In organic production, attack by carrot leaf blights (Fig. 1), caused by fungal (Alternaria dauci,Cercospora carotae)or bacterial pathogens (Xanthomonas campestris pv. carotae), can result in yield loss. The choice of resistant varieties is crucial; however, commercial varieties differ greatly in their susceptibilities. Additionally, official variety testing does not test for the suitability for organic growing conditions, and often takes place under ideal conditions on a limited number of locations.
Solution
Organic farmers can set up a simple, on-farm cultivar trial to assess the varieties’ susceptibility to leaf blight. For this, different varieties are cultivated in 2-3 strips, distributed in the trial field, to minimize the influence of heterogeneous soil conditions. A variety with a well-known
tolerance level serves as a reference. Farmers can use their own machinery for cultivation. Throughout the growing season, scoring of disease symptoms (Fig. 2) should be conducted. Symptoms include:
• Spots at the margin of leaves
• Lesions on petioles and stems
• Curling of leaves
• Defoliation
Description
Practical recommendations
Other strategies include:
• Use of vigorous and decontaminated seeds
• Avoid favourable micro climate through wider row spacing and planting on raised ridges
• Crop rotation,with at least 2-3 carrot-free years in crop rotation
• Use of organic fertilisation and irrigation if necessary, to reduce plant stress and promote foliage development
• Harvesting on time, to reduce crop loss
• Ploughing of crop debris to avoid survival of inoculum
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