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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. [ABSTRACT FROM AUTHOR]

In woody plants, pioneer roots are the main roots used to expand the root system horizontally and vertically whereas fibrous ‘feeder’ roots are chiefly used in the absorption of water and nutrients. Because of their different roles, we expected newly emerged pioneer and fibrous roots to respond differently to restrictions in soil moisture. We hypothesized that fibrous roots would exhibit greater growth plasticity and greater physiological impairment from soil moisture deficits, especially under heterogeneous conditions. We compared the responses of fibrous and pioneer roots of olive seedlings (Olea europaea) to localized and uniform soil moisture deficits in transparent containers in the greenhouse. In comparison with uniformly wet conditions, uniformly dry conditions caused reduced shoot photosynthesis and reduced shoot growth, but no significant effect on root morphology, root respiration (measured in aerated buffer solution using excised roots) or electrolyte leakage as a function of root age. Under heterogeneous soil moisture conditions, root growth tended to preferentially occur in the moist sector, especially in the pioneer roots. In comparison with pioneer roots in the moist sector, pioneer roots in the dry sector had higher tissue density and higher suberin content, but no shift in root respiration, non-structural carbohydrates or electrolyte leakage. In contrast, fibrous roots in the dry sector exhibited evidence of impaired physiology in older (>38 days) roots compared with similar age fibrous roots in the moist sector. While we anticipated that, compared with pioneer roots, fibrous roots would be more sensitive to soil moisture deficits as expressed by higher electrolyte leakage, we did not expect the strong growth plasticity of pioneer roots under heterogeneous soil moisture conditions. Differentiating the responses of these two very different root types can improve our understanding of how different portions of the root system of woody plants cope with soil moisture deficits. [ABSTRACT FROM AUTHOR]