Leaf Spots and Remedies
Does your tree, plant or flower have these spots on them? It could be one of these diseases. Here's how to control them.
Does your tree, plant or flower have these spots on them? It could be one of these diseases. Here's how to control them.
Black Knot
Symptoms: Black knobby growths on branches and stems.
Pathogen: Dibotryon fungus.
Susceptible plants: Ornamental and edible plums, cherries and other stone fruits.
Favorable conditions: Cool—near 55°—and wet.
Look-alikes: Canker.
Control strategies: Sanitation may help slow the spread. Unfortunately, there are many wild plums and cherries to infect your trees. Prune at least 2 to 4 inches below the black knots. Remove immature knots in late winter to prevent spore release.
Some pathologists recommend cutting the knots out of major branches and trunk at least 1 inch below and 1/2 inch beyond the knot, but this may be more hazardous to the tree than the disease.
Dormant, spring and early summer fungicide treatments may provide some relief. If you have badly infected trees with knots on the trunk, you may want to eliminate them for the health of your other plants.
Crown Gall
Symptoms: Knobby growth on stem or roots; plants may be stunted, decline and eventually die.
Pathogen: Several species of Agrobacterium, a soil-borne bacterium brought into the landscape on infected plants or soil.
Susceptible plants: A variety of ornamental and fruit plants; often seen on euonymus, grapes, raspberries and apricots.
Favorable conditions: Bacteria in soil
Look-alikes: Insect galls.
Control strategies: Thoroughly examine plants you buy; choose only those that are disease-free. Avoid injuring roots and stems during planting. Remove infected plants and the soil around them. Disinfect tools.
Use resistant plants such as andromeda, barberry, birch, boxwood, cedar, firethorn, golden-rain tree, holly, maidenhair tree, mimosa, mountain laurel, redbud, smoke tree, sweet gum and tulip tree. Select plants suited to your growing conditions and landscape design.
Damping Off
Symptoms: Seeds fail to germinate; seedlings suddenly collapse and die; older plants may appear wilted.
Pathogen: Several fungi.
Susceptible plants: Seedlings.
Favorable conditions: Contaminated and cool, wet soils.
Look-alikes: Bird or animal damage may be blamed for poor sprouting; other rot diseases.
Control strategies: Avoid problems by starting with quality seeds. Indoors, use sterile soil and clean containers. Bottom heat speeds germination and reduces disease risk. Outdoors, plant in well-drained soils. Use row cover fabrics to warm soils for plants that need warm soil to germinate. Cultural controls are usually sufficient. When these techniques fail, fungicides labeled for damping off can be applied as a soil drench.
Dutch Elm Disease
Symptoms: Tips of a few branches wilt, yellow and brown from early spring through summer. Symptoms spread throughout the tree. Death can come quickly, within one season, or take several years.
Pathogen: Two species of Ophiostoma fungi can attack the vascular system, entering the tree through root grafts or on the European elm bark beetle.
Susceptible plants: All native elms. American elm is most susceptible; red or slippery elms are somewhat less susceptible.
Favorable conditions: Diseased trees in the vicinity provide the pathogen. Pruning cuts and wounds attract disease-carrying insects.
Look-alikes: Elm yellows and bacterial leaf scorch.
Control strategies: When adding new elm trees, plant resistant hybrids or the American Liberty variety to minimize disease risks.
For existing trees, practice good sanitation. Remove weakened or susceptible branches immediately. Remove infected trees as soon as possible to avoid spreading the disease. Sever root grafts between healthy and diseased trees before removal. Use infected firewood immediately or pull off the bark and tarp securely for storage.
Chemical injections of some fungicides can prevent the disease and even cure a tree if less than 5% of it is infected.
Fireblight
Symptoms: Leaves suddenly wilt and turn brown or black as if burned. Stem ends curl like a shepherd's crook. Cankers develop on infected branches.
Pathogen: Erwinia bacterium that overwinters in cankers on the bark. Transferred by wind, bees and pruning tools to other trees.
Susceptible plants: 75 species in the rose family, including apples, crabapples and pears.
Favorable conditions: High humidity and temperatures near 65°.
Look-alikes: Drought stress; borers.
Control strategies: Plant resistant varieties and species whenever possible. Avoid excess nitrogen, which can promote the lush, succulent growth that's most susceptible. Avoid pruning during wet weather when the disease is active. Prune out infected branches at least 12 inches below the canker. Disinfect tools between cuts.
Bordeaux—the fungicide, not the wine—and copper-containing fungicides applied at the silver tip stage (when buds swell and show silver through the tips) may provide some protection.
Southern Blight
Symptoms: Leaves yellow and wilt due to basal or root rot. A white filamentous growth (mycelium) may be present near the soil line.
Pathogen: Sclerotium fungus.
Susceptible plants: A wide variety of annuals ane perennials; some fruit, trees and shrubs.
Favorable conditions: Hot, humid weather. Has been found to survive in northern regions with mild winters or good snow cover.
Look-alikes: Other root rots.
Control strategies: Hard to control because of the wide range of hosts. Prevent problems by buying disease-free plants. Move plants or soil from infected gardens to disease-free planting beds. Disinfect tools and equipment when moving from diseased to non-diseased areas.
Use resistant species such as abutilon, alyssum, baby's breath, bells of Ireland, cleome, cockscomb, English daisy, four o'clock, freesia, fritillaria, globe amaranth, hyacinth, lavender, moss rose, primrose, statice, tansy, Virginia bluebells and wild geranium.