Hot water treatment of vegetable seed – an alternative seed treatment method to control seed borne pathogens in organic farming

In future, conventionally produced seed will not be allowed for organic farming. Therefore, effective non-chemical methods are needed to control seed-borne diseases. Five important vegetable crops (carrot, cabbage, celery, parsley, lamb´s lettuce) and their most important seed-borne pathogens (Alternaria spp., Phoma spp., Septoria spp., Peronospora valerianellae, Xanthomonas spp.) have been investigated in laboratory, model and field trials. Hot water treatments were made at temperatures of 40 °C and 50 to 55 °C for 10 to 30 minutes, in some cases to 60 minutes. In most cases, seed health tests were conducted according to ISTA guidelines. In case of seed infestation with Septoria species and P. valerianellae, the number of spores or oospores were counted in order to assess efficacy. Seed-borne pathogens could be reduced without significant losses of germination by hot water treatments at 50 °C for 20 to 30 minutes up to 53 °C for 10 to 30 minutes. At higher temperature, however, treatment time must be lowered to avoid reduced germination of sensitive crops. In most cases efficacy of hot water treatments against Alternaria species (A. dauci, A. radicina, A. alternata, A. brassicicola) was high (efficacy >95%). Treatment was also very efficient against Phoma species (Ph. lingam, Ph. valerianella) (80-95%). The reduction of Ph. valerianella on the seed of lamb´s lettuce correlated in the first test year with the reduction of the disease in the field. The number of spores in the pycnidia of S. apiicola and S. petroselini was significantly reduced by hot water treatment. This correlates with the reduction in disease incidence and yield increase. The hot water treatment reduced the number of oospores of P. valerianellae in trials on weakly infected seed, but was ineffective on highly infected seed. For Xanthomonas campestris on carrot and cabbage, laboratory trials yielded good effects at 50 °C for 30 minutes.