Clubroot in an intensive market garden
An intensive 2.0 ha market garden near Levin grows brassicas, lettuce, carrots and green manure crops. The main 0.6 ha brassica block has shown stunted, wilted broccoli with swollen, distorted roots and galls, confirmed as clubroot (Plasmodiophora brassicae). Soil pH is 5.9. The block has grown brassicas in three of the last four years. Drainage is poor in the lower corner, which stays wet. The owner plans to grow broccoli again in spring on the same block because of a supermarket contract, and proposes a three-year rotation, 500 kg/ha of lime, and a fallow. Options include moving the brassica crop to a new 0.4 ha paddock with pH 6.5 and no history of brassicas, extending the rotation, improving drainage, or growing a non-host crop.
1. Evaluate the owner's plan for the infested block.
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The plan is not adequate. Clubroot spores survive for 10 years or more in soil and in brassica weeds, so a three-year rotation is far too short in an infested block. Liming from pH 5.9 toward 6.5 will reduce root-hair infection, but the target for clubroot suppression is closer to pH 7.0–7.2, and 500 kg/ha of lime will not move pH that far on most soils. The lower corner with poor drainage is the highest-risk area, because clubroot zoospores need water to swim and waterlogged roots are more susceptible.
The main risk is continuing broccoli on the same block for the contract. That will increase spore numbers and spread the infestation. The best judgement is to move the spring broccoli to the clean 0.4 ha paddock, provided water, access and contract volume allow, and keep the infested block for non-hosts such as lettuce, carrots or a grass/clover ley. The contract may justify buying in land or reducing the area rather than losing the crop. The owner should treat the clean paddock as a limited resource and protect it with hygiene.
2. Design a rotation and hygiene programme for the infested block.
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Rotation length should be based on spore load and host frequency, not on a fixed three years. In a badly infested block, aim for at least six to seven years without brassicas. That includes green manure brassicas such as mustard and radish, and brassica weeds such as shepherd's purse and wild turnip, which must be controlled.
Use non-host break crops and a grass ley to build soil structure and reduce spore activity. Lime to pH 7.0 or above and maintain it; apply lime well before planting and incorporate it. Improve drainage in the lower corner or take that area out of production, because wet soil favours infection.
Hygiene matters as much as rotation. Wash machinery and boots between blocks, avoid moving soil, and use clean transplants. Remove and destroy galled roots rather than spreading infected debris. These measures reduce the risk but do not eliminate it, so the owner should not promise a clean crop from the infested block. Keep records of host crops and lime applications so the rotation can be audited.
3. The supermarket contract requires a spring broccoli crop. What would you advise, and why?
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Grow the contracted broccoli on the clean 0.4 ha paddock and negotiate volume with the buyer, accepting a smaller area if necessary. The expected loss from clubroot on the infested block is high, because infected plants are stunted and unmarketable, and the disease will spread with soil movement and water.
The clean paddock has pH 6.5, which is adequate but not ideal. Lime it now toward pH 7.0 if there is time before planting, and keep machinery out of the infested block before entering the clean paddock. If the clean paddock cannot be used, the next best option is to plant only the best-drained part of the infested block and accept some loss, while starting the long rotation. Planting the whole 0.6 ha and hoping is the worst option.
Record the decision, the yield from each area, and the level of disease. That information will support future contract negotiations and show whether the clean paddock can be kept clean.