Safe Ways to Banish Vegetable Garden Diseases

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Late blight on a tomato plant

When it comes to garden diseases, you never know what nature will throw your way. Last summer after rabbits ate my cucumbers, my friend Kelly Erb invited me to pick some of hers. The robust vines hid dozens of fruits of three different varieties. Only one week later, the scene was completely changed as the plants fought a fast-spreading leafspot disease, most likely cucumber downy mildew, which was unknown in this area until a few years ago.

Such is life in the garden, but what to do next? In addition to getting the diseased plants out of the garden, which Kelly did without delay, what are other smart responses to vegetable garden diseases?

“Plant
Vegetable garden disease problems can develop when the three points of the triangle come together

Understanding Garden Diseases

Every garden hosts diseases, which are very specific pathogens that infect certain vegetable crops. Plant pathologists use the phrase “disease triangle” to describe the three things that must happen for a garden disease to develop into a problem.

  1. Host: The plant must be susceptible to the disease, and at a life stage when it is easily infected. Most vegetables become more susceptible to disease as they age.
  2. Pathogen: The microscopic organism that causes the disease must be present.
  3. Environment: Weather and other environmental factors must prevail that favor the disease.

You can use our disease guides to identify a disease in your garden. From there, your first and best line of defence is to start growing resistant varieties. Rotating crops is immensely helpful, too, because moving veggies around prevents the proliferation of crop-specific soil pathogens in the soil. But there is little you can do about the weather, or the appearance of unusual garden diseases you’ve never seen before that blew in on the wind, like our downy mildew.

From now on, we will be planting cucumbers as we do tomatoes, in scattered patches rather than in one large group, so that when diseases come, the entire crop won’t be such easy prey.

“Cucumber
Growing large numbers of plants together makes it easy for disease to spread rapidly

Disposing of Diseased Plants

Can you dispose of diseased plants in the compost pile? In most cases the answer is yes, because most plant diseases perish along with the plants they infect. Plus, in the real world of the garden, almost every plant you pull and compost carries some form of disease. Rather than lurking in the compost heap, most diseases, including all viruses, overwinter on wild weeds or in the bodies of insects, or they may be spread on the feet or mouthparts of pollinators visiting nearby flowers. Microbially speaking, the garden is never a clean space.

Yet every climate has some terrible disease that strikes at the wrong time, like late blight of potato and tomato, the same pathogen that caused the Irish potato famine of 1840s. I see it every few years, and rather than having late blight oospores splashing about in the garden, I promptly pull and dispose of diseased plants by taking them to the brush pile in the woods. There they dry into skeletons a safe distance from the garden.

“Mustard
A green manure crop of mustard can help restore soil with disease issues

Another method for disposing of diseased plants is to dig a hole outside the garden and bury them under 6 inches (15cm) of soil. I once did this with apple leaves infected with cedar apple rust, the best way to be sure the spores would not become airborne. It was once common practice to burn diseased plant parts, but burial is more climate friendly. No air pollution, and carbon is buried in the soil.

In situations where a disease seems to be present in soil, remove all plant material and dispose of it outside the garden before piling on compost and sowing a green manure crop of mustard greens. When chopped under, mustard greens act as a natural fumigant for troubled soil. After the mustard greens rot, the soil should be ready to grow vegetables again.

“Washing
Use warm, soapy water to remove salts and organic matter from planting containers

Cleaning Garden Equipment

When you wash pots, seed trays and other garden equipment in warm, soapy water, you remove salt deposits, soil, and millions of microbes. Removing soil is crucial, because fungi hide in organic matter. Once cleaned, items with smooth surfaces, such as plastic seed trays or metal stakes, are not likely to harbour plant pathogens, but the crevices in wood stakes can provide safe havens. After removing dirt, spraying wood stakes with a weak bleach solution (9 parts water) will keep diseases from staging a comeback next season. Better to be safe than sorry.

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