The ghouls and goblins of late October may be gone, but there are still some perils lurking in our autumn gardens: disease outbreaks, which occur when plants are weakened or under stress, environmental conditions are favorable and a pathogen is present.
Plant pathogens fall into many large groups—viruses, bacteria, fungi such as molds and mildews, phytoplasms or nematodes. Organisms may attack selected plant parts like roots or leaves only, or they may be opportunists and take over any vulnerable place. Correct diagnosing is a major step in restoring your garden’s health.
The Missouri Botanical Garden website has clear images that will help you identify disease symptoms. The first step is to identify the plant—many diseases are host-specific. The next is to define the damage. Tissue samples, such as branch tips with infected leaves or fruits, may be brought to the Kemper Center, but must be fresh and sealed inside a freezer baggie to prevent spreading contaminants.
Lurking in the Leaves
Problems with bacterial and viral leafspots, mildew, aster yellows (a phytoplasm) and mites (very tiny creatures like insects) can become worse if old leaf litter remains. Excellent garden sanitation, including cutting back perennials and thoroughly cleaning the beds of debris, can help prevent outbreaks. If you have a disease problem, dispose of all infected materials. For example, the large-scale composting at Edie’s Mulch Pile in Ladue reaches high enough temperatures to kill most pathogens (home composts usually don’t).
Common Bad Guys
Aster yellows and mites cause very similar damage in cone flowers (Echinacea) and many perennials—distortion of shape, color or blossom size. They can cause these flowers to be green and have firework-like shoots coming out of the cone. Any time you see a deformed flower like this the entire plant should be removed and discarded immediately.
Powdery mildew whitens foliage and soon kills the leaves. The dead leaves fall to the ground and hold spores that will re-infect the host plant the next growing season. Choose disease-resistant varieties when you can. Watering with soaker hoses helps keep foliage dry and can reduce mildew occurrence.
Botrytis, the Noble Rot or gray mold, can affect nearly every part of a plant. It frequently causes rot in peaches, strawberries and tomatoes. Deformed, wilted or blasted flowers in roses, lilies and peonies, or dark purple spots on peony leaves, can also be its handiwork. On casual inspection, botrytis resembles bread mold, a related organism. If you get bulbs that look moldy, don’t plant them. And avoid overhead watering, as the splashing of water droplets will spread the fungus.
Fire blight is a serious bacterial diseases found in local gardens. It causes twig die-back that eventually kills the tree or shrub. Specific to certain rose family members, this pathogen can be transferred from plant to plant with infected clippers. Sterilize clippers with rubbing alcohol or diluted bleach after each cut to avoid contamination.
Bacterial stem rots and wilts are common in hostas and irises. These microscopic pathogens enter through existing wounds and are often associated with insect pests, like stem borers on iris, that open up holes for them to enter.
Rusts are a group of fungi that can perpetuate themselves in the garden for years. The orange spots on your hollyhock leaves are one example of their effects. Some rusts must alternate between hosts; for example, cedar-apple rust goes back and forth between those two plants for each generation. Ever seen a large, misshapen orange blob like a spiny orange gumball on a cedar tree in your yard? That’s rust fungus. All rusts can be avoided or lessened by thoroughly cleaning up leaf litter, spent fruit and flowers and other debris.
Anthracnose leaf and fruit-spotting diseases are caused by related fungi. The species are host-specific, so the disease that affects a maple is not the same one that attacks a dogwood. On woody plants, anthracnose typically causes leaf lesions. For dogwoods, prevent anthracnose leaf spot by using hybrids with Cornus kousa parentage, which are somewhat protected. Another kind of anthracnose causes fruit rot in tomatoes and peppers. It starts as a watery-looking, round lesion that darkens and eventually destroys the fruit. Hot, humid weather like we had this summer accelerates the decay. Thoroughly cleaning out vegetable garden debris and rotating crops can help lessen the problem.
Black spot is another fungal disease infamous for wiping out roses. Try the old-fashioned ‘wild’ or ‘cemetery’ roses that have survived in spite of our conditions or the new hybrid shrub roses that have been specifically selected for disease resistance.
Soil-borne and Serious
My mother’s garden always included the fragrant Daphne, a small, sweet shrub grown here only in glasshouses. Unfortunately, Daphne suffers from a problem we call Daphne Sudden Death Syndrome; it would prosper for a decade and then suddenly collapse into a tiny heap of brown leaves. I have a similar situation with my leather leaf viburnums. One day, the left shrub turned gray, wilted and died in about four days. The one on the right side followed suit two weeks later. Without a complete pathogen assessment, I won’t know for sure what took them, but like the dead Daphne, the most likely culprit is a soil-borne fungus Phytophthora (phi-top-ther-a). It is one of the most wicked and insidious soil-borne diseases. Sudden oak death, the scourge of California, is caused by a species of Phytophthora.
Soil pathogens like Phytophthora and its evil twin sister, Rhizoctonia, are carried on our feet and hand tools, tent pegs and car tires. They spread down the garden with moving water and up the hill on your gardening shoes. If you suspect these diseases, sanitize your shoes and shovels with chlorine bleach (diluted to 5 percent) and do not transplant divisions from infected areas.
Keeping the Garden Clean
Promote the healthiest garden possible by using good cultural practices including balanced nutrition, careful watering, proper pruning, improving soil drainage and pH, proper sanitation including tool cleaning and debris disposal. Plant spacing is important for mold and mildew control, so prevent overcrowding by dividing clumps and pruning for openness.
Control Insect Vectors
Many plant diseases are spread by insects. Controlling those populations helps halt the transmission of pathogens. Leaf hoppers carry many diseases from plant to plant. Infected plants should be taken out by the roots and destroyed; the same goes for eriophyid mites.
Plant diagnosis at the Missouri Botanical Garden
Phone-in Horticultural Answer Service: 9 a.m. to noon, Monday through Friday, 577-5143
Kemper Center Plant Doctors: Bring in a sample of your sick plant (in sealed plastic bag), 10 a.m. to 3 p.m., Monday through Saturday. Service is free with Garden admission.