Michelle Grabowski, University of Minnesota Extension Educator
As the Minnesota growing season is winding down and first frost is just around the corner, many gardeners are starting to prepare their landscapes for the long winter ahead. Plants are making physiological changes that will allow them to survive Minnesota's cold winter and so are the plant pathogens that have plagued the garden this year. Many plant pathogens overwinter in infected leaves and herbaceous stems that will soon fall to the ground and become part of the leaf litter. Other pathogens survive in live plant tissue like perennial crowns and tree branches. Reducing the number of pathogens that survive from one season to the next is a great way to reduce potential disease problems in the next growing season. Sanitation, the removal of infected plant parts, is an important part of disease prevention. Although sanitation will not prevent all future disease (pathogens can often move in from long distances away), it can reduce the severity of disease or delay its appearance.
In the Flower Garden
Photo 1 (left): Peony with a fungal infection on leaves and stems. M. Grabowski.A wide variety of fungal and bacterial plant pathogens cause leaf spot and blight diseases in annual and perennial flowers. After the first frost hits, these infected leaves, stems, and flowers fall to the ground. The pathogens spend their winter nicely insulated within this old plant tissue, covered by snow. In the spring, warming temperatures and moisture from spring rains and melting snow stimulate the pathogens to start reproducing. Fungal spores and bacterial pathogens are then soon being splashed or blown onto the newly emerging plant shoots, starting the disease cycle all over again.
What can a gardener do?
- Scout your gardens now and take note of any plants with leaf spot or blight diseases. Even mild cases are worth addressing now.
- Do not bother spraying fungicides at this time. It is too late. Fungicides are protective and preventative in nature. They may be useful early in the season to protect young growing tissue but are unnecessary as the plants go dormant.
- After the first hard frost, go back to each infected plant and collect all of the fallen leaves. Cut herbaceous stems back to the ground. Remove all of this infected plant material from the garden.
- Infected plant debris can be composted if the compost pile heats up to 160 degrees F. Don't forget to check with municipal compost facilities. These are often more intensely managed than a backyard compost pile and are often an acceptable place to take infected plant material. If composting is not an option, infected plant material can be buried to speed up decomposition or thrown away with the trash.
- If any perennials are found to be infected with a virus or with aster yellows, remove the plant. These pathogens survive in the live crown of the plant. Once infected, the plant will always carry the disease. Virus and aster yellows infected plant material can be composted since these pathogens do not survive without a live host plant.
Trees and ShrubsAs in the flower garden, fungal and bacterial pathogens that infect tree leaves, survive the winter on these same leaves once they have fallen to the ground. New fungal spores and bacterial pathogens will be produced in the following spring to infect the newly emerging leaf buds. Other pathogens survive within live branches, the trunk or roots of the tree. Depending on where the pathogen is living, the control strategy is different
Photo 2 (right): Black knot on Prunus. M.Grabowski
- Scout trees and shrubs for any signs of infection before the first hard frost. Make notes as to where the pathogen is found. In leaf spot diseases, the pathogen infects the leaves. With cankers or galls, the pathogen survives in the infected branches.
- After leaf drop, rake up and remove all leaves from trees suffering from any leaf spot diseases. If there are too many leaves to collect, use a mulching lawn mower to chop of the leaves and speed up their breakdown.
- Infected leaves can be composted, burned (if allowed in your city) or buried.
- If cankers and galls are found, mark them for later removal. The best time to prune out cankers and galls in Minnesota is in February and March. At this time pathogens and insect pests that might infect the pruning wound are not active. For more information about dormant season pruning, see: http://www.extension.umn.edu/projects/yardandgarden/YGLNews/YGLNewsMar12007.html
- It is also important to help trees prepare themselves for winter by providing adequate water until the ground freezes. The trees root system will continue to take up water even after leaf drop. Research has shown that trees stressed by dry soil conditions are more likely to suffer from frost cracks. For more information about frost cracks visit http://www.extension.umn.edu/yardandgarden/YGLNews/YGLNews-Oct0106.html#frost
- If the base of the tree is mulched, make sure that the mulch is not piled up against the tree trunk. Volcano mulch cones provide excellent hiding places for rodents in the winter and can result in damage from rodent feeding along the trunk of the tree. Rake any mulch away from the trunk so that an air space occurs between the trunk and the mulch.
LawnsSnow mold is a common disease of Minnesota lawns. Although this disease most commonly causes problems in early spring, there are many cultural control practices that can be implemented now to reduce problems next year. The fungi that cause snow mold love moist cool conditions.
Photo 3 (left): Lawn with pink and gray snow mold B.Mugaas
- Reduce mowing height to 2.0 - 2.5 inches. This will allow the turf to remain upright underneath the snow cover. Longer grass flops over creating moist pockets that favor snow mold growth.
- Remove all leaves from nearby trees and shrubs from the lawn. This can be done by using a mulching lawn mower to break up the leaves or by raking and removing the leaves. Clumps of leaves left on the grass, do not allow the grass leaves to dry properly and create conditions favorable to snow mold.
- If making a second application of fertilizer, wait until the grass blades have stopped actively growing. This typically occurs about mid to late October in the Twin Cities. The roots and crowns of the grass plant will still be active and will use the fertilizer nutrients to build up the plant's food reserves. If fertilizer is applied very late in the season, while the turf is still growing above ground, new leaves may not have time to harden off before winter. This soft succulent tissue is especially susceptible to snow mold.