Carol O'Meara - Colorado State University Extension

Carol O’Meara – Colorado State University Extension

BOULDER –  Getting powder all over the furniture, leaving a trail of slime, some guests just don’t know when they’ve worn out their welcome. When the party’s over, instead of a fond farewell, they snuggle into the cover of leaves to sleep through winter, emerging as the weather warms to pick up the party on your plants next season.

If you’re letting leaves lie you might be adding to plant problems in spring, when insects and fungi shake off the cold to infest the garden. Now that fall snows finally quieted the garden, clean out the guests before winter sets in to ensure your plants get off to a healthy start next year.

Powdery mildew overwinters as cleistothecia, big, obvious black specks that look like someone sprinkled pepper on the leaves. If not cleaned out, it will re-infect the plants next spring, and soon the coating of powder-white fungus will encase the leaves and attack stems, buds, flowers and fruit.

Afflicting cottonwoods and aspen in our yards, Septoria fungus causes a spotty foliar disease. From tan, circular spots with black pimples in the center to irregular brown to black spots that coalesce into large areas, symptoms of the disease vary between tree species, particularly lanceleaf cottonwoods. In moist, humid areas, the fungus can attack twigs and main stems.

Septoria spends winter as pycnidia – a saclike spore case – on infected fallen leaves. With spring and its warm, wet weather, the pycnidia releases spores to be carried by wind onto emerging leaves. Though early infections aren’t often serious, if the weather remains warm and moist, these infections release spores that create a wide swath of infestation. These secondary infections appear later in the season and can cause trees to drop their leaves early.

Though Septoria is common, it’s Marssoninna fungus that is a true thug attacking Aspen and cottonwoods. Marssonina causes dark brown leaf spots, often with yellow halos and a white center. In wet weather, spots enlarge and merge forming black, dead patches.

Marssonina spends the winter on fallen leaves, producing spores in spring with warmer, wet weather. Like Spetoria, if the weather conditions are right, spores from these infections spread, causing large secondary infections that become obvious later in the season.

If you don’t care about your trees but do love your tomatoes, fall cleanup of plant debris is key in controlling early blight (alternaria solani). Early blight fungus attacks older leaves of the tomato first, spreading upward as the season progresses. Leaves develop brown to black dead spots with yellow halo and as the spots grow, they form concentric rings, or “bull’s-eye” pattern.

Early blight overwinters as spores in leaf or stem tissue left in the garden, so if you can’t rotate your plants, be meticulous about cleaning up debris.

Two fungi invading lawns, necrotic ring spot and Ascochyta, attack at different points:  ascochyta invades the leaves, while necrotic ring spot goes after the crown. They colonize the soft new growth of the lawn in spring, just as the plant starts actively growing.

Research at Colorado State University has shown that early season fertilization of lawns encourages both diseases by pushing growth of young leaves.  Delay fertilization of lawns in spring, waiting until early May if you used a winterizer this fall. This is one way of reducing the amount of soft tissue the fungi can commandeer.

Keep your plants healthy by tidying up the leaves and applying a fresh covering of mulch in perennial beds to keep diseases at bay.

Colorado State University Extension, together with Boulder County Parks and Open Space, provides unbiased, research-based information about consumer and family issues, horticulture, natural resources, agriculture and 4-H youth development.  For more information contact Extension at the Boulder County Fairgrounds, 9595 Nelson Rd., Box B, Longmont, 303.678.6238, or visit