A hypothesis on physiological alterations during plant ontogenesis governing susceptibility of winter barley to ramularia leaf spot

The occurrence and epidemic spread of ramularia leaf spot (RLS) caused by Ramularia collo-cygni was studied with relation to the plant age-dependent alterations in the antioxidative defence systems in the leaves during mature stages of field-grown winter barley. The breakdown of enzymatic activities of dehydroascorbate reductase, monodehydroascorbate reductase and glutathione reductase in the two uppermost fully expanded leaf layers correlated well with a decrease in the pool of the non-enzymatic antioxidants ascorbate and glutathione. The general decline in the antioxidative systems occurred between 31 May and 8 June after ear emergence stages and preceded the first visible symptoms of RLS. The activities of peroxidases corresponded to an increase in phenolics and lignification which seemed to enhance the infectivity of the fungus rather than to protect the plant. The first R. collo-cygni conidia were trapped in the field about 4 weeks before the latent infection was first detected by ELISA, and 5 weeks before the first disease symptoms became visible. Both the counts of airborne conidia and the weather data recorded between April and June suggest that neither inoculum nor the weather conditions are limiting factors for infection by R. collo-cygni at earlier growth stages of barley. It is concluded that the relatively late development of RLS in the field in mid June is governed by the significant degradation of the antioxidative protection systems in the leaves of barley at the onset of ripening stages, rather than by environmental factors or the availability of airborne inoculum.