The effects of silver on microbial contamination of agar medium and on interactions between mildew and barley leaf segments with and without the mlo gene

Segments of primary leaves of several barley varieties with and without the mlo gene were placed in Petri dishes on an agar medium containing benzimidazole, mineral nutrients and 0, 0.1, 0.3, 1 and 3 ppm of AgNO3. Three Petri dishes were prepared of each concentration. The segments were uniformly inoculated with 103 conidia/cm2 of the partially mlo-virulent powdery mildew culture PV-32. Subsequently, one open Petri dish of each Ag-concentration was exposed for 1 hour to a different potential con-tamination environment: one in the laboratory (low load), one in a humid cellar close to stored vegetables (medium load) and one on the top of a compost heap of decaying garbage (heavy load). Germination of mildew spores on the medium surface declined slightly with increasing concentration of AgNO3. Mildew infection was evaluated 7 days after inoculation. The number of mildew colonies per leaf segment and the differential interaction of the Mlo- and mlo-varieties with the mildew culture was apparently not affected by the AgNO3 concentration. Contamination of the medium by airborne micro-organisms was evaluated 12 days after exposure both microscopically and by eye. The contamination of the medium increased with environmental load and with decreasing AgNO3 concentration. 0.1 ppm AgNO3 markedly retarded the growth of contaminant colonies from all three environments, but did not prevent contamination. At 1 ppm AgNO3, no contamination was observed on the media exposed to low and medium load, but several dozen small contaminant colonies developed on the medium exposed to heavy load. At 3 ppm AgNO3, only three small contaminant colonies developed on the medium exposed to heavy load, while the media exposed to medium and low load remained clean. It can be concluded that adding 1 ppm AgNO3 to a mineral-agar medium efficiently suppresses its contamination under low and medium load,without apparently affecting the growth of mildew or the interaction between mildew and mlo-barley on leaf segments placed on the medium.