Leaf reddening as suitable symptom of pear decline for remote sensing

Pear decline (PD) is a devastating disease of pears in Europe and North America associated with the presence of ‘Candidatus Phytoplasma pyri’. Reduction of insecticide treatments against the pear psyllid vectors and drought stress due to climate change increase its spread and severity in central Europe. Timely uprooting of infected trees is one of the best measures to limit the spread of the disease. However, monitoring of PD is hampered by the lack of specific symptoms and molecular detection is often less reliable due to irregular distribution of the phytoplasma in the tree. Thus, for large-scale monitoring new strategies are needed. The early leaf reddening in late summer was evaluated as suitable symptom for remote sensing of PD. In 2019 - 2022, more than 6,300 pear trees in 13 older orchards in the Palatinate region of southwest Germany were monitored. Trees with leaf reddening were analysed in August, September and October by PCR for ‘Ca. P. pyri’ presence. More than 80% of reddening trees were shown to be infected with PD. However, latent infection was common as 40% of asymptomatic trees tested positive as well. The correlation of phytoplasma detection and leaf reddening was highest in September. Testing green and red leaves of single infected trees showed that phytoplasmas could only be detected in the red-coloured leaves. However, quantitative PCR analysis indicated that there is no tight relationship between phytoplasma concentration in the leaf and intensity of the reddening. In conclusion, early leaf reddening in pear is a sufficiently reliable indicator of PD to be used in remote sensing approaches.