Copper is still the most effective permitted means of protection for plants against leaf blight in organic potato production. However, this heavy metal has the great disadvantage of accumulating in the soil and damaging soil organisms in the case of higher input. The annual maximum quantity of pure copper as specified by EU organic regulations is set at 6 kg per ha. For members of national organic associations lower maximum quantities may apply.
Solution
In order to minimise the negative effects of copper on the environment, and to avoid exceeding the current maximum quantities per hectare and year, the dosage of copper and the intensity of treatment can be adapted to specific levels of infection, as well as weather conditions.
Description
• Lower accumulation of copper in the soil.
• Less damage to microorganisms in the soil.
• Potential saving in costs of spraying agent.
• Improved distribution of spraying agent over the required period of treatment until exhaustion of the permitted maximum quantity.
Practical recommendation
Adapting dosage to state of infestation: As long as there is no infestation in a radius of 50 km, refrain from treating. Observe national information and alert services. As soon as the first case of infestation in the region is reported, protect potatoes with 200 to 250 g of pure copper per hectare. If potatoes in your own or neighbouring fields are afflicted by leaf blight, increase the dosage to 800 g and do not wait longer than a week in between treatments.
Adapting dosage to time of infestation: If yield production is already advanced at time of infection (often after mid/end of June), reduce the copper dosage or forgo its use. Carry out a sample digging to evaluate yield production.
Adapting treatment to the weather: Apply the copper shortly before periods of bad weather to ensure protection during an occurrence of rain for as long as possible. Repeat the treatment once the plants are dry again.
Tips
• Due to the purely protective effect of cupper, a regularly applied film on the upper and lower surface of the leaves is crucial for an effective protection of the potato plants. New application techniques (e.g. upper- and
lower-leaf sprayer) help in achieving a regularly applied copper film.
• New copper compounds give a better distribution on the leaf surface, as well as an increased rain-resistance, which enables lower dosages.
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