BBA, Institute for Integrated Plant Protection, Stahnsdorfer Damm 81, D-14532 Kleinmachnow, Germany

Background and objectives
Seed treatment with agrochemicals is currently the only method to protect plants from pathogens disseminated by seed alone, and from those seedborne pathogens that cause poor emergence and root or stem rot in surviving plants. About 80% of the seeds of important arable crops are treated with fungicides in the EU. Besides the required phytosanitary effect, seed treatment in winter wheat alone causes inputs of 5000 tonnes of chemicals into the ecosystems of the member states. On the other hand, insufficient fungicides are offered for minor uses as a result of increasing costs for the development and authorization of pesticides. In both situations, alternative, non-chemical methods for seed treatment are required. Physical techniques, such as hot-water dressing and microwave treatment, are available, but are rather non-productive and energy-consuming, or have been successful only on a laboratory scale. A well-known preservation method, aimed at killing all harmful flora on and in the treated material, is irradiation of food. This requires full irradiation using gamma radiation as well as high-energy electron beams. In contrast, low-energy electrons penetrate only the outer layers of seed so that resident pathogens are eliminated without damage to the embryo. Penetration of the low-energy electrons is determined by the thickness of the seed coat. In a preliminary evaluation, therefore, the thickness of the seed coat of various agricultural and horticultural crops was measured to calculate the acceleration voltage. This is one of the main parameters of irradiation; the other is the radiation dose. In order to offer data for the latter, lethal doses for seedborne pathogens were determined. Combining both parameters, several irradiation treatments were created and their effects on seed vigour, emergence and yield, as well as on the viability and pathogenicity of seedborne pathogens, were examined.

Materials and methods
Crops and seedborne pathogens included in the investigations were winter wheat infected with Tilletia caries, Septoria nodorum, Fusarium culmorum and Microdochium nivale; bean infected with Pseudomonas phaseolicola; and carrot infected with Alternaria spp. and Xanthomonas campestris. The following methods were used. (i) Lethal doses - agar plate test for examination of colonies developed from irradiated seed or spores. (ii) Thickness of seed coat - freeze-cut parts of seed measured under a microscope. (iii) Efficacy and phytotoxic harmlessness - methods according to ISTA.

Results and conclusions
Although the seeds examined had quite different morphologies, the thickness of the seed coat ranged between only ca 0.04 and 0.08 mm. However, to pass these distances the electrons require an average energy of 60 keV (40-80 keV). The lethal dose for seed-contaminated and separated spores of T. caries is 2 kGy. This irradiation dose reduced spore germination to below 1%. Seeds infected by S. nodorum, F. culmorum and M. nivale were irradiated at doses of up to 10 kGy. While rare mycelial growth of M. nivale was observed at 6 kGy, a dose of 7 kGy inhibited growth to undetectable levels. Alternaria sp. was only irradiated as an in vitro culture. The relatively low dose of 4 kGy, required to eliminate growth, is therefore understandable. Since bacteria, in comparison to fungi, are less developed organisms, this seems to be the reason for P. phaseolicola to be more resistant against radiation. Radiation doses of ca 10-13 kGy eliminated growth of this bacterium completely. Treatments with low-energy electrons characterized by the highest crop-specific lethal dose and acceleration voltage around 60 kV were evaluated with regard to efficacy and phytotoxic harmlessness in tests under controlled conditions. The treatment optimized for beans demonstrated an efficacy against P. phaseolicola of nearly 100%. On carrots, efficacy against Alternaria spp. was only about 50%. However, the emergence of infected seeds was distinctly improved. Electron treatment of wheat was tested in about 150 trials. The field emergence of irradiated seeds infected by S. nodorum and F. culmorum was improved considerably, and the efficacy against T. caries was >99%. At present, mobile plants for electron tratment with a capacity of 15 t are being designed in order to introduce the method in practical farming.