UC Riverside

In recent decades, the production of nuts has increased due to the higher consumption demand globally. Almonds and walnuts are one of the most profitable crops worldwide. The emergence of fungal diseases (Botryosphaeria and Diaporthe) coupled with extreme weather conditions increase the vulnerability to diseases outbreaks and affect tree longevity. Therefore, it is important to elucidate the etiology of diseases, develop disease management strategies and understand how water stress impact tree longevity in mature and young almond and walnut orchards. The following studies presents i) the etiology of wood diseases in Chilean production; ii) the epidemiology of the causal agents of wood diseases in California almond and walnut orchards; and iii) the effect interaction between water stress and wood pathogen interaction on the physiology, morphology, and water potential on English walnut trees. The findings in Chile reveal the presence of three Botryosphaeriaceae (Diplodia mutila, Diplodia seriata and Neofusicoccum parvum) and two Diaportheaceae species (Diaporthe australafricana and Diaporthe cynaroidis) in walnut orchards between Valparaiso and Maule region with Neof. parvum as the most virulent pathogen. Our second study consisted of comparative profiling of fungal canker pathogens from spore traps and wood samples in almond and walnut orchards in California. Our findings show five genera in the Botryosphaeriaceae family, one genus of the family Diaportheaceae in both almond and walnut orchards along with the insect-vectored Ceratocystis as the major pathogen from almond trees. The data also shows that orchard age, wet events, and precipitation highly influenced disease severity. Our last experiment consisted of determining the effects of water stress and wood pathogens on walnut tree physiology. Our results reveal that water deficit treatments severely affect physiological and morphological traits. Furthermore, as water stress progressed, mid-day water potential increased thus becoming a factor to infection by Botryosphaeria spp. As a consequence of this event, whole tree canopy photosynthesis was greatly reduced. In summary, our studies show the importance of adopting integrated disease management strategies to minimize the level of disease incidence and incorporate the optimum water management to reduce infection of fungal trunk pathogens.