Main conclusion The chlorophyll fluorescence parameter Φ NO is an excellent metric for the non-destructive monitoring of disease progression, measured over a broad range of light intensities.
The suitability of the slow induction chlorophyll fluorescence parameters Φ PSII, Φ NPQ, and Φ NO to monitor in vivo disease progression in a host-root pathogen pathosystem was evaluated and compared to the established method of monitoring disease by measuring F v /F m . Using the infection of ginseng plants (Panax quinquefolius L.) with Pythium irregulare Buisman as a model, light response curves were used to establish the optimal irradiance for the resolution of differences between fluorescence parameters Φ PSII, Φ NPQ and Φ NO. As infection progressed only changes in Φ NO remained consistent with increased irradiance, and increased as infection progressed. Furthermore, Φ NO showed a high sensitivity for distinguishing increased disease load. In contrast, the magnitude in change of Φ PSII and Φ NPQ were sensitive to irradiance levels. The magnitude of increase in Φ NO per unit disease score was equivalent to the corresponding decline in F v /F m values. Thus Φ NO is as sensitive as F v /F m in monitoring biotic stress. The ability to measure Φ NO under a wide range of light intensities, including natural light, potentially without the need for dark adaptation, means that it can be used in the development of a general protocol for non-invasive, in vivo monitoring of plant health, from the laboratory to the field scale.