September is National Organic Month, so I thought I’d use this month’s PDDC web article to explore techniques for plant disease management that fall within the philosophy of organic gardening. This philosophy tends to steer gardeners away from the use of chemicals for plant disease control. There are many disease management techniques that are appropriate for organic gardening, and quite frankly, when I am making control recommendations for virtually any client, the primary techniques that I recommend are organic in spirit. These include:
Practicing good garden clean up
Disease-causing organisms often survive in the remains of infected plants. Removing and properly disposing of this debris can help reduce pathogen populations in a garden or landscape. Whether you do this clean up in the fall after plants have gone dormant for the year or in the spring before plants begin to emerge for the new growing season is up to you. There are pros and cons to either choice. Disposal methods for this debris include burning (not the most environmentally friendly option), burying (a somewhat laborious task), or hot composting (probably the best technique if done properly).
Using resistant plant varieties
Individuals of a particular plant species can be highly variable in terms how they react to disease-causing organisms. Plant enthusiasts have exploited this variability by watching for plants that develop less severe symptoms and interbreeding these healthier individuals to develop disease resistant plant varieties. I often recommend that home gardeners plant apple trees that have been bred for resistance to apple scab and fire blight, or rose shrubs that have been bred for resistance to black spot. Monarda, phlox, and cucumber varieties that are resistant to powdery mildew can be useful in home gardens as well. One downside to using resistant varieties, is that these plants may not have other horticultural characteristics (e.g., flower color, size, scent, flavor) that gardeners are looking for.
Buying healthy plants and seeds
You can easily introduce disease-causing organisms into a garden via infected (or otherwise contaminated) plant materials. Carefully inspect new plants for evidence of disease issues (e.g., leaf spotting, sunken areas on branches or trunks, fuzzy growth on upper or lower leaf surfaces, etc.), and avoid buying problematic plants. Seeds can also be a source of disease-causing organisms but can be much more difficult to assess, as they may not exhibit obvious symptoms. Try to purchase seeds from a reputable grower/company (although even the best of growers can occasionally have disease issues). If you have a suspect batch of seed (particularly vegetable seed), consider using hot water treatments to help eliminate disease-causing organisms. These treatments can be particularly effective for controlling seed-borne bacteria.
Planting trees and shrubs (and plants in general) in the right environment
Plants that are under environmental stresses tend to be more prone to disease issues. Before purchasing plants, make sure they are well adapted to the light, moisture, and fertility conditions at your location. Put the right plant in the right place. Pagoda dogwood is a tree that is often sited poorly in urban landscapes. I see these trees in the middle of an open yard with grass growing up to the trunk where there is a lot of light, excessive heat, and limited water. Pagoda dogwood is an understory tree that prefers shady, cool, moist conditions. When planted in a hot, dry, sunny environment, this tree tends to be more prone to developing golden canker, a serious and often lethal fungal disease. Also, be cautious about planting pin oaks or red maples. In much of Wisconsin, these trees are prone to chlorosis, because soil pH is so high that the trees have trouble taking up adequate iron (pin oak) or manganese (red maple).
Using proper plant spacing and thinning of trees and shrubs
Plant diseases tend to be more of an issue when plants are crowded. Crowded plants trap humid air, which slows leaf drying. Wetter leaves favor infection by disease-causing fungi and bacteria. Planting herbaceous plants farther apart, routinely dividing large clump plants (e.g., peonies), and regularly pruning trees and thinning shrubs can help create a drier environment that is less favorable for disease development. Regular pruning also removes diseased branches, thus eliminating a source of pathogens and reducing pathogen spread.
Avoid using sprinklers for watering your garden, as this method wets leaves and creates a favorable environment for fungal or bacterial infection. Use of overhead watering can eliminate any benefits you might gain by properly spacing and pruning/thinning plants (as described above). Instead of a sprinkler, water with a drip or soaker hose that applies water directly to the soil and keeps moisture off of leaves.
Keeping weeds under control
Weeds compete with garden and landscape plants for nutrients, leading to stress that can predispose plants to infection. Weeds also crowd other plants, trapping moisture and creating an environment conducive to infection and disease development. Finally, weeds can serve as reservoirs for disease-causing organisms that can eventually move from the weeds to your favorite ornamentals and vegetables. So, weed, weed, weed. Fewer weeds translates into a healthier garden.
The points that I’ve outlined above are just a few of the many techniques that you can employ to achieve a healthier and more aesthetically appealing garden or landscape. If you have questions about these or any other plant-disease-management techniques, feel free to contact me at firstname.lastname@example.org or (608) 262-2863.
Now, go forth and garden!!