ФPSII and NPQ to evaluate Bremia lactucae-infection in susceptible and resistant lettuce cultivars

• ФPSII and NPQ was analysed to facilitate detection of Bremia infection.
• Detection of infection of susceptible lettuce cultivars was clearly possible.
• Distinction between susceptible and resistant cultivars was possible in early stage.

Downy mildew, caused by Bremia lactucae, is an important disease in lettuce production. The fungal pathogen is characterised by a pronounced damage potential and a high ability to rapidly overcome specific resistances. This problem may partially be overcome by continuous breeding of new resistant lettuce cultivars. For successful and rapid breeding, early identification of susceptible cultivars is very important as is the effective selection of infected seedlings. The presented study aimed to develop and optimize a chlorophyll fluorescence imaging-based technique for facilitated and rapid early detection of Bremia lactucae infections in lettuce. During the initial phase of Bremia-infection, only a slight sporulation could be visually rated on leaf surface. In this stage, effects of infection on metabolic pathways such as photosynthesis were difficult to evaluate. Chlorophyll fluorescence imaging can overcome this problem if appropriate fluorescence parameters and specific algorithms are used. Indeed, analyses of changes and variations in ФPSII and NPQ facilitated detection of Bremia infection in artificially inoculated plants of susceptible butterhead and batavia lettuce cultivars already between days eight to twelve after inoculation. Further, differentiation between susceptible and resistant lettuce cultivars was possible with this method before distinct visual infection symptoms appeared.