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Excerpt from the Popular Garden Series magabook Orcharding with permission from its publisher, BowTie magazines, a division of BowTie Inc. Purchase Orcharding here (deep link http://www.shopanimalnetwork.com/product.aspx?cid=85&pid=838)
Fire blight is a serious bacterial disease of apples and pears. Bacteria are usually spread by water splashes and/or flowing water, but can also be spread by moisture and management practices. Fire blight bacteria don’t produce spores like fungi, but can survive on dead tissue in the absence of a live host.
Fire blight gets its name from the shoot blackening that it causes; limbs literally look burned. The first sign of fire blight is an ooze – tan and watery – as growth begins; the ooze then turns dark. Fire blight is usually noticed when shoots start to blacken and die. Fire blight can also cause cankers on bark. The pathogen overwinters in these cankers and spreads by oozing in the spring; the ooze can be carried by insects or by water splashed on new flowers. Temperatures of 75 to 85 degrees Fahrenheit and humidity or rain create ideal conditions for fire blight infection. Once blooms are infected, fire blight can spread to twigs and branches.
Managing fire blight starts with choosing less-susceptible varieties. Keep trees healthy, but avoid over-fertilizing; the spread of fire blight is related to the rate of growth. If symptoms of fire blight appear, prune out the affected shoots eight to 12 inches below where you see signs of injury. Remember that you can spread the disease, so sterilize your pruning equipment with a 10 percent bleach solution between cuts. Dispose of infected material by burning it or sending it to a landfill. In the event of a nearby fire blight infection, sprays of a weak solution of copper or Bordeaux as blossoms open can help reduce, but won’t completely prevent, the spread of fire blight.