Gloved hands placing a seedling in the soil
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Preventing Plant Diseases

Cultural practices and knowing your soil type are key ingredients to disease prevention success!

If the plant tag says “prefers full sun and well draining soil”, then placing that plant in a shady area with clay soil is probably not going to be good for the plant. Light and water are two things plants need, but they need varying degrees of each, depending on the plant and your soil type.

Soil Type


  • Water is held by clay particles for long periods of time, and can become waterlogged if over-watered. You will not need to water as often since the soil will hold the water more efficiently.
  • Soil can become crusty when dried out making it difficult for seedlings to emerge.
  • It can take a longer time to re-wet clay soils since water has a tendency to run off the surface, rather than penetrate. So once clay soils become crusty, spend some time to ensure water penetrates down at least 4 inches. Incorporate compost into the soil to help loosen and create more pore spaces.


  • Water drains quickly in sandy soils, making it less available for plants.
  • It will take less time to water deeply in sandy soils than in clay soils.
  • For water-hungry plants like tomatoes, you can bury logs approximately three feet down in your garden. When the wood breaks down, it will slow down water drainage. At the same time, the decayed matter adds nutrients to the soil. You also simply add compost to your garden soil for more water retention.

Remember that regardless of what type of soil you have, adding organic matter to the soil enables it to store water for your plants! This is a good thing in your vegetable garden, but might not be so good for your sedums, rockrose, lavender, or other plants that don’t like wet feet. Make sure these soils are well draining.

Ideal soil is a combination of organic matter and mineral soil, blended to create a well draining medium that also holds water long enough for plants to utilize it.  Create healthy soil for the first line of defense against plant diseases.

Cultural Practices

Avoid overhead irrigation

This one is very difficult to do in the Pacific Northwest since we cannot control the rain. But when you do need to water, water the roots of the plant at the soil level, and avoid getting water on the leaves. Water in the morning when the leaves have a chance to dry out before nightfall. A wet environment will promote fungus. There are many techniques you can use to keep rain off your plants, including cloching them with a hoop system covered in plastic. This is especially helpful for tomato plants which are susceptible to late blight (Phytophtora infestans). Controlling the watering completely and never letting the water splash soil onto the leaves will prevent this devastating disease from hitting your plants.

Weed patrol

Weeds compete with plants for sunlight, moisture, and space. Weeds can also host pests and diseases that can spread to other plants.  The Cucumber Mosaic Virus for instance can be carried by chickweed and groundsel.  Thrips are often found on their alternate host of annual sowthistle and plants in the carrot family will be beacons for carrot rust fly. Removing these weeds from the garden will help reduce the incidence of problems associated with these insects and allow the sun, water and soil nutrients to be available for your growing plants.

Sanitation and Air Circulation

Many plant diseases spend the winter clothed in foliage and plant material left from the summer. A good way to reduce winter survival of the disease organisms is to remove old plant material and compost it in a hot pile or place it in your yard waste bin for commercial composting. Take the time to prune plants to open them up for air circulation; this is especially important when the plants are fully leafed out in the summer. When choosing places to plant, keep in mind the need for air movement through the plant and avoid overcrowding.

Choose disease resistant plants.

There are cultivars of plants that are resistant to insects and diseases such as verticillium wilt, apple scab, anthracnose, dutch elm disease, scale, and others. Become familiar with what might be a problem for the plants you grow so that you can pre-empt issues.

Crop Rotation

Several crop families need to be rotated from bed to bed, from growing season to growing season in order to prevent disease buildup and insect infestations. These include all plants in the Solanaceaefamily (tomato, potato, pepper, eggplant, tomatillo, groundcherry), Brassicaceae family (broccoli, cabbage, collards, turnips, rutabagas, radish, mustard, Brussels sprouts, kale, cauliflower, bok choy) and the Allium genus (onion, leek, garlic, chives, shallots). Keep detailed plans of which bed you are growing them in each season and rotate them into a new bed the next growing season. Avoid inter-mixing these so you can control this more easily. Since we do many successive sowings of plants in the northwest make this rotation based on the annual cycle and keep the same types of crops growing in their respective beds through the season.

Plant Right for Your Site

Pay attention to the site placement of your plants. Take advantage of different sun and shade spots in your yard to create a more diversified garden. Hostas, ferns, and hellebore love shadier placements compared to daylily, Echinacea and yarrow for instance. Get to know the native environment of the plants you are growing to get clues about what they like. When trees grow they will alter the sun environment in your garden, likewise when someone builds a taller building in the neighborhood. You might find plants will need to be moved after a few years to accommodate their need for sun or shade.

Creating a habitat that works well for your plants will always be a good strategy to prevent disease issues in your garden. And the wonderful thing is that be default you will develop a more diverse and beautiful yard that also will need less maintenance. A win-win situation!