Soil Borne Disease

Healthy soil is teeming with microscopic life. Most soil organisms are beneficial, but some of them carry diseases.

Hands holding soil and seedling (Pexels) Pixabay

The biggest problem with soil-borne diseases is knowing they are there. You can’t see the pathogens, so damage can occur before you know anything is wrong. Also, symptoms of soil-borne diseases can look a lot like nutrient imbalances, chemical overspray, and poor environmental conditions.

Fungi and nematodes are behind most soil-borne diseases, but other conditions are at play, and some discoveries are new.

​Nematodes

​Nematodes are microscopic, unsegmented worms. Some of them are beneficial, and some carry diseases. Beneficial nematodes kill cutworms and corn earworm moths. Disease-carrying nematodes include needle nematodes, root-knot nematodes, and stubby root nematodes. The real problem with nematodes is that there are so many of them. For every person on Earth, there are approximately 60 billion nematodes. [Thank goodness they aren’t all bad!]

Juvenile root-knot nematode penetrates tomato root (William Wergin, et al) USDA CC BY 2.0

Bacterial diseases

Bacterial diseases  are less likely to be soil-borne because it is difficult for bacteria to survive in the soil. Also, they need a wound or natural opening to get inside your plants. The following soil-borne diseases can occur in your garden:

• Bacterial wilt
• Crown gall
• Corky root of lettuce
• Potato scab
• Soft rots

Bacterial wilt of muskmelon (Eeshie) CC BY-SA 3.0

Fungal diseases in the soil

​Fungi cause most soil-borne diseases. Fungal diseases are more likely to occur in areas with high moisture and poor drainage. These diseases include:

• Crown rot
• Fusarium root and crown rot
• Fusarium wilt
• Leaf spot
• Root rots
• Stem canker
• Take all disease

There are nearly 20,000 fungi that cause diseases in plants, but we can thwart most of them with good cultural practices.

Peas infected with Fusarium wilt

Oomycetes

Oomycetes used to be considered fungi but are now a separate class. Oomycetes cause soil-borne diseases such as damping off, downy mildews, late potato blight, some root rots, and sudden oak death.

Oomycete responsible for late blight ‘Phytophthora infestans’ (Matteo Garbelotto, et al) CC BY 2.5

Phytomyxea

There is another class of soil-borne disease carriers called Phytomyxea [FI-toe-muh-kia]. Scientists used to think they were a type of slime mold, but genetic testing and electron microscopes have taught us that they are a unique group. Phytomyxea are plant parasites that can cause clubroot in cruciferous vegetables and powdery scab in potatoes.

Viral diseases

Soil-borne viral diseases are rare. In most cases, nematodes and certain fungi carry these pathogens. Lettuce necrotic stunt and wheat mosaic, which causes stunting and mosaics in wheat, barley, and rye, are two common soil-borne viral diseases.

How to prevent soil-borne disease

In nature, plant diseases rarely get out of hand. Plants’ defense mechanisms and other organisms keep most pathogens in check. But, when we select plants, spray chemicals, and disturb the soil, we interrupt those natural processes. The primary cause of soil-borne diseases taking hold is an imbalance in soil populations. Reduced biodiversity gives pathogens the upper hand.

One way to reintroduce that biodiversity is by top dressing with aged compost. Research has shown that top dressing with compost is very effective at suppressing soil-borne diseases in greenhouses, though less so in the field. There is a direct correlation between how much compost was applied and its effectiveness. Interestingly enough, sterilized compost is less effective. I think we can assume the effect is at least partially biological.

​As with most diseases, three factors must be present for a problem to occur: the host plant, the pathogen, and the right environmental conditions. Remove any one side of the disease triangle, and the disease is prevented or controlled. Crop rotation is an excellent way to break this disease triangle. Your rotation schedule will vary depending on the plants and pathogens in your garden.

While you can sometimes apply treatments directed toward specific pathogens, they don’t always work. Most of these treatments consist of other microorganisms that prey on the pathogens. These only work if your soil already has everything the introduced microorganisms need. The funny thing is, if all those things were already there, so would the predators. Biodiversity. Mycorrhizal fungi (good guys) often create protective mats containing antibiotics and pathogenic toxins around plant roots. They also help plants absorb nutrients.

Use these tips to prevent soil-borne diseases in your garden:

• Do not plant grocery-store produce or live herbs from the grocery store.
• Use disease-resistant varieties and certified disease-free seeds and seedlings.
• Select plants suited to your microclimate and soil pH.
• Avoid damaging root systems and employ no-dig gardening.
• Improve drainage.
• Regularly test soil to help you maintain proper nutrient levels.
• Monitor plants frequently for signs of disease.
• Treat fungal diseases with fixed copper or sulfur according to the plant species.
• Remove and dispose of infected plant material.
• Keep host weeds at a distance.
• Rotate crops.
• Sanitize tools, planter pots, and trellising between uses.
• Avoid walking through areas affected by diseases.

Finally, as tempting as they may be, chemical treatments are rarely a good choice for backyard gardeners. Pathogens are developing resistance to these treatments. Increasingly powerful chemicals are applied, resulting in a dangerous escalation. Whenever possible, use some other method of controlling soil-borne diseases.