Fire blight on Plants

Symptoms

The first sign is a watery, light tan bacterial ooze on cankers and infected tissue. Infected flowers and flower stems wilt and turn black on pear trees. On apple trees, infected flowers and flower stems wilt and turn brown instead. Young shoots wilt as well, and they curl over into the shape of a shepherd's crook. Dead, blackened leaves and fruit cling to the branches through the season. When spring weather turns wet, cankers ooze a cloudy liquid. Cankers can girdle and kill branches, and they can kill the whole tree. The signs range from wilted flowers and shoots to dead branches and cankers on the tree. The flower color you see depends on the tree, black on pear and brown on apple. Watch for the ooze on cankers in particular, since it is the first sign listed. The black and brown flower colors described here are recorded for pear and apple trees. Flowers, shoots, leaves, fruit and branches can all show signs of the disease. Look at the flowers, the young shoots, the leaves and fruit, and the branches for these signs.

Causes

Fire blight is caused by the bacterium Erwinia amylovora. Water droplets in rain and irrigation spread the bacteria from place to place. Pollinating insects spread it as well. Rainy or humid weather favors the disease, especially when daytime temperatures run around 75-85°F (24-29°C). Excess nitrogen fertilization promotes susceptible, succulent growth. Heavy pruning promotes the same kind of susceptible, succulent growth. Two of these causes are things you do yourself: fertilizing with too much nitrogen and pruning heavily. The other causes are biological or tied to weather: the bacterium, the insects that spread it, rain and humid air. Irrigation matters as much as rain, since both spread the bacteria in water droplets. Succulent new growth is the tissue that fire blight infects most easily. Irrigating trees during bloom is a cause worth avoiding, since the bacteria spread in water droplets. A tree can therefore be exposed by weather, by insects or by your own care. Warm, damp spells matter because the disease is favored by rainy or humid weather.

Diagnosis

No look-alike problems are named in the facts for this page, so this section compares fire blight with ordinary care faults. Fire blight is a bacterial infection by Erwinia amylovora, not a watering or feeding mistake. Look for the signs that point to a bacterium: a watery, light tan ooze on cankers and infected tissue. Look for shoots that curl into a shepherd's-crook shape, and for dead, blackened leaves and fruit that cling to the branches. Check the flowers too, which wilt and turn black on pear and brown on apple. The host plant matters, since fire blight is recorded on apple, pear, crabapple, quince, hawthorn, cotoneaster, firethorn and rose, among others. Rainy or humid weather with daytime temperatures around 75-85°F (24-29°C) favors the disease. Excess nitrogen fertilization or heavy pruning also fits the picture, since both promote susceptible, succulent growth. Cankers that ooze a cloudy liquid in wet spring weather are another sign to look for. A tree with several of these signs together is showing the pattern described for fire blight.

Treatments

Start with the cultural steps, which are the first group of treatments listed for fire blight. Avoid irrigating trees during bloom, since the bacteria spread in water droplets from rain and irrigation. Avoid excess nitrogen fertilization and heavy pruning, since both promote the succulent new growth that fire blight infects most easily. Monitor your trees regularly, and remove and destroy fire blight infections promptly as they appear. Prune out infected branches during dry weather in dormancy, cutting well below the base of the visible canker. Dry weather in dormancy is the time the facts name for pruning out infected branches. Disinfect your pruning tools frequently between cuts to avoid spreading the bacteria. When you remove cankers from trunks or major limbs, cut well beyond the visibly discolored, infected tissue. Copper is the only chemical treatment in the facts for this page. Copper-based sprays, such as a weak Bordeaux mixture or other copper product, can be applied to open blossoms during bloom. The aim is to help prevent new infections. Copper sprays are a preventive step for blossoms, not a cure for infected wood. Copper will not cure wood that is already infected, and that wood still needs to be pruned out. Copper can also cause russeting or scarring on fruit. The product label is the legal instruction, and you must follow it for the crop and site being treated.

Prevention

Prevention starts with the conditions around the tree. Keep irrigation away from trees during bloom, because the bacteria spread in water droplets from rain and irrigation. Hold back on nitrogen fertilization, since excess nitrogen promotes susceptible, succulent growth. Avoid heavy pruning for the same reason. Watch the weather, because rainy or humid conditions with daytime temperatures around 75-85°F (24-29°C) favor the disease. Monitor your trees regularly so infections are found and removed promptly. Disinfect pruning tools between cuts, so you do not spread the bacteria from branch to branch. Pollinating insects and water droplets are the two ways the bacteria travel. Prune only in dry weather during dormancy, when infected branches are cut out.

Plants most often affected

Fire blight is recorded on apple, pear, crabapple, quince, hawthorn, cotoneaster, firethorn, rose, serviceberry, mountain ash, loquat, spirea, toyon, raspberry, blackberry, strawberry, apricot, medlar and flowering quince. Apple and pear are both on that list, and their flower colors are described in the symptoms section. The list holds 19 hosts in all.

• American Red Raspberry
• Black Chokeberry
• Callery Pear
• Cherry Plum
• Chinese Pear
• European Crab Apple
• European Mountain Ash
• Saskatoon Serviceberry