Treatment with Streptomyces sp. AcM20 increases the resistance of Arabidopsis thaliana against brassica black spot. Arabidopsis thaliana seedlings were preinoculated on roots with streptomycetes or water at d-7 and postinoculated on leaves with Alternaria brassicicola at d0. Treatment with Streptomyces sp. GB 4-2 was included as a positive control, since treatment with GB 4-2 is known to increase the plants’ Fv/Fm value and its disease resistance. In the control treatment no bacteria were inoculated on the roots. (a). Plant stress level was estimated according to chlorophyll fluorescence (maximal photon yield of photosystem II), Fv/Fm. At d14, the values with GB 4-2, AcM20 and AcM11 were significantly different from the control treatment (one way analysis of variance, p < 0.05). (b). Alternaria black spot disease development was determined. At d5, d7, d11 and d14, the values with GB 4-2, at d5, d11 and d14, the values with AcM20 and at d5 and d14, the value of AcM11 were significantly different from the control according to one-way analysis of variance (p < 0.05). Streptomycete strain names are arranged in the top down order of decreasing disease index. Note that a low disease index indicates low amount of fungal infection.