Bulletin #2753, Managing Diseases in the Home Garden

planting seeds in a garden with drip irrigation

By Alicyn Smart, D.P.M., Plant Pathologist & Director of the Plant Disease Diagnostic Laboratory, Katherine Garland, Horticulturist, University of Maine Cooperative Extension, and Katie Ashley, University of Maine Graduate Student

For information about UMaine Extension programs and resources, visit extension.umaine.edu.
Find more of our publications and books at extension.umaine.edu/publications/.

With all the time and effort you put into your garden, it’s important to limit the diseases your plants could encounter. The best way to manage plant disease is through prevention. Here are simple suggestions that can make a big impact.

Identify past disease problems and select varieties with the most disease resistance for the upcoming growing season.

For example, if you had an issue with a disease on your tomatoes last year, contact your local Extension office to determine the likely pathogen, then select tomato varieties resistant to the culprit (Table 1). If this is your first season with a crop, research some of the common disease problems and what varieties might be more resistant. If you plan to save seeds, only save the seed from plants that performed well and showed no symptoms of a disease. See bulletin #2750, An Introduction to Seed Saving for the Home Gardener. Also, remember that seeds can sometimes harbor diseases and resistance may not carry to the next generation of plants.

Crop	Disease	Resistant Varieties
Tomato	Early Blight	Mountain Magic, Defiant PhR, Jasper, Juliet, and Verona
Tomato	Late Blight	Mountain Magic, Defiant PhR, Mountain Merit, Pruden’s Purple, Matt’s Wild Cherry, and Plum Regal
Potato	Early Blight	Elba, Ranger Russet, and Lamoka
Potato	Late Blight	Elba, Kennebec, Sebago, Allegany, and Chieftain
Cucumber	Powdery Mildew	Nokya, Quirk, Salt and Pepper, and Tasty Jade
Plum	Black Knot	Damson, Bluefree, Shiro, Santa Rosa, Formosa, Methley, Milton, Early Italian, Brodshaw, and Fellenberg
Peach	Peach Leaf Curl	Frost, Indian Free, Muir, and Q-1-8
Spruce	Rhizosphaera Needlecast	Norway spruce and Red spruce
Rose	Black Spot	Blushing Knock Out, Brite Eyes, Double Knock Out, Golden Eyes, Hansa, Kashmir, Moje Hammarberg, My Girl, White Dawn, and Wildberry Breeze

Purchase seeds and plants from reputable sources.

Take the time to carefully examine the plant when you buy it. Make sure it’s not root bound, there is no discoloration of the leaves, there are no broken limbs, and the plant’s overall health is good. Avoid “rescuing” questionable, enticingly low-cost plants bound for the trash or compost pile.

Keep it clean!

Good sanitation is an essential step in avoiding diseases. Clean re-used pots and seedling trays by brushing off debris, rinsing with water, and then submerging in a 10% bleach solution or other approved sanitizer for the time indicated on the label. If the label does not state a specific amount of time, then soak for at least 10 minutes and rinse with water. Similarly, a cleaning solution should be used to clean tools and garden structures, such as trellises or stakes, potentially exposed to diseased plant tissue. Metal cutting tools should be cleaned with 70% alcohol, which will not cause corrosion. Certain garden infrastructure that cannot be sanitized (e.g., burlap) and may have been exposed to diseased plants should be replaced to limit the chance of introducing the disease to healthy plants. Equipment, such as a rototiller, should be rinsed with water if moved from one area to the next. Make sure last year’s garden and yard remnants have been cleaned up and diseased and damaged woody stems are removed as soon as they are noticed. Leaving diseased tissue in place allows pathogens to overwinter and reinfect your garden the following spring.

Practice proper planting methods.

