Soil Borne Disease

Healthy soil is teeming with microscopic life. Most soil organisms are beneficial, but some of them carry disease. The more you know about soil borne diseases, the better you can protect your plants.

Hands holding soil and seedling (Pexels) Pixabay

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The biggest problem with soil borne diseases is knowing they are there. You can’t see the pathogens. Damage can be done 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. This is why it is so important to monitor your plants regularly.


Fungi and nematodes are behind most soil borne diseases, but there are other players and some of them are relatively new discoveries.


Nematodes

Nematodes are microscopic, unsegmented worms. Some of them are beneficial and some carry disease. 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. It is estimated that, for every person on earth, there are 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

​Fungal diseases, et al

Fungi cause most soil borne diseases. Fungal diseases are more likely to occur in areas with high moisture and poor drainage. These diseases include:
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• crown rot
• Fusarium root and crown rot
• Fusarium wilt
• leaf spot
• root rots
• stem canker
• take all disease​

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Oomycetes used to be considered fungi but are now their own 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

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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 their own group. Phytomyxea are plant parasites that can cause clubroot in cruciferous vegetables and powdery scab in potatoes.


Bacterial diseases

Bacterial diseases are less likely to be soil borne because bacteria have a hard time surviving in the soil. Also, they need a wound or natural opening to get inside your plants. That being said, 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

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Viral diseases

Soil borne viral diseases are rare. In most cases, they are transmitted by nematodes and certain fungi. Soil borne viral diseases include lettuce necrotic stunt and wheat mosaic, which causes stunting and mosaics in wheat, barley, and rye.


How to prevent soil borne disease

In nature, plant diseases rarely get out of hand. Soil pathogens are usually kept in check by other organisms and plants’ defense mechanisms. However, as we select plants, spray chemicals, and stir up the soil, we interrupt those protections. The main 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 aged compost is very effective at suppressing soil borne diseases in greenhouses, though less so in the field. In both situations, the more compost was added, the more effective it was. Interestingly enough, if the compost was sterilized beforehand, it was 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. This is called the disease triangle. Remove any one of the three 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 disease.


While you can, in some cases, 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. Funny thing is, if all those things were already there, so would most of the predators. Biodiversity is your friend. In fact, mycorrhizal fungi (good guys) often create protective mats which contain antibiotics and pathogenic toxins around plant roots, all while helping your 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
• incorporate 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, depending on the plant species
• remove and dispose of infected plant material as soon as it is seen
• keep host weeds at a distance
• incorporate crop rotation
• sanitize tools, planter pots, trellising between uses
• avoid walking through areas affected by disease

Finally, as tempting as they may be, chemical treatments are rarely a good choice for backyard gardeners. Pathogens are developing resistance to these treatments which means stronger chemicals must be used. Whenever possible, use some other method of controlling soil borne diseases.