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

- In all plants presenting blister bark symptoms Pseudomonas syringae pv. syringae van Hall was isolated.
- Plants characterized by the poorest physiological status were attacked in the presence of a low inoculum.
- Vigorous plants were infected in the presence of high environmental inoculum.
- Symptoms occurred earlier in autumn on the stressed plants, and after winter frost in the vigorous plants.
- Plant vigour was related to soil type and consequent root behaviour.

Blister bark is a disease of increasing virulence representing a serious threat for apple orchards in several 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 loam soil, sandy loam soil and peat were tested in two different locations: one showing many cases of blister bark symptoms occurrence and the other (up to present) symptoms-free. Leaf gas exchange, chlorophyll a fluorescence and leaf pigment content were measured in 2014 and 2015. Canopy growth, yield and plant biomass were measured and root biomass allocation pattern was described. Plant sanitary status was described and the possible presence of P. syringae was monitored. Silt soil reduced plant development while peat ensured the wider and healthier root system. Silt and sandy soils exposed plants to the risk of stress with a significant reduction of leaf gas exchanges in the presence of inaccuracies in water management. During autumn early blister bark symptoms occurred in sandy and silt soils only in the previously symptoms-free area. During next spring several plants grown in sandy soil and in peat, resulted symptomatic 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 early attacks even in the presence of low environmental inoculum. Conversely a vigorous vegetative growth might further expose plants to the risk of spring attack in locations with high inoculum pressure.