Landscape - Fire Blight

Fire blight is being seen in ornamental pears and other landscape plants at this time in Delaware. The following is a good article on the subject from Rutgers University.

Fire blight is a relatively common disease caused by a bacterium (Erwinia amylovora). Fire blight can occur on many rosaceous plants, including crabapple, cotoneaster, hawthorn, mountain ash, pyracantha, and pear. Although we see this disease most often in pyracantha, some ornamental pears in central New Jersey have been recently diagnosed with the disease. Development of fireblight usually begins in the spring as bacterial cells ooze in a yellow-amber liquid from existing cankers on infected plants. These cells are spread to susceptible flowers, leaves, fruit, and stems by insects, wind, splashing rain, or pruning equipment. Insects are especially attracted to this sweet, sticky, bacterial ooze; bees and other pollinating insects commonly pick up bacteria while visiting the cankers and subsequently deposit the cells on developing flowers. Erwinia bacteria penetrate tissue through wounds and natural openings such as stomates and nectaries. Twigs and branches infected with the pathogen die rapidly and appear scorched, hence the name “fireblight.” Tender shoots tend to droop and bend as they die, developing a symptom commonly associated with the disease called a shepherd’s crook. As the disease advances, cankers form at the base of infected branches, and highly susceptible plants may die. Conditions optimal for fireblight development are extended periods of warm (greater than 65 to 75 F), wet weather during spring.

To manage the disease, improve plant vigor, but avoid heavy spring fertilization that would promote succulent growth. In late summer (after bacterial oozing no longer occurs), prune all diseased wood at least 6 to 8 inches below the infection, surface sterilizing tools between cuts. Remove water sprouts, and dispose of any infected plant material which may harbor the pathogen. The following bactericides have labels for fire blight (check for host, timing, and rates): copper (Badge, hydroxide, metallic, salts, sulfate), fosetyl- Al (pre-bloom and repeat every 7 days until bloom), Junction (5-day intervals during bloom), OR phosphite (7-day intervals through bloom). Finally, consider repeatedly diseased, highly susceptible plants with material that is more resistant to this disease. Highly susceptible callery pears include Aristocrat, Autumn Blaze, Capital, Fauriei, and Redspire; moderately susceptible cultivars include Cleveland Select, Earlyred, and Whitehouse. Although Bradford is reported to be moderately resistant in certain locations, it will develop fireblight under conditions optimal for disease development, as we noticed last year.

Fire blight on ornamental pear. Photo from the Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, Bugwood.org.

Information from Ann B. Gould, Ph.D., Specialist in Plant Pathology, Rutgers University.

Nursery and Landscape - What to do About Fire Blight Infections

Fire blight strikes are evident in landscape plantings. Fire blight affects rose family plants, most commonly apple and pear, but also crabapple, hawthorn, mountain ash, cotoneaster, pyracantha, and spirea. What should be done with fire blight infections? The following are recommendations.

Growers and landscapers dealing with infected trees are often tempted to remove fire blight infected branches as soon as they see them. In many cases, this would be the wrong strategy, because removing branches can encourage new shoots to develop and these new shoots would also be susceptible to new infections. If fire blight strikes are discovered early, before leaves have turned completely brown, timely removal of infected shoots can help slow the spread of the disease. However, most growers do not discover the disease early enough for this to be helpful. So what is to be done with infected trees now?

Growers should just let the disease run its course, allowing the tree defenses to stop fire blight spread within the tree. Dead shoots and branches should be removed later in summer, after the plant has stopped the infection or better in winter when there is little chance of spreading the disease.

Some growers may feel compelled to cut out fire blight infections, possibly for cosmetic or aesthetic reasons. What then? If pruning is begun after obvious symptoms appear, cut back in the direction of a healthy internode of at least two-year-old wood, leaving a stub several inches long. Rely on the tree's natural defenses to prevent further movement into the branch. If needed, paint the stub with bright paint to make it more obvious. This stub can then be safely removed in the winter. Leaving infected stubs rather than pruning all the way back to the main branch reduces the chances for development of undesirable water sprouts in response to pruning.

The reason not to prune infected branches back to a spur or crotch in summer is because it may not be noticed in winter and could be overlooked. It should not be necessary to sterilize cutting tools between cuts if only blighted shoots are being removed.

Do not engage in normal summer pruning and training at the same time as fire blight removal without wiping the cutters with sterilizing solutions like Lysol, 70% alcohol or 10% bleach. Don't forget to remove the infected stubs along with dead shoots and cankers next winter.
Do not apply chemical controls such as streptomycin. They are only effective if used during the normal bloom period. Remove trailing blooms to prevent late spring and summer infections.

Fire Blight Symptoms. Photo by Division of Plant Industry Archive, Florida Department of Agriculture and Consumer Services, Bugwood.org.

Adapted from "FIRE BLIGHT-WHAT NOW?" By John Hartman in the May 31, 2005 edition of the Kentucky Pest News from the University of Kentucky, College of Agriculture.

Landscape - Spring Diseases

Periodic rainy days and nights have been occurring recently in Delaware. Wet weather leading to prolonged leaf wetness as new foliage is emerging is very favorable for many landscape tree diseases. Symptoms of scab, rust, and anthracnose diseases are already beginning to appear in many landscapes.

Apple scab. Unsprayed, susceptible flowering crabapples and some backyard apples are already showing symptoms of leaf infections by the scab fungus, Venturia inaequalis. Long (15-18 hours) periods of leaf wetness are were favorable for new apple scab infections in. If trees were not already infected, expect symptoms developing from recent infections to appear in 10-12 days.

Cedar apple rust and cedar quince rust. The causal fungi, Gymnosporangium juniperi-virginianae and Gymnosporangium clavipes are still active on infected cedar trees. Symptoms on flowering crabapple and apple are already visible in some locations.

Fire blight (Erwinia amylovora). The primary infection season on flowering pears and crabapples and backyard apples and pears is over since most blooms have dropped flower petals by now. Depending on location, landscapes in Delaware were exposed to one or two fire blight primary infection periods based on favorable weather conditions this spring. If fire blight infections occurred earlier this season, symptoms in the fruiting spurs should be visible now. Look for blossom blight symptoms in the form of blackened, dead fruitlets in the fruit spurs throughout the tree. If there are not too many of them, infected clusters can be broken out by hand. Shoot blight symptoms, if they are going to occur, will appear in the coming weeks.

Anthracnose diseases. Where cool, rainy weather has prevailed, it is likely that sycamore (Apiognomonia veneta), oak (Apiognomonia quercina), maple (Discula sp. or Kabatiella apocrypta), and dogwood (Discula destructiva) anthracnose diseases will be active.

Adapted for Delaware from "SPRINGTIME DISEASES IN THE LANDSCAPE" By John Hartman in the May 5, 2008 edition of the Kentucky Pest News from the University of Kentucky College of Agriculture.