Optical detection of downy mildew in grapevine leaves: daily kinetics of autofluorescence upon infection

Abstract

A 15-day survey of autofluorescence has been conducted upon infection by downy mildew [Plasmopara viticola (Berk. & M.A. Curtis) Berl. & de Toni] of leaves of a susceptible grapevine genotype. Different autofluorescence signals were followed from the cellular to the whole-leaf level by using four types of devices for fluorosensing: a macroscope, a spectrofluorimeter, a portable field optical sensor (the Multiplex 3), and a field fluorescence sensor prototype with 335nm excitation. It was shown for the first time, by the three different techniques and at three different scales, that the stilbene-dependent violet–blue autofluorescence (VBF) had a transitory behaviour, increasing to a maximum 6 days post-inoculation (DPI) and then decreasing to a constant lower level, nevertheless significantly higher than in the control leaf. This behaviour could be sensed from both sides of the leaf. On the abaxial side, VBF could discriminate the presence of infection from 1 DPI, and on the adaxial side from 3 DPI. There was a constant increase in blue-excited green fluorescence starting from 8 DPI, concomitant with a decrease in leaf chlorophyll content sensed by one reflectance and two fluorescence indices available on the Multiplex 3 sensor. These results show that a pre-symptomatic and symptomatic sensing of downy mildew is possible by autofluorescence-based sensors, and this is potentially applicable in the field.

Proximal Sensing of Plant-Pathogen Interactions in Spring Barley with Three Fluorescence Techniques

“…These changes were more pronounced in the susceptible varieties Belana and Tocada, as compared to the resistant varieties Conchita and Marthe. Similar results are reported in the literature [7,32]. Modifications of the SFR_G were caused by a first disruption of the photosynthetic quantum conversion and consequently in a later decrease of the chlorophyll content [33].…”

First detection of the presence of naturally occurring grapevine downy mildew in the field by a fluorescence-based method

“…The downy mildew infectious agent is an oomycete named Plasmopara viticola (Berk & M.A. 3,4 In vivo, they are slightly shifted to longer wavelengths, with the excitation maximum at 330 nm 3,5 and the emission maximum at around 400 nm. 2 After one to two weeks of being present in the leaf it produces visible symptoms known as oil spots.…”

“…Curtis) Berl & de Toni. 5 This autofluorescent property of the stilbenoid phytoalexins, which is absent from healthy leaves, 5 was exploited to detect the presence of downy mildew in greenhouse-grown plants, [4][5][6] in outdoor-grown plants 5,6 and in the field. One of the reactions of plants to both downy and powdery mildew is the synthesis of a variety of stilbenoid compounds.…”

“…One of the reactions of plants to both downy and powdery mildew is the synthesis of a variety of stilbenoid compounds. 3,4 The development of a portable fluorescence sensor, 5 Multiplex 330 (FORCE-A, Orsay, France), hereafter Mx-330, allowed the application of this diagnostic method to leaves attached to the plant. In vitro, the excitation maximum is at 320 nm 3,4 and the fluorescence emission maximum is at 380 nm.…”

“…A useful characteristic of grapevine phytoalexins is that they produce a UV-induced violet-blue fluorescence (VBF). 5 In the field, infected leaves could be discriminated from the control ones starting from DPI 6 on both sides. 3,4 In vivo, they are slightly shifted to longer wavelengths, with the excitation maximum at 330 nm 3,5 and the emission maximum at around 400 nm.…”