Soil-borne plant diseases persist in the soil for years, so prevention combines long rotation with drainage, a soil pH matched to the disease, clean transplants, tool and boot hygiene and resistant varieties, and diagnosis starts by pulling a sick plant and examining the roots and cut stem. Rotation alone does not clean the soil: it only lowers the pathogen, and the pH that suppresses clubroot can encourage potato scab in the next crop.
Soil-borne diseases are plant diseases caused by organisms that live in the soil or crop debris and infect roots and stems from there.
What Makes Them Hard
A soil-borne pathogen survives without its host in resting structures: spores, sclerotia or hardened fungal bodies. There is no spray fix inside a field, the infestation is patchy and moves on soil, so a boot, tool or wheel carries it to the next bed. The best management is to stop it spreading and to lengthen the gap between hosts (Crop Rotation for Disease Break shows how a seven-year break still leaves about 30% of clubroot spores, on a reported half-life of about 4 years).
Common Soil-Borne Diseases
| Disease | Crops | Sign | Main controls |
|---|---|---|---|
| Clubroot | Brassicas | Wilting at midday, swollen club-shaped roots | Long break, high pH, resistant varieties, drainage, clean transplants |
| Fusarium and Verticillium wilts | Tomato, potato, many others | Yellowing, wilting on one side, brown streaks inside the stem | Resistant varieties, rotation, clean seed and transplants |
| Damping-off | Seedlings of most crops | Seedlings collapse at the soil line | Clean mix, no overwatering, airflow, sow into warm soil |
| Potato scab | Potato, beet, radish | Rough corky patches on skin | Keep soil moist at tuber set, avoid liming just before |
| Root-knot nematodes | Tomato, carrot, beans | Knotted, galled roots; stunting | Long rotation, resistant varieties, solarisation |
| Allium white rot | Onion, garlic, leek | Yellowing leaves, white fluffy rot on the base | Never import contaminated stock; no short rotation will clear it |
Diagnose By Pulling The Plant
- Pull a wilting plant rather than guessing. Look at the roots.
- Swollen, club-shaped roots on a brassica point to clubroot. Do not confuse them with the small round nodules on legume roots: nodules rub off with a thumb and are nitrogen factories (Nitrogen Fixation); galls from nematodes or clubroot do not rub off.
- Cut the stem lengthwise just above the soil. Brown streaking in the inner tissue suggests a wilt fungus rather than drought. A drought wilt recovers after watering; a wilt disease does not.
- Record where it occurs. Mark the patch on the bed map; the pattern shows the direction in which tools and water carried it.
Preventive Measures
Drainage. Waterlogged soil encourages root rots and damping-off. Raise beds and shape paths to shed water.
Soil pH. Clubroot is suppressed by raising soil pH toward neutral or above, which is normally done with lime or wood ash (recalled extension guidance puts the target around 7.2 or higher; check against your own soil tests) (Soil pH, Wood Ash). Potato scab is the reverse: it is worse in dry, alkaline soil, and extension guidance suggests keeping potato soil below about pH 5.2. A bed limed for brassicas should therefore not carry potatoes the following year. Put this in the rotation plan, not in your memory (Family-Based Rotation Schedules).
Clean transplants. Raise your own seedlings in clean mix; do not buy or take plants from unknown sources into a clean garden (Sanitation and Quarantine Protocols).
Tool and boot hygiene. Scrape soil off before leaving an infested bed; work infested areas last.
Resistant varieties. Resistance is the single most effective tool for wilts and clubroot. Save seed from plants that survive an outbreak (Selecting for Adaptation vs Uniformity).
Compost and organic matter. Well-made compost improves drainage and supports organisms that compete with pathogens; it helps but is not a cure.
Solarisation. Clear plastic laid tight over moist bare soil for 4-6 weeks in the hottest sunny weather heats the top few centimetres enough to kill many pathogens, but not deeper ones. It needs plastic, and it is a hot-season job, so it fits fallow beds (Fallow Rotation and Soil Recovery).
Failure Modes
- Moving soil. Soil on tools, wheelbarrows and boots spreads the patch.
- Weeds as hosts. Shepherd’s purse and wild mustards carry clubroot between brassica crops.
- Liming blindly. It can fix one disease and encourage another.
- Treating symptoms as water stress. A wilting plant that does not recover after watering is not thirsty.
- Replanting the dead patch. Leave the infested area to a non-host crop.
Lime and wood ash burn
Quicklime and slaked lime burn skin and eyes and are harmful if inhaled, and wood ash is caustic when wet. Wear eye protection and gloves, keep upwind when spreading, and wash splashes with plenty of water.
What’s Next
- Learn next: Fungal Disease Prevention and Management — the leaf-disease side
- Combine with: Nutrient Cycling Through Rotation — rotation for fertility as well as health