Michelle Grabowski, University of Minnesota Extension Educator
Whether you are looking for tomato transplants, annuals for a front garden bed or a new tree or shrub, one of the most important things you can do to ensure the future success of the plant is to start out with a healthy disease free plant.
Some plant pathogens live in our gardens in plant debris or soil, waiting for the right plant and the right environmental conditions to come along. Other plant pathogens come into the garden on wind, rain, or are carried by insects. Unfortunately many plant pathogens can be brought into the garden on infected plant material.
This later group of plant pathogens can be avoided by a disease management strategy known as exclusion. Exclusion is a strict ‘no pests allowed’ policy. For gardeners, this is one of the simplest pest management strategies to implement.
Photo 1: Petunia plants wilting due to Rhizoctonia root rot. RK Jones NCSU, Bugwood.org
First, purchase plants from a reputable nursery or seed company. In Minnesota, the Nursery Law (MN statutes, Chapter 18H) protects consumers, by requiring that all perennial nursery plants be inspected annually and certified pest free before being sold. This includes perennial flowering plants, trees, shrubs and other perennial landscape plants. Anyone selling perennial nursery plants in Minnesota must have a nursery certificate. You can verify any vendor’s certificate status at the Minnesota Department of Agriculture’s website.
Although you may find a great deal on plants on e-bay, Craig’s list or at a garage sale, remember these plants have not been inspected and many gardeners have found that their bargain plant came with a free plant pathogen. These unwanted pathogens can harm the new plant and potentially spread to other established plants in the garden.
Photo 2: New Chysanthemum plants wilted from root rot. RK Jones NCSU, Bugwood.org
At the nursery or garden center, carefully inspect every plant prior to purchase. Look at the upper and lower leaf surfaces for spots, discoloration, unusual growth or other signs of disease. Examine the stem. Are there any open wounds, excessive sap, discolored or soft mushy areas? These could all indicate a disease problem. Either in the store or prior to planting, examine the roots. They should be firm and light colored. Dark mushy areas may be infected with a root rotting pathogen. Reject any plants showing symptoms of disease. If many plants in a group appear diseased, do not purchase any of the plants within the group. Nursery plants are often grown close together and share many maintenance practices like pruning, watering and fertilizing. If there are infected plants within a group, it is easy for the plant pathogen to move from one plant to another in a nursery setting.
Read over the plant tag. Is this plant hardy in your area? Will the plant thrive in the conditions of your garden? Plants stressed by cold, drought, too much sun or shade or other environmental problems often are more susceptible to disease problems in addition to environmental stress.
Photo 3: Tomato plant with a large crown gall. FLDACS, Bugwood.org
Whenever possible purchase plants that are bred to be disease resistant. Many common disease problems like apple scab on crab apple, powdery mildew on phlox and Verticillium wilt on tomato can be avoided altogether by growing disease resistant varieties. Disease resistance is often listed on the plant label.
In addition, some plants like roses and hostas are screened by some nurseries to verify that they are free of viruses like Rose Mosaic Virus and Hosta Virus X. Ask at the nursery if these plants have been screened or ‘indexed’ for virus prior to purchase.
Careful selection of plants early in the season can help gardeners avoid future disease problems.
Splitting and sharing perennial plants is a tradition as old as gardening itself and can be a cost effective way to grow your garden while connecting to friends in the gardening community. If you are receiving plants from a split perennial, accept plants from someone you know and trust. Volunteer to help dig and split plants. This will allow you to see the mother plant. Select vigorously growing plants for division. Examine the plant carefully for any disease signs or symptoms. Look for spots, discoloration, unusual growth, or rot on all plant parts. Reject plants that appear to be infected with a disease. Healthy looking, vigorously growing plants can be split and shared. Gardeners may choose to take extra precautions against introducing a disease problem by isolating the new plant in an area away from the main garden bed for a year to watch for potential problems. This way, if a problem does come up, the plant is easily removed and it is less likely to spread to your other well established perennials.