DISTRIBUTION OF 'CANDIDATUS PHYTOPLASMA MALI' IN INFECTED APPLE TREES IN BELGIUM.

'Candidatus Phytoplasma mali' is a quarantine organism in the European Union which is consequently monitored and controlled in apple tree nurseries and orchards. Although symptoms like witches' broom, large stipules or small fruits can help to visually detect infected trees, PCRs should be performed on corresponding samples to confirm this first visual diagnostic or to detect latent infections. However, because of the uneven distribution of phytoplasmas within the trees, infected trees can still be missed by PCR. In order to improve the official sampling procedure applied in Belgium, PCR detectability of the pathogen was followed in 17 'Ca. Phytoplasma mali' infected trees from an orchard located in the province of Namur during late summer early autumn for two years. On the one hand, 5 trees were sampled in October 2011 at the four cardinal points in the crown and at two cardinal points in the roots to further understand the distribution of phytoplasmas in the tree for a given date. On the other hand, 12 infected trees were sampled randomly in 2013, once in the crown and once in the roots at three different dates to study the influence of these factors on the probability of detection. DNA was extracted from leaf midribs, petioles or roots and amplified by PCR using the universal primer pair fU5/rU3. Despite the limited number of data collected, this study showed that: because PCR detectability of 'Ca. Phytoplasma mali' seems more constant and more likely in the roots, root sampling should be favoured; the sampling date had a significant influence on PCR outcome but, at least in the leaves, this seems to vary a lot from year to year; more than one random sample should be taken from the same tree to increase the detection efficiency.