Detection of latent apple proliferation infection in two differently aged apple orchards in South Tyrol (northern Italy)

Apple trees acquiring the agent of apple proliferation, 'Candidatus Phytoplasma mali', remain infected for their entire life span although symptom manifestation can vary considerably or the disease might never break out. Thus, symptom based assessments do not necessarily reflect the true infection rate of a particular orchard. In order to reveal the level of latent AP infections, root samples from a total of 450 trees were collected in a productive orchard and in a young orchard in South Tyrol (Italy) to be tested by real-time PCR. In the orchard in production the true infection rate of 3.8% was 2.6 times higher than the one that was revealed by symptom observation. In the young apple orchard 10.5% of the trees were infected already during the first year, most probably by the insect vector Cacopsylla picta. These trees remained latently infected for at least one and a half years, while two years after detection all positively tested trees of the orchard manifested disease symptoms.