. Dwarfing Apple Rootstock Susceptibility be minimal, while other trees may be severely injured or killed, especially if bacteria enter a susceptible rootstock Apple Resistance Apple Resistance (Table 1). In the heat of summer and over the winter, the Rootstock Level Rootstock Level bacteria “rest” in infected tissue until trees come out of Bud 9 (B.9) susceptible M.7 resistant dormancy and temperatures rise the following spring. moderately highly B.118 M.9 susceptible susceptible WINTER Geneva 11 highly resistant M.26 (G.11) susceptible Pruning/Removal highly G.16 M.27 susceptible Removal of overwintering cankers from dormant trees is resistant one of the most important steps in managing fire blight. highly moderately G.41 MM.106 It reduces the amount of inoculum in the orchard, thus resistant resistant reducing future infections. highly moderately G.65 MM.111 resistant resistant Prune trees in winter or early spring when the weather is dry. Infected branches are easily identified because the For more information about resistance, see the tables for leaves remain attached well into winter (Fig. 3). Limbs apple variety resistance and apple rootstock resistance in the with bark that appears wet, off-color, or sunken indicate Intermountain Tree Fruit Guide website. a canker (Fig. 1). Cut these limbs at least 8 inches below Fig. 4. Some trees may need heavy pruning, which is best Fig. 3. Leaves remain attached on infected twigs well into done in winter. If larger branches are removed in spring, leave winter. a 4- to 6-inch stub to be removed the following winter. Fire Blight Annual Management Guide Page 2 SPRING Predicting Fire Blight Infections USU provides access to a predictive fire blight tool on Copper the USU Climate Center website at climate.usu.edu/traps for a range of locations in Utah. The tool uses a model The efficacy of copper as delayed-dormanta developed by Washington State University (WSU) called application has been proven effective by researchers at “Cougarblight,” and is based on several orchard and Oregon State University. They found that copper does environmental factors (Smith, 2010): not kill bacteria on existing bark cankers, nor does it prevent bacteria from oozing from cankers. It works 1. Local blight history of the orchard: The presence by providing an inhibitory barrier on bark and buds, or absence of blight in the orchard or nearby will preventing widespread colonization of bacteria on these affect the risk potential for infection. Infection is surfaces (Elkins et al., 2015). most likely to occur if there was fire blight in the orchard or in neighboring orchards last year. A delayed-dormant copper application is applied before bloom, between bud swell (silver tip) and when 2. Daily maximum temperature: Although bacteria leaves just start emerging (green tip) (Table 2). Its effect can start multiplying at 50°F, the most accelerated is related to how it is applied and the weather after bacterial division occurs between 78 and 90°F. To application. It must be applied as a high-volume spray, provide a prediction, the tool calculates a “four- thoroughly wetting all exposed surfaces in the orchard, day risk value” related to each day’s maximum including any resistant trees growing near susceptible temperature. Temperatures from four days are used trees (e.g., Red Delicious). It should not be applied when because stigma tips, on which bacteria colonize, are ° temperatures will fall below 45 F that evening. viable for about that long. The four-day value then indicates a risk level of low, caution, high, extreme, Studies have not been conducted on the use of copper or exceptional. as a delayed-dormant application in Utah. The grower’s decision to use it at that timing will depend on weather, 3. Moisture: If flower stigmas are colonized with cost, amount of cankers in the orchard, and personal bacteria, infection will not occur without sufficient experience. moisture to wash the bacteria into the floral cup. Although the fire blight model provides a Some copper formulations have a lower metallic copper predicted risk level of infection, it does not account concentration and can be effective as ablossom for flower-wetting. (Wetting that triggers flower application (Table 2). Utah State University (USU) infection happens from a minimum of two hours researchers have tested several coppers as stand-alone of rain, dew, or light wetting from any form of treatments during bloom with limited success. sprinkler irrigation.) Therefore, growers should use the predicted risk level in combination with Table 2. Copper Formulations and Application Timing a local weather forecast for rain or check for dew Example % When to formation to decide on spraying. Keep in mind Active Ingredient Brands Copper Use that a heavy downpour does not seem to trigger Basic Copper 53, silver to infection, perhaps because the blossoms are basic copper sulfate 20-50% green tip actually washed free of bacteria colonies (Johnson, basic copper sulfate 2015). Cuprofix silver to plus C-O-C-S 51% green tip For a detailed description of the Cougarblight model, how four-day risk values are calculated, and descriptions Kocide, Champ, silver to copper oxychloride 25-50% Nu-Cop green tip of risk levels, see the Cougarblight fact sheet provided by WSU (Smith, 2010). copper hydroxide silver to Badge 20% green tip copper hydroxide + Antibiotic Treatments at Bloom silver to copper oxychloride half-inch An antibiotic should only be used based on the results Previsto 3% green and/ of the fire blight model on theClimate Center website copper hydroxide or bloom and if moisture is present. Streptomycin (Agri-Mycin silver to and others) is the most effective fire blight antibiotic Cueva, Grotto, copper octanoate half-inch available, but in parts of Utah, fire blight bacteria have Fertilome 2% (“copper soap”) green and/ developed resistance, limiting its effectiveness.