Understanding interactions between Ramularia collo-cygni and barley leaf physiology to target improvements in host resistance and disease control strategies (PhD)

Ramularia leaf spot (RLS) is an increasingly problematic disease of barley. As the causal fungus, Ramularia collo-cygni, has developed resistance to several major fungicide groups, control options are limited. The fungus can grow systemically from infected seed, with visible symptoms often only appearing after flowering. As the relative contribution of the latent and symptomatic stages of the fungal life cycle to reduced barley yields is not known with any certainty, it was the focus of this research. 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.

In controlled environment (CE) experiments, leaf photosynthetic activity was measured – using infra-red gas analysis (IRGA), chlorophyll fluorescence analysis and imaging – in inoculated seedlings before and after visible symptom development. No reduction in photosynthesis was observed in infected leaves, compared to non-infected leaves, during the latent phase of infection. After visible symptoms appeared, photosynthesis within lesions reduced as they developed. However, this did not reduce photosynthetic activity across the whole leaf area. This result suggests that for whole leaf photosynthetic activity to be affected, visible symptoms must develop into mature lesions and coalesce to cover larger areas of the leaf surface.

In field experiments, plots were either untreated, treated with a full fungicide regime, or inoculated with R. collo-cygni and treated with fungicide to which R. collo-cygni is resistant. RLS was the only disease of significance that developed in untreated or inoculated plots, with symptoms appearing after flowering (around growth stage 72). Fungicide-treated plots remained free of disease. Plants showed no effect of infection on the maximum quantum efficiency of Photosystem II (Fv/Fm) before visible symptoms, consistent with results from CE experiments. Grain yield was predicted from radiation use efficiency (RUE) and utilisation of soluble sugar reserves, and post-flowering healthy (green) leaf area light interception. Grain yields predicted from the difference in post-flowering light interception between treated 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 suggested that symptom expression was associated with leaf senescence. CE 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 developed fewer lesions than control plants. Fungal growth was also restricted in the treated plants. 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.