Etiology of Branch Dieback and Shoot Blight of English Walnut Caused by Botryosphaeriaceae andDiaportheSpecies in Southern Spain

Abstract

English walnut (Juglans regia L.) is considered an economically important fruit crop worldwide. In Spain, little attention has been given to walnut diseases owing to the minor economic importance of the walnut crop in the country until recently. In 2017, typical symptoms of branch dieback and shoot blight of English walnut were observed in southern Spain. From 2017 to 2018, 10 commercial walnut orchards showing disease symptoms were surveyed. Botryosphaeriaceae and Diaporthe fungi were consistently isolated from affected shoots. Cytospora isolates were also recovered with minor relevance. Representative isolates of each fungal group were characterized based on colony and conidial morphology, optimum growth temperature, and comparison of DNA sequence data from the internal transcribed spacer, elongation factor 1-α, and β-tubulin genomic areas. Pathogenicity tests were performed on detached and attached shoots and on detached fruit by inoculating them with mycelial plugs. Botryosphaeriaceae and Diaporthe isolates had higher optimum growth temperatures (≈25 to 27°C) than Cytospora sp. (19.5°C). The following species were identified: Botryosphaeriaceae: Botryosphaeria dothidea, Diplodia seriata, Dothiorella sarmentorum, Dothiorella sp., Neofusicoccum mediterraneum, and N. parvum; Diaporthe: Diaporthe neotheicola, Dia. rhusicola, Diaporthe sp., and Phomopsis amygdali; and Cytospora sp. Botryosphaeriaceae isolates were the most aggressive fungi to walnut in all tissues evaluated, followed by Diaporthe isolates and Cytospora sp. N. parvum was the most virulent among the remaining species tested in any of the tissues evaluated, followed by B. dothidea or N. mediterraneum. This work is the first report to identify the fungal species causing this complex disease of English walnut in Spain and Europe.

Water Stress Enhances the Progression of Branch Dieback and Almond Decline under Field Conditions

“…On the other hand, Dia. rhusicola and species belonging to Cytospora and Neoscytalidium genera, are associated with branch dieback and tree decline in other nut crops, such as English walnut or pistachio [ 6 , 15 , 16 , 17 ], but to our knowledge, these species have not been previously reported in association with branch dieback and almond decline in Spain. However, their pathogenicity to almond should be demonstrated in the future to confirm they are canker pathogens of almond.…”

“…These two species have been previously reported, associated with branch dieback and shoot blight of English walnut in California [ 28 ] and southern Spain [ 16 ], and the first one was also isolated from pistachio in southern Spain [ 17 ] and recently reported associated with twig cankers and shoot blight of almond in Spain [ 26 ]. Usually, Diaporthaceae species occur simultaneously with Botryosphaeriaceae in the same orchards, with Botryosphaeriaceae being always the most frequent [ 16 , 17 , 28 ]. In addition, studies conducted in California by Agustí-Brisach et al [ 29 ] suggest that coinfections between Botryosphaeriaceae and Diaporthaceae species result in antagonistic interactions on infection and disease development on English walnut.…”

“…However, the role of this fungus as a trunk pathogen is still uncertain, given it has been reported as a latent pathogen of global importance for its endophytic behaviour in woody plant health [ 20 ]. In fact, studies conducted recently in southern Spain, which compared the pathogenicity of B. dothidea on inoculated detached and attached shoots of almond, the English walnut and pistachio, demonstrated that, in every case, the fungus is significantly higher aggressive on detached shoots than on attached [ 5 , 16 , 17 ]. It was confirmed that B. dothidea could remain latent in woody plants until trees become weakened as a consequence of different biotic and/or abiotic factors.…”

“…or Phaeoacremonium W. Gams, Crous and M.J. Wingf. Branch dieback and tree decline has been described as a common syndrome worldwide in a broad diversity of woody crops including grapevine [ 10 , 11 , 12 ], olive [ 13 , 14 , 15 ] and tree nuts [ 6 , 15 , 16 , 17 ]. In any cases, the main fungal species associated with tree decline belongs to Botryosphaeriaceae and/or Diaporthaceae Höhn.…”

“…In any cases, the main fungal species associated with tree decline belongs to Botryosphaeriaceae and/or Diaporthaceae Höhn. ex Wehm., with the first ones being the most aggressive [ 6 , 9 , 15 , 16 , 17 , 18 ]. However, the role of this wide diversity of fungi that has been causing tree decline is still uncertain.…”