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The aspen tree's smooth white bark and trembling light green leaves epitomize Colorado's native beauty. However, those same leaves are often unsightly reminders of several diseases that diminish the tree's aesthetic value.
Black leaf spot caused by the fungus Marssonina, is one such disease. This fungus affects aspen and other poplars. Symptoms on the leaves are dark brown spots, often with yellow halos or borders. Spots may be seen on leaf stems and tender shoots as well. Mature spots may have a white center. On severely infected leaves, these spots fuse to form dead patches.
Small trees may have all the leaves infected, whereas larger trees usually show symptoms lower in the crown. Diseased leaves can become stunted. Leaf drop, twig death and reduced growth can occur during severe outbreaks. Affected trees become prime targets for other insect and disease problems.
Marssonina survives the winter in fallen leaves and debris. Warm, wet spring weather starts spore formation. The infection begins when wind-borne or rain splashed spores come in contact with newly opened leaves. Continued warm, wet weather favors secondary disease cycles. The disease can spread to adjacent previously healthy leaves. In some years, these cycles can reach epidemic level, although tree death is rare.
Trees that lose their leaves early in the season may re-leaf without suffering serious harm. However, mid-season re-foliation can be a disadvantage. Trees that do not harden off sufficiently before winter are subject to frost damage and twig die back. If the tree drops its leaves late enough in the season, it usually will not re-foliate.
Management of black leaf spot begins with fall removal and destruction of leaves and debris to reduce the amount of over-wintering fungi. Make a note on your October calendar now to remind yourself to do this. When watering, avoid wetting the leaves with overhead irrigation systems. Early morning watering will allow any wetted leaves to dry out during the day.
Trees that are overcrowded are more likely to get the disease. Increase the air circulation around your aspens by pruning the surrounding vegetation. A fungicide spray can be used as a preventive measure but not a cure. Apply as the leaves begin to open but well before symptoms develop. Check with your local nursery or garden center to locate available sprays. Be sure and follow the directions on the label carefully and use only the proper dose. In the case of pesticides, "more" is not necessarily "better."
Aspens are favorite landscape trees that require close monitoring to maintain their health and beauty along the Front Range. Our soils and weather conditions stress these trees to a point that diseases are a common problem.
For further information on aspen leaf problems see Fact Sheet #2.920 - Aspen and Poplar Leaf Spot. You can also call Planttalk Colorado toll free at 1-888-666-3063 and listen to recording #1403 - Aspen leaf spot.
Q: I've got spring fever. If I don't mind the mud, is it okay to plant when the soil is wet?
A: Even if your green thumb is throbbing to get out and plant, resist
working the soil when it is wet. Even walking on wet soil can increase
compaction, but if you get out there and dig in the mud you're likely to
destroy the soil structure and create a difficult environment for the roots
of your new plants.
Q: Do I need to protect my spring flowering bulbs when the temperature
dips below freezing?
A: Spring flowering bulbs are generally hardy characters that thrive in Colorado. They don't mind the cold. However, if snow falls and weighs down the flowers of tulips or daffodils, sometimes the stem can break or the blossom can shatter. If spring snowflakes fly, you might want to go out and tip an empty box over the flowers of your prize bulbs.
Q: For the first time this year, I wrapped the trunks of my young trees to protect them from sunscald. When should I remove the wrap?
A: It is safe to remove tree wrap on tax day or soon after.
Q: I'd like to get a head start on planting my annual garden. Can you suggest some seeds that I can plant outdoors now?
A: Look for seeds that are described as hardy annuals. These include pansies, cosmos, bachelor buttons, and larkspur. These seeds can be safely sown outdoors now.
You don't have to throw out that lily (Lilium longiflorum) you received for Easter. After it is finished blooming, plant it in a sunny spot in the garden. Remove the flowers but let the foliage and stalk die naturally. Water and fertilize regularly and it should reward you with blooms next year in late summer.
Crabgrass, foxtail and goosegrass are annual grassy weeds that can be a problem in your lawn. Non-herbicidal controls include mowing 2 1/2" to 3" high during the summer to keep the soil surface shaded. Mow frequently, irrigate deeply, fertilize at regular intervals and core aerate at least once a year. Preemergent herbicides must be applied before the weed seeds germinate –when the soil reaches 50 degrees. Water well to wash the herbicide off the grass and onto the soil surface. Do not apply these herbicides to the soil if you are going to seed a new lawn or lay sod.
Anxious to get into the yard? It's a great time to plant bare root shrubs for hedges and other dormant, bare root trees, roses and small fruits like raspberries, grapes, gooseberries and strawberries. Soak the roots overnight before planting. Improve the soil with compost, water in well and use an organic mulch on the soil.
Start seeds indoors for tomatoes, eggplants, and peppers. The
varieties available by seed are almost endless. Use a "soilless"
mix or sterilized soil to avoid damping off problems. Invest in fluorescent
lights to supply 14 – 16 hours of daylight for good, stocky growth and
avoid the leggy plants that usually result from trying to grow seedlings
on the window sill.
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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