When sowing seeds in containers, moisten the media first before filling the trays. This will help reduce compaction, which can cause poor germination and developmental delays. Compaction can also occur if you stack the trays, so carry each one separately. Follow the instructions on the seed packet to determine proper planting depth. See Bulletin #2751, Starting Seeds at Home for more information on seed starting. Planting depth is also an important factor when installing perennials, shrubs, and trees—not too shallow, not too deep, but just right! Learn more about selecting, planting, and caring for woody plants. Be sure to also follow spacing recommendations. A common mistake many gardeners make is to plant seeds, seedlings, and more mature plants too close to one another. Seeds can be sown closely, but they should be thinned to proper spacing early in development. Thinning is ideally done before more than two sets of true leaves have developed.

Select the right plants for the location.

Sun exposure and drainage are two key site factors to assess. Carefully choose plants well adapted to the specific conditions of your site (see links to listings of plants for challenging sites below). Installing plants not well adapted to the site conditions may cause plants not to thrive and be more susceptible to pathogens. With most vegetables, especially any plants grown for their fruit, at least 8 hours per day of sunlight and good drainage is essential. Learn how to assess the drainage of your soil (PDF). Sites prone to water accumulation can harbor water mold pathogens that cause root diseases.

Rotate crops based on plant families.

Closely related plants are often susceptible to the same insect and disease pressures. Rotating plant families to different locations from year to year will help to avoid the buildup of pathogen and insect populations. Even if symptoms of a particular disease aren’t noticed, the pathogen may have been present and can eventually accumulate in the site if plants in the same family are grown in the same spot annually. Keeping a written or photographic garden journal with a planting map can help ensure crop rotation.

Increase air circulation within the plant canopy and water wisely.

Wet leaves are more prone to foliar diseases. Improving air circulation will allow leaves to dry more quickly and can be accomplished by properly spacing plants, keeping weeds down, providing adequate support for taller and vining plants (YouTube), and pruning woody plants according to their specific growth requirements.

Overhead irrigation can promote foliar diseases because the water lands primarily on the leaves, rather than the soil. Instead, apply water directly to the soil and limit water splashing from the soil onto the lower leaves. Remember that plants take water in through their roots, not their leaves. Mulches, such as straw and shredded leaves, can make a good barrier between the soil and foliage to block pathogens from moving onto the plant.

Be sure to not overwater. If your irrigation turns on at a scheduled time, consider adding a rain sensor to override the timer when there is sufficient precipitation. Morning watering is preferred to evening because foliage that gets wet will be more likely to dry during the daylight hours.

Finally, make sure plants have access to healthy soils.

Nutrient levels, pH, and organic matter content can all be assessed by a standard soil test. The report offers gardeners important “next steps” to create the conditions for optimal plant health. A healthy plant is more resilient in the face of disease pressures.

Carefully selected plants growing in the appropriate site and cultivated using proper sanitation practices have the best shot at staying vigorous and productive throughout the growing season. However, if you think you might have a disease in your garden, you can submit a sample to the UMaine Extension Plant Disease Diagnostic Laboratory. We can help you identify the problem and give you management recommendations.

Information in this publication is provided purely for educational purposes. No responsibility is assumed for any problems associated with the use of products or services mentioned. No endorsement of products or companies is intended, nor is criticism of unnamed products or companies implied.

Call 800.287.0274 (in Maine), or 207.581.3188, for information on publications and program offerings from University of Maine Cooperative Extension, or visit extension.umaine.edu.

In complying with the letter and spirit of applicable laws and pursuing its own goals of diversity, the University of Maine System does not discriminate on the grounds of race, color, religion, sex, sexual orientation, transgender status, gender, gender identity or expression, ethnicity, national origin, citizenship status, familial status, ancestry, age, disability physical or mental, genetic information, or veterans or military status in employment, education, and all other programs and activities. The University provides reasonable accommodations to qualified individuals with disabilities upon request. The following person has been designated to handle inquiries regarding non-discrimination policies: Director of Equal Opportunity, 5713 Chadbourne Hall, Room 412, University of Maine, Orono, ME 04469-5713, 207.581.1226, TTY 711 (Maine Relay System).