Suitability of various copper-free leaf treatment products for the control of Phytophthora infestans in organic potato farming

Summary

Potato late blight (Phytophthora infestans) causes severe losses in potato yield and quality in organic production. Still, the disease can only be effectively controlled by the application of copper fungicides. Due to copper accumulation in the soil and expected detrimental effects on the environment and non-target organisms, a reduction in the usage of Cu fungicides is urgently required. Within the course of a project aiming at the reduction and avoidance of Cu in organic farming, trials were performed to investigate the efficacy of different Cu-free products for the control of P. infestans leaf infections, and to identify potential alternatives replacing or amending copper based fungicides in the future. We have tested several commercial and non-commercial organic products in in vitro leaf assays, potted plant assays and field trials. In total, 23 Cu-free alternative products were tested so far. In the detached leaf assay, leaves were inoculated after spray application of the product. A water agar plug kept the droplet of a sporangial suspension of P. infestans in place, ensuring equal infection pressure in all variants. Leaves were incubated at 15°C for 5 days and disease rated as the number of leaves and % leaf area affected. Potted plants grown from tubers or eye plugs were infected by spraying with a sporangial suspension after application of the product. Plants were incubated at 20°C for 7 days, with plastic bags over the plants for the first 48 hrs. Disease was rated as the number of leaves and % leaf area affected. Only few of the substances tested in our laboratory assays proved to be effective against Phytophthora leaf infections. Among the most promising in the leaf test were chitosan, a knotweed product, a fertilizer with low Cu content, a liquorice, a horsetail and a citrus test product. Some of these products were almost as effective as the Cu control with regard to infection incidence and severity. All of these products were also effective in the growth chamber test with potted potato plants, where they could significantly reduce infection rates of the plants. In addition to the lab assays described above, field tests were performed with a selection of Cu-free leaf treatment products in 2012 (Bacillus subtilis, chitosan and citrus) and 2014 (citrus, horsetail, chitosan, liquorice, Cu fertilizer and knotweed). The field trials showed that even products that provided good protection in the lab were not necessarily effective under field conditions, but also highlighted the potential of some of the products as substitutes or amendments to copper fungicides. Chitosan, the liquorice and the horsetail test products together with the Cu fertilizer in particular might become part of a management strategy to reduce the extent of leaf infections and thus to minimise the deposition of sporangial inoculum on the soil surface and the potato crop in organic farming.