This year has been a tough one on our landscapes. The fluctuating temperatures this winter caused death and dieback in euonymus and willow. Then the prolonged cool, wet spring has brought about fungal infections including ash rust and cedar/apple rust. Now a bacterial infection is rearing its ugly head in apple, crabapple, and their relatives. Find out what fire blight is, the symptoms to be on the look-out for, and what can be done to trees once infected.
Fire blight is the oldest and most serious bacterial disease of apple and pear. It has a wide host range including many species in the rose family. Apple, crabapple, pear, cotoneaster, hawthorn, firethorn, and mountain ash are the most common species infected in Nebraska. The infection enters into the plant through natural openings in blossoms and leaves or through wounds in the bark. Infections continue to develop until the spring flush of growth stops or until about a month after flowering.
The symptoms are fairly easy to recognize. The most common symptom are in the twigs and leaves. Infected leaves quickly wilt and turn brown or black, but the key characteristic is that they remain attached to the branches. The infected twigs often form a cane-like shepherd’s crook at the tips of the branches. The infection moves its way down from the blossoms and twigs to the older branches. Cankers show up as sunken-in, discolored areas on the branches. The bacteria may continue to spread into scaffold limbs and eventually into the main trunk. The severity of the disease is related to the cultivar and weather conditions at the time of the infection.
Unfortunately, there isn’t any cure for this disease, but there are things you can do to slow the spread of the pathogen. Prune and discard all infected twigs and branches that have cankers. Cuts should be made at least 12” below the infected area. The disease can also be spread through the use of contaminated pruning equipment and tools. Pruning tools should be disinfected after each cut by dipping the cutting surface into disinfecting solution of either 70% rubbing alcohol or a 10% bleach solution for at least 30 seconds. Ideally the pruning should take place when the plant is dormant to avoid spreading the bacteria.
Chemical controls are usually not successful and are more of a preventative treatment than a curative one. Copper-based fungicides and the antibiotic, streptomycin, are somewhat effective in controlling fire blight. The copper fungicides are best applied at the green tip stage to help reduce the inoculum produced in carryover cankers on branches. Control of blossom blight is done by spraying with streptomycin during flowering. Always read and follow label instructions and precautions.
There are some species or cultivars that are more resistant to fire blight. If you are selecting a new tree for the landscape, try to select those that are resistant. Most edible pear varieties are also susceptible, but the following varieties show some resistance: Kieffer, Garber, Seckel, Tyson, Lincoln, Dutchess, and Moonglow. The least susceptible apple varieties include Red Delicious, Golden Delicious, Dayton, Liberty and Jonafree. Because of the widespread occurrence of this disease on ornamental pear, homeowners may wish to consider an alternative tree for the landscape. Fire blight is highly infectious in the following ornamental pear cultivars: Aristocrat, Autumn Blaze, Capital, Fauriei, and Redspire. Bradford pear is moderately resistant to the disease, but has other issues in the landscape and should be avoided.
Fertilization of trees can promote this disease. Heavy fertilization promotes rapid, succulent plant growth, and increases the probability of fire blight and intensifies its severity. Generally, trees don’t require much additional fertilization. A tree in a location surrounded by turf will be provided with more than enough nutrients from what it can absorb from the lawn.
Be on the look-out for the tell-tale signs of fire blight in your susceptible species. Be prepared to do some pruning. Consider a replacement that is more resistant. Who knows maybe next year will be a ‘normal’ Nebraska year.
Elizabeth Killinger is the Horticulture Extension Educator with Nebraska Extension in Hall County. For more information contact Elizabeth at firstname.lastname@example.org, her blog at https://huskerhort.com/, or HuskerHort on Facebook and Twitter.