One of the most damaging and most perplexing diseases of the Front Range is fireblight. It is a bacterial disease affecting apples, crabapples, pears, Mt. Ashes, pyracanthas, quinces, hawthorns and occasionally cotoneasters. Fireblight does not affect cherries, plums, peaches or shade trees.
When the bacterium with the lovely name Erwinia amylovora enters the trees through an opening such as a wound, a pruning cut, the open blossom or leaf pores, the tissues quickly die causing blossoms and leaves and young twigs to suddenly wilt and turn black as if scorched by fire (hence the name “fireblight”). As the bacteria spread through the tissues, secondary infection continues through the season and bark areas die, become sunken and turn black or reddish-orange. These areas are called “cankers”. Sometimes large numbers of leaves turn black but the infection stays confined to small twigs; sometimes entire large branches die; occasionally the entire tree will die.
It is important to understand how and under what conditions this dastardly disease progresses and prospers because improper actions, regardless of the good intentions, can actually make the disease far worse. For example, many people do not realize that fireblight is a very infectious disease . Not only can this disease be spread by insects, birds, wind and rain, but also by pruning tools. The bacteria overwinter in blighted branches and cankers and in the spring with warmer temperatures and rainfall, the bacteria multiply and push out through cracks and pores appearing as a gummy “bacterial ooze”. Because of its nutritional value to insects, they contact it and then carry it to the open blossoms and wounds where it infects other areas. In the same way, the bacteria can be carried on pruning shears or saws from an infected area to a previously unaffected area.
Likewise it is important to know that young, soft growth is the most vulnerable to fireblight. The well-meaning tree-owner who pours on the nitrogen fertilizer, hoping to strengthen the sick tree, makes matters far worse by stimulating fast succulent growth. Therefore it is better to fertilize with milder,organic fertilizers that are not water-soluble or with just compost, or not at all.
Also we must learn to water intelligently. John Spaulding, of Spaulding Horticulture in Louisville said that whereas fireblight is the worst when we have humid, cool spring weather, the disease spreads even more because of drought stress. In doing his landscape assessments, he has observed that our summer showers rarely penetrate into the ground very far, but that people think the trees have been watered; the resulting drought stress makes the trees more vulnerable to fireblight. Therefore John recommends giving trees an occasional long, deep soaking during July and August or any especially dry period. He also believes pruning for good air circulation is helpful. At the same time, keeping the soil saturated with water does not let the roots breathe and is recognized as a condition which promotes fireblight.
So what can be done to control this difficult pest? This is a good question and unfortunately there is not one simple, sure solution. The chemical spray remedy is to apply streptomycin to the trees during bloom time. Orchard managers are encouraged to spray at 50% bloom, again at full bloom, again 3 days after full bloom, and again at 50% petal fall. Homeowners are told to spray twice during bloom time and more if humidity is high or if the blooming continues long. Even experts say it “may” help or that it works 50% of the time.
Another good idea is to use resistant varieties. James Klett, professor and cooperative extension landscape horticulturist at CSU has, since 1986, been testing various crabapple cultivars for fireblight resistance. Twenty three other centers in the US have also been testing them. It has been learned that the CSU site has had the worst fireblight of all the centers. The program at CSU started with 60 varieties and they are now down to 12-15 varieties of which 8 are described as “good bets.” Professor Klett does not feel there is any crabapple that is totally resistant to fireblight. His best bets are: Bob White, Centurion, Coralburst, David, Indian Summer, Profusion, Robinson, and Zelkirk. Crabapples having had severe fireblight in their tests were: Mary Potter, Dolgo, Ormiston Roy, Red Barron, Red Jade, Royalty, Sentinel, Silver Moon, and Strawberry Parfait.
Scott Skogerboe, woody plant propagator at Ft. Collins Nursery and fruit explorer and researcher agrees that “fireblight resistant” only means resistant under more conditions. His studies including the research done at the Cheyenne Horticultural Field Station indicate these fruiting apples as fireblight resistant: Williams Pride, Pricilla, Freedom, Liberty, Cortland, Dolgo, Haralson, MacIntosh, Linda Sweet, Patricia and Lodi. He explained that although the dwarfing stock M7 is the most resistant, the standard variety on the Russian rootstock Antonovka has more vigor, vitality and drought resistance, therefore more fireblight resistance than the dwarf varieties.
CSU Cooperative Extension recommends as resistant: Red Delicious, Winesap, King David, Duchess and Turley in apples and Magness and Moonglow and Starking Delicious for pears. Their warnings were for the susceptible apples: Jonathan, Yellow Transparent, Wealthy, Rome Beauty and Cortland; for susceptible pears: Bartlett, Bosc, Comise and Duchesse; for susceptible crabs:Bechtel, Hopa and Snow Drift.
Craig Hayes of Hayes Bros. Tree Service in Boulder believes the best method for controlling fireblight is judicious pruning. He thinks it is best to avoid pruning fireblighted trees in May, June and July, waiting until at least August but mostly until fall and winter when the chances of spreading this infectious disease will be lessened by the dryer and dormant conditions. Craig sterilizes his tools after each cut with Lysol.(other recommendations are for 70% alcohol or 1 part bleach with 9 parts water) Craig prunes back 6” from the blighted areas, preferring to do a follow-up on the tree and then to do more pruning only if necessary. This approach keeps more leaves on the tree for photosynthesis and smaller pruning cuts which will heal faster. He also cuts and scrapes the cankers out of the larger branches to avoid having to remove the branches entirely or letting the cankers spread.
CSU Cooperative Extension recommends pruning 12” down from any sign of fireblight, removing newly infected twigs as soon as possible and removing all blighted branches in the winter 4”-6” below any sign of infection.
Having dealt with fireblight in my own tree-care business for 20 years, I am inclined to mostly agree with Craig’s method. I would also recommend core or drill aeration around the drip line of the tree, filling the holes with compost; and I would suggest people do their thorough watering early in the day so that the humidity stays lower in the evening when our temperatures cool. Most importantly I would say that people should neither ignore the problem nor react violently to it by over-pruning. Also I would like to see more research done on the fireblight relative Erwinia herbicola which has been tried at Utah State to outcompete the fireblight. Scott Skogerboe told me this was fairly successful. Also I hope people will try Tea Tree Oil or Neem sprays, possibly even horticultural oil to control or lessen the spread of this difficult disease.
If this information has left you feeling let down, frustrated and somewhat confused, join us professionals in hoping a lot more research will go into solving this still-existing problem.