Understanding interactions between Ramularia collo-cygni and barley leaf physiology to target improvements in host resistance and disease control strategy

Abstract

Ramularia Leaf Spot (RLS) is an increasingly problematic disease of barley. Control options are limited as the causal fungus, Ramularia collo-cygni, has developed resistance to several of the major fungicide groups. Developing new methods for controlling this disease is therefore a priority. R. collo-cygni can grow systemically in barley plants from infected seed, without inducing visible symptoms. In the field, visible symptoms normally only appear after flowering. The relative contribution of the latent and symptomatic stages of the fungal lifecycle to reduction in barley yield is not currently known with any certainty. Two possibilities are that the effect of asymptomatic infection on pre-flowering photosynthetic activity, and the development of grain sink capacity, plays an important role; or that reduction in photosynthetic activity during grain filling, resulting from lesion development and loss of green leaf area, is the predominant factor. This research aimed to increase our understanding of the impact of different phases of the fungal lifecycle on barley photosynthesis and yield formation, to better target host resistance and disease control strategies. Controlled environment and field experiments were used to determine the relative effects of asymptomatic and symptom-expressing phases of R. collo-cygni infection on photosynthesis and yield formation in spring barley. In controlled environment experiments leaf photosynthetic activity was measured in seedlings inoculated with suspensions of R. collo-cygni mycelia. Measurements were made before and after visible symptom development using Infra-Red Gas Analysis (IRGA), chlorophyll fluorescence analysis and chlorophyll fluorescence imaging. No reduction in photosynthetic activity was observed in leaves infected with R. collo-cygni, compared to those of non- infected leaves, during the latent phase of infection. After the appearance of visible symptoms, photosynthetic activity within lesions reduced as the lesions developed. However, this did not lead to reductions in photosynthetic activity when measured across the whole leaf area, suggesting that for there to be a significant effect of disease on whole leaf photosynthetic activity, visible symptoms must develop into mature lesions and coalesce to cover larger areas of the leaf surface. In field experiments plots were treated with a full fungicide regime, left untreated, or inoculated with R. collo-cygni and treated with fungicide to which R. collo-cygni is resistant (the latter as a precaution against lack of natural RLS disease that year and/or other diseases developing on untreated plots). RLS was the only disease of significance that developed in untreated or inoculated plots. Symptoms first appeared after flowering, around Zadoks Growth Stage 72. Fungicide-treated plots remained free of disease. Chlorophyll fluorescence analysis of field plants showed no effect of infection on the maximum quantum efficiency of Photosystem II (Fv/Fm) before visible symptom development, consistent with results from controlled environment experiments. Grain yield of untreated and fungicide-treated plots was predicted from fixed common values of radiation use efficiency (RUE) and utilisation of soluble sugar reserves, and measured values of post-flowering healthy (green) leaf area light interception. Grain yields predicted from the difference in post-flowering light interception between fungicide-treated plants and untreated or inoculated plants displaying symptoms of RLS were comparable with the measured yield response to fungicide. This suggests that yield loss to RLS is primarily associated with a reduction in light capture during grain filling, resulting from lesion development and loss of green leaf area. Results from controlled environment and field experiments suggested that symptom expression was associated with leaf senescence. Further controlled environment experiments tested this relationship by using treatments to vary the onset and rate of leaf senescence. Seedlings that were treated with cytokinin to delay senescence after inoculation with suspensions of R. collo-cygni mycelia developed fewer lesions than control plants. Fungal growth, as measured by quantification of R. collo-cygni DNA in leaves, was also restricted in plants treated with cytokinin. Collectively these results suggest that prevention of visible symptom development, rather than prevention of asymptomatic growth, is the most important target for management of this disease. Control methods targeted at delaying senescence could be a useful avenue for further investigation.