Comparative Profiling of Wood Canker Pathogens from Spore Traps and Symptomatic Plant Samples Within California Almond and Walnut Orchards

Abstract

Fungi causing wood canker diseases are major factors limiting productivity and longevity of almond and walnut orchards. The goal of this study was to compare pathogen profiles from spore traps with those of plant samples collected from symptomatic almond and walnut trees and assess if profiles could be influenced by orchard type and age, rainfall amount and frequency, and/or neighboring trees. Three almond orchards and one walnut orchard with different characteristics were selected for this study. Fungal inoculum was captured weekly from nine trees per orchard using a passive spore trapping device, during a 30-week period in the rainy season (October-April) and for two consecutive years. Fungal taxa identified from spore traps were compared with a collection of fungal isolates obtained from 61 symptomatic wood samples collected from the orchards. Using a culture-dependent approach coupled with molecular identification, we identified eighteen known pathogenic species from ten fungal genera (Ceratocystis destructans, Collophorina hispanica, Cytospora eucalypti, Diaporthe ampelina, Dia. chamaeropis/rhusicola, Dia. eres, Dia. novem, Diplodia corticola, Di. mutila, Di. seriata, Dothiorella iberica, Do. sarmentorum, Do. viticola, Eutypa lata, Neofusicoccum mediterraneum, Neof. parvum, Neoscytalidium dimidiatum, and Pleurostoma richardsiae), plus two unidentified Cytospora and Diaporthe species. However, only four species were identified with both methods (Di. mutila, Di. seriata, Do. iberica and E. lata), albeit not consistently across orchards. Our results demonstrate a clear disparity between the two diagnostic methods and caution against using passive spore traps to predict disease risks. In particular, the spore trap approach failed to capture; (i) insect-vectored pathogens such as Ceratocystis destructans that were often recovered from almond trunk and scaffold; (ii) Diaporthe chamaeropis/rhusicola commonly isolated from wood samples likely because Diaporthe species have a spatially restricted dispersal mechanism, since spores are exuded in a cirrus; and (iii) pathogenic species with low incidence in wood samples such as Pleurostoma richardsiae and Collophorina hispanica. We propose that orchard inoculum is composed of both endemic taxa that are characterized by frequent and repeated trapping events from the same trees and isolated from plant samples, as well as immigrant taxa characterized by rare trapping events. We hypothesize that host type, orchard age, precipitation, and alternative hosts at the periphery of orchards are factors that could affect pathogen profile. We discuss the limitations and benefits of our methodology and experimental design to develop guidelines and prediction tools for fungal wood canker diseases in California orchards.

Temporal susceptibility of almond pruning wounds to infection by fungal canker pathogens in California

“…This study revealed important differences between the taxa detected using spore traps versus those detected from tissues with symptoms. Although the study of Jimenez Luna et al (2022) confirmed the presence of the fungal canker pathogens described by in diseased almond tissues, only Diplodia species were frequently identified using a passive spore trap methodology; this leaves important epidemiological questions unanswered, such as the origin of infectious propagules infecting almond trees. However, previous spore trapping studies on similar canker pathogens in vineyards suggest that fungal spores are dispersed mainly during rain events in California (Fujiyoshi et al, 2021;Úrbez-Torres et al, 2010).…”

“…This could be explained by the abnormally dry conditions that the region has been experiencing during recent years. A separate recent study, using spore traps and sampling of tissues with symptoms, evaluated the presence of wood canker fungi in almond and walnut orchards from October to April in California (Jimenez Luna et al, 2022). This study revealed important differences between the taxa detected using spore traps versus those detected from tissues with symptoms.…”

Biocontrol of almond canker diseases caused by Botryosphaeriaceae fungi

“…22 Disease symptoms consist of trunk and branch cankers, extensive gummosis, discoloration and necrosis of internal tissues, plant dieback and, occasionally, plant death. 23 The main external sources of inoculum consist of neighboring crops and ornamentals, which are alternative hosts for Botryosphaeriaceae species, 24 as well as the plant propagation materials from nurseries. [25][26][27] Young nursery stock may have asymptomatic latent infections that lead to disease once established in the field.…”

“…The main external sources of inoculum consist of neighboring crops and ornamentals, which are alternative hosts for Botryosphaeriaceae species, 24 as well as the plant propagation materials from nurseries 25–27 . Young nursery stock may have asymptomatic latent infections that lead to disease once established in the field.…”

Plant-Associated Neoscytalidium dimidiatum—Taxonomy, Host Range, Epidemiology, Virulence, and Management Strategies: A Comprehensive Review

“…In the context of dragon fruit canker, a devastating affliction affecting dragon fruit plants worldwide, the initiation of fungal infection ensues with the development of appressoria on the surface, followed by direct penetration into epidermal cells [45]. The transmission mechanisms of N. dimidiatum encompass seeds, propagation materials, soil, and airborne dispersal, with a marked tendency to persist within soil alongside infected debris [39,[45][46][47][48][49].…”