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Fire blight is a bacterial disease that attacks apple, pear, crabapple and mountain ash trees, cotoneaster, quince and pyracantha shrubs and other members of the rose family. The first symptoms appear in spring and include flowers that wilt early and quickly turn dark brown or black. The leaves, even though wilted and dark remain on the tree giving the tree a scorched appearance, hence the name "fire blight." The tips of these branches often curl to form a crook. A white creamy substance containing the bacteria sometimes oozes from the affected twigs.
Living along the Front Range of Colorado our dry and often warm weather is something gardeners often grumble about. But these conditions protect our plants from many diseases and insect problems. Spring, however, is the time when we can have much of our rainfall and this humidity and precipitation create the perfect environment for the onset and spread of the disease.
The disease is often spread by bees that have pollinated the blossoms of an infected tree. Some of the disease spores will become active when the weather is wet and the temperatures are in the upper 60's to lower 80's.
When symptoms such as described above are noticed and seem to be spreading quickly, scientists with Colorado State University recommend cutting off the infected limb at least 12 inches below the visible damage. To avoid further spread of the disease it is very important to sterilize the cutting tool between EVERY cut. Dip the tool in a solution of 1 part bleach to 4 parts water. If larger branches need to be treated, a full explanation is needed. This is covered in the Fact Sheet listed at the end of this article.
What can we do to prevent fire blight? First of all when purchasing trees susceptible to this disease look for varieties which are resistant to fire blight. Although no varieties are immune there are many less likely to be infected. The tag on apple, pear or crabapple will say if it is a resistant variety. For example red delicious and winesap trees are resistant whereas yellow delicious and jonathon are susceptible. There are also a number of beautiful flowering crabapples that have proven relatively resistant to fire blight in studies conducted at Colorado State University.
Moderate use of fertilizer will also help prevent fire blight since the bacteria tends to attack young succulent growth that is encouraged through the heavy application of fertilizers. The use of drip irrigation will avoid excess humidity caused by over-head irrigation which also increases susceptibility and spread.
Some preventative sprays can be effective against fire blight. They should be considered if spring conditions are right for the growth of the disease such as high humidity and relatively cool temperatures or there has been a lot of new succulent growth or hail damage. Streptomycin and copper sprays used alternately are the recommended treatment.
Although it is too late this season for sprays to be of benefit they can be kept in mind for future years. But remember, always follow label directions for applying any pesticide.
For more information contact the Larimer County Office of Colorado State University Cooperative Extension at 498-6000 and ask for Fact Sheet #2.907 - Fire Blight You can also call Plantalk Colorado at 1-888-666-3063 and select message #1411 on fire blight.
Q: The bark of my aspen trees has developed a rough scaley appearance in places. What could this be?
A: This sounds like oystershell scale, an insect that emerges around this time of year. If possible, try scraping them off GENTLY with a plastic scrub pad. If needed, spray diazinon or sevin when you see the insects moving. Remember, diazinon is very toxic to birds. Read labels carefully before applying any pesticide.
Q: When can I spray my apple tree to avoid a crop of wormy fruit?
A: Spraying with a summer weight horticultural oil or with carbaryl or diazinon should have been started shortly after petal fall. However, starting now and spraying every 10 to 14 days for the next couple of months will catch the most damaging generation of the codling moth which is the worm in wormy apples.
Q: My crabapple tree has fuzzy white spots on its bark. What causes this?
A: This could be an infestation of the woolly apple aphid. It is an insect best controlled earlier in the season with a dormant oil spray. However, at this time the best treatment is to spray insecticidal soap or carbaryl. Be sure and read labels carefully before applying any pesticide.
Q: How can I keep my perennials blooming longer?
A: Many perennials will continue to bloom longer if you remove dead blooms. This also helps annual plants become more bushy and full of blooms.
The All- America Selection organization (AAS) has been testing vegetables and flowers since 1932. There are 198 display gardens across the country including one at Colorado State University Plant Environmental Research Center (PERC) on Lake Street. The winners are outstanding plants that will grow well all across the country.
May to mid-June is the time to fertilize all species of turf grass. Bluegrass needs 1 pound of Nitrogen (N) per 1000 square feet. Turf-type tall fescue, fine fescue, buffalo grass or blue grama need one-half to 1 pound of Nitrogen (N) per 1000 square feet. If you leave grass clippings on the lawn after mowing, reduce the amount of fertilizer by one-fourth to one-third.
Plant Select (R) 2000 plants are now available at local garden centers. The newest plants include Spanish Gold (tm) Broom - a hardy shrub, Princess Kay Plum - a flowering tree, two perennials - Prairie Jewel Penstemon and Coral Canyon Twinspur and a ground cover - Pawnee Buttes Sand Cherry. Plant Select (R) is a cooperative plant introduction and marketing program developed by the Denver Botanic Garden and Colorado State University to find and develop plants suited to our unique climate.
Beans, eggplant, melons, peppers and pumpkins do not grow well when the temperatures are below 70 degrees. It's usually safe to plant these vegetables around May 25, later if you live at a higher elevation. However, as unsettled as our weather has been lately, it would be wise to wait another week or so.
Fall bearing raspberries, especially Heritage, Fall Red and Fall Gold are recommended for Front Range gardens up to 8,500 feet. The canes of fall-bearing varieties are mowed off at ground level after the fall harvest.
The author has received training through Colorado State University Cooperative Extension's Master Gardener program and is a Master Gardener volunteer for Larimer County.
Fact Sheets are available at the Larimer County Extension Office, 1525 Blue Spruce Drive, Fort Collins, Colorado, telephone (970) 498-6000, or contact us by e-mail at larimer@coop.ext.colostate.edu
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