The effect of three herbicides on the number of spores of Rhynchosporium secalis on barley stubble and volunteer plants

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Abstract
SUMMARY: Single sprays of paraquat, glyphosate or dinoseb‐in‐oil, were applied to barley stubble or volunteer plants and their effects on the number of spores of Rhynchosporium secalis were studied weekly. Spore concentrations were measured by washing samples of stubble or volunteer plants and counting spores using a haemocytometer.The number of spores recovered from stubbles was relatively small; both paraquat and glyphosate caused some reductions but the results were not consistent. Treatment with paraquat generally caused a transitory increase on volunteer plants; this effect was not found after glyphosate. Both herbicides caused persisting reductions from 4 wk after treatment. Volunteer plants treated with dinoseb‐in‐oil had fewer spores for 6 wk after treatment, but the plants were not killed and when new growth became infected, spore production recovered to equal that on untreated plants.The fungicide captafol caused some reductions in spore number on stubble but results were not consistent, applied after paraquat it had no effect. It was generally ineffective when applied to volunteer plants; applied after paraquat it tended to increase spore number, but the differences were seldom significant.Spores washed from volunteer plants treated with paraquat or glyphosate were inoculated to pot‐grown barley seedlings; neither herbicide had any effect on spore viability. Viable spores were recovered in February from stubble or volunteer plants treated the previous autumn with paraquat or glyphosate respectively.