The effect of growth substrate on apple plant status and on the occurrence of blister bark symptoms

tBlister bark is a disease of increasing virulence representing a serious threat for apple orchards in sev-eral alpine valleys. Pseudomonas syringae pv. syringae van Hall was indicated as a potential causal agent,among others. Plant growth conditions might affect disease bearing. Three growth substrates: silt loamsoil, sandy loam soil and peat were tested in two different locations: one showing many cases of blisterbark symptoms occurrence and the other (up to present) symptoms-free. Leaf gas exchange, chlorophylla fluorescence and leaf pigment content were measured in 2014 and 2015. Canopy growth, yield andplant biomass were measured and root biomass allocation pattern was described. Plant sanitary statuswas described and the possible presence of P. syringae was monitored. Silt soil reduced plant develop-ment while peat ensured the wider and healthier root system. Silt and sandy soils exposed plants to therisk of stress with a significant reduction of leaf gas exchanges in the presence of inaccuracies in watermanagement. During autumn early blister bark symptoms occurred in sandy and silt soils only in the pre-viously symptoms-free area. During next spring several plants grown in sandy soil and in peat, resultedsymptomatic in the location characterized by previous attacks. In all symptomatic plants P. syringae pv.syringae was isolated. This study indicates that a poor plant physiological status exposed plants to earlyattacks even in the presence of low environmental inoculum. Conversely a vigorous vegetative growthmight further expose plants to the risk of spring attack in locations with high inoculum pressure.