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

Biocontrol of almond canker diseases caused by Botryosphaeriaceae fungi

BACKGROUND

Botryosphaeria dieback is a canker disease caused by fungal species of the Botryosphaeriaceae family that threatens almond productivity. The most common control measure to prevent canker development is the application of fungicides which are being phased out by European Union regulations. In the present study, two sets of bacterial strains were evaluated for their antifungal activity against pathogenic Botryosphaeriaceae species through in vitro and in vivo antagonism assays.

RESULTS

The rhizospheric bacteria Pseudomonas aeruginosa AC17 and Bacillus velezensis ACH16, as well as the endophytic bacteria Bacillus mobilis Sol 1-2, respectively inhibited 87, 95, and 63% of the mycelial growth of Neofusicoccum parvum, Botryosphaeria dothidea, Diplodia seriata, and Macrophomina phaseolina. Additionally, they significantly reduced the length of lesions caused by N. parvum and B. dothidea in artificially inoculated detached almond twigs. All these bacterial strains produce hydrolytic enzymes that are able to degrade the fungal cell wall. P. aeruginosa AC17 also produces toxic volatile compounds, such as hydrogen cyanide. This strain was the most effective in controlling Botryosphaeria dieback in planta under controlled conditions at a level similar to the biocontrol agent Trichoderma atroviride and standard chemical fungicide treatments.

CONCLUSION

Pseudomonas aeruginosa AC17 is the best candidate to be considered as a potential biocontrol agent against Botryosphaeriaceae fungi affecting almond. © 2023 Society of Chemical Industry.

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

Neoscytalidium dimidiatum, a plant- and human-associated fungus, has emerged as a substantial global ecological and agricultural threat aggravated by global warming. It inflicts various diseases, including canker, blight, dieback, leaf spot, root rot, and fruit rot, across a wide spectrum of fruit trees, field crops, shrubs, and arboreal species, with a host range spanning 46 plant families, 84 genera, and 126 species, primarily affecting eudicot angiosperms. Six genera are asymptomatic hosts. Neoscytalidium dimidiatum exhibits worldwide distribution, with the highest prevalence observed in Asia and North America, notably in Iran, Turkey, and California. Rising disease prevalence and severity, aggravated by climate change, particularly impact tropical arid places across 37 countries spanning all 7 continents. This comprehensive review encapsulates recent advancements in the understanding of N. dimidiatum, encompassing