The resistance to abiotic stress is increasingly recognised as being impacted by maternal effects, given that environmental conditions experienced by parent (mother) trees affect stress tolerance in offspring. We hypothesised that abiotic environmental maternal effects may also mediate the resistance of trees to biotic stress. The influence of maternal environment and maternal genotype and the interaction of these two factors on early resistance of Pinus pinaster half-sibs to the Fusarium circinatum pathogen was studied using 10 mother genotypes clonally replicated in two contrasting environments. Necrosis length of infected seedlings was 16% shorter in seedlings grown from favourable maternal environment seeds than in seedlings grown from unfavourable maternal environment seeds. Damage caused by F. circinatum was mediated by maternal environment and maternal genotype, but not by seed mass. Mechanisms unrelated to seed provisioning, perhaps of epigenetic nature, were probably involved in the transgenerational plasticity of P. pinaster, mediating its resistance to biotic stress. Our findings suggest that the transgenerational resistance of pines due to an abiotic stress may interact with the defensive response of pines to a biotic stress.

Summary: Climate change and pathogen outbreaks are the two major causes of decline in Mediterranean holm oak trees (Quercus ilexL. subsp.ballota(Desf.) Samp.). Crown‐level changes in response to these stressful conditions have been widely documented but the responses of the root systems remain unexplored. The effects of environmental stress over roots and its potential role during the declining process need to be evaluated. We aimed to study how key morphological and architectural root parameters and nonstructural carbohydrates of roots are affected along a holm oak health gradient (i.e. within healthy, susceptible and declining trees). Holm oaks with different health statuses had different soil resource‐uptake strategies. While healthy and susceptible trees showed a conservative resource‐uptake strategy independently of soil nutrient availability, declining trees optimized soil resource acquisition by increasing the phenotypic plasticity of their fine root system. This increase in fine root phenotypic plasticity in declining holm oaks represents an energy‐consuming strategy promoted to cope with the stress and at the expense of foliage maintenance. Our study describes a potential feedback loop resulting from strong unprecedented belowground stress that ultimately may lead to poor adaptation and tree death in the Spanish dehesa.

Pitch canker, caused by the fungus Fusarium circinatum, is an introduced non-native disease on pines in natural and planted stands of Europe. Research has not been conducted to test whether a European native pine species shows genetic variation in susceptibility to this disease. Half-sib families from 39 Pinus pinaster clones and seedlings from one unimproved seed source (control) were evaluated for resistance. Pitch canker resistance was not genetically related to tree growth, but seed weight and germination rates were predictive of time-to-death. Heritabilities and associated genetic gains calculated from the greenhouse experiment were consistent, $hi2$ = 0.18 and 0.45 for time-to-death and for tree mortality, respectively. These heritabilities are high enough to allow pitch canker to be reduced through appropriate genetic strategies. Results indicated that selection for growth of P. pinaster trees in breeding programs would not necessarily imply an increase of susceptibility to F. circinatum. This research may allow the use of native pine individuals as breeding stock or as sources to produce seeds with moderate levels of tolerance to F. circinatum.