Welcome spring! Trees are awakening, bulbs are sprouting, and lawns are beginning to green up. Warmer weather stimulates the growth and development of the plants and flowers we are longing to see. Well guess what! Many disease-causing organisms (pathogens) like the change in season, too. They are waking up from their winter rest and they are hungry. Some precautions, such as sanitation, balanced fertilization, and preventative fungicide application can play a major part in plant disease management.
Several pathogens survive the winter by hiding out in leaf litter, thatch, and infected twigs, branches, and trunks. Cleaning up last year's debris can reduce the potential for disease by removing a good portion of those elusive pathogens. It's time to clean up last year's debris if you haven't done so already. Remove dead annuals by pulling up the roots if possible. Cultivate the soil in vegetable gardens to bury remaining debris. This will speed up the decomposition process. Rake up fallen leaves from last year and remove debris from around perennials. Raking can also break up snow mold fungi that possibly grew beneath the snow on lawns this winter.
Environmental and cultural stresses are important factors in the occurrence of many plant diseases. Maintaining healthy, vigorous plants may help keep their defense mechanisms intact and encourage the development of strong healthy root systems that can carry them through stressful situations, such as drought. Balanced fertilization, adequate moisture, core aeration (turf), and other cultural practices are key in keeping plants healthy.
If fungicides and/or bactericides are incorporated in your plant health program, remember they are, in most cases, used for disease prevention. Once plants become infected, chemical treatment will be of little use. Read the label and follow rate and timing instructions. Chemical treatments are most effective when combined with sound cultural practices.
A few diseases to be aware of this spring are fire blight, oak wilt, summer patch, and various conifer diseases.
Fire blight is a bacterial disease of apple. Pyracantha (fire thorn), cotoneaster, mountain ash, hawthorn, quince, raspberry, and pear are all susceptible to fire blight. If infected branches remain on trees, the bacteria will multiply very rapidly and ooze out of infected branches and trunks when the weather warms up. Insects, birds, and equipment can transfer the bacteria to healthy plants. Prune out existing infected branches, before they begin to ooze, to prevent the spread of the bacteria. Cut at least 10 inches below the margin of healthy and diseased wood. This should be done as soon as possible before temperatures warm up. If it becomes too late in the season, there is a greater possibility that bacteria will be spread by pruning tools.
Several products are available for the management of fire blight, but they must be used in conjunction with good cultural practices, such as pruning and sanitation.
There is resistance to fire blight, so plant selection is important in managing the disease if you are planting new trees.
Prevention is the best "cure" for oak wilt. Avoid pruning oaks between April 1 and July 1, when oak trees have a high risk for infection. If pruning is necessary during this period, promptly treat wounds with a lanolin-based wound treatment compound.
Why think about sick lawns now? Although turf problems usually don't become apparent until summer, some disease-causing fungi become active and infect plants in the spring. In many cases fungi will infect only stressed plants. This is why it is important to maintain plant vigor. Slow release nitrogen fertilizers should be used in the spring. Avoid excess fertilization during summer months. Seventy-five percent of the season's nitrogen should be applied in the fall. Also, it is time to core aerate, which is most effective during the spring or fall rather than the summer.
In an area with a history of summer patch (a fungal disease of Kentucky bluegrass), fungicide use may be an option. Timing is essential for effective control. The first application should be made in the spring when soil temperature is consistently above 65 F at a depth of 2 inches, usually late April or early May.
Winter injury and disease can look similar. Inspect trees carefully for little black pimples on needles, resin flowing from the trunk or branches, or whole branches dying back. If fungicides are included in your disease management program, now is the time to start considering fungicide applications for conifer diseases such as Diplodia (Sphaeropsis) tip blight, Rhizosphaera needle cast, Swiss needle cast, and Dothistroma needle blight. The first application for Diplodia (Sphaeropsis) tip blight should be made at bud swell. Applications for Rhizosphaera needlecast and Swiss needle cast should begin during the last two weeks of May. Dothistroma fungicide applications should begin mid May. The second application for each of these works best if scheduled 4-6 weeks later.
Pine wilt, a disease caused by a complex of organisms, has lately become an important problem in Iowa. Dead pines can harbor organisms involved in this disease. Remove and dispose of any pines diagnosed with pine wilt. Avoid saving this material for firewood, because beetles, one of the major players in this disease, can still emerge from cut logs.
Samples can be submitted to your county or area extension office or the Iowa State University Plant Disease Clinic, 323 Bessey Hall, Iowa State University, Ames, IA 50011, ((515) 294-0581). There is a $10.00 diagnostic fee.
This article originally appeared in the April 6, 2001 issue, pp. 30-31.