An apple orchard in late May, just as the trees’ deep pink buds expand into pale pink blossoms, is one of the most beautiful places on earth. I won’t tell about the smells or the songs of birds to further describe this heaven, because this story is about fire blight, a bacterial disease that first strikes right at this magical spring moment. When fire blight bacteria land on an apple blossom’s nutrient-rich stigma (sometimes delivered by pollinating honeybees), they move right in and multiply. The blossoms soon will look a bit water-soaked; then they will wilt, shrivel up, and turn brown.
This is unpleasant for the casual observer, but it can be disastrous for an orchardist. In addition to infecting blossoms, fire blight bacteria also colonize shoots, branches, and rootstocks.
When fire blight invades new shoots, all the leaves die and then turn black, making them appear to have been badly scorched in a fire. The end of the shoot often droops to resemble a shepherd’s crook.
Apple orchards nowadays are generally planted at high density, and many of the size-controlling rootstocks that have been developed for modern orchards turn out to be highly susceptible to fire blight. If the bacteria make below-graft inroads into these rootstocks, the tree is likely to die.
And when strong winds or hailstorms create multiple wounds in the leaves and stems of an apple tree, fire blight bacteria are able to enter many places at once, causing what is called trauma blight. Each infection can spread, and sometimes the tree is soon fighting a losing battle.
Fire blight only affects members of the rose family, but this very large group has many important members. Hawthorn and pears are also susceptible, as are numerous wild tree species and horticultural species. Now found in 43 countries, fire blight is native to North America and in 1883 was the first bacterial plant disease to be discovered and described.
Most commercial fruit growers take fire blight seriously and counterattack on a number of fronts. Planting resistant varieties is an obvious choice, but pear growers don’t have many alternatives. As for apples, two resistant apples, Freedom and Liberty, fortunately have many other desirable traits. Several choice new apples – Gala, Braeburn, and Fuji – are quite susceptible, however, as are some old favorites, such as Rome, Ida Red, and Jonathan. It’s not just cultivars that are susceptible; wild apple trees can become infected by fire blight.
Copper fungicides are effective at reducing new infections when applied at four- or five-day intervals during blooming, but the inevitable build up of copper in the soil and its harmfulness to earthworms limit its usefulness.
The antibiotic streptomycin has also been used, and when carefully scheduled, taking degree days and humidity into account, used to be very effective. The recent emergence of streptomycin-resistant fire blight bacteria in some parts of the country does not bode well for its usefulness in the future.
Pruning and burning any and all infected parts is the most important way to control the amount of fire blight inoculum in an orchard. Paul W. Steiner, from the University of Maryland, recommends the “ugly stub” method of fire blight removal. As soon as fire blight is identified, the affected branch is removed, at least 8 inches below any infections, but not back to the next branch intersection. The “ugly stub” that is left is flagged and, in winter, when fire blight is dormant, a proper pruning cut is used to remove the stub. It is important to disinfect pruning tools between each and every cut by dipping them in a 10 percent bleach solution.
One hopeful development is the use of another bacterium, such as Pseudomonas fluorescens, which is antagonistic to Erwinia. Honeybees can be dusted with these organisms as they leave their hives; when they are out seeking nectar, they deliver the bacteria to apple blossoms. There they aggressively compete for the nutrients on flower stigmas and prevent fire blight from becoming established. There is a sprayable version of this treatment, called Blight Ban. Timing is tricky and critical: the fire blight competitors must arrive on the blossoms before the fire blight.
Even though fire blight is native to North America, apples and pears are not. This disease is relatively new to them, and so far they have not yet developed a good defensive strategy. Add to this the rich opportunity that fire blight encounters whenever its preferred food is planted as a monoculture. Our native rose family members – the serviceberries and hawthorns, for instance – are seldom killed by fire blight. These have coexisted with the disease for thousands of years.
Sanitation, antibiotics, antagonistic bacteria, and copper all have some effect on fire blight, but the weather during apple bloom is of overriding importance. As Michael Philips, author of The Apple Grower, says, “When temperatures rise above 80°F and humidity is high, fervent prayer may be the best bet going.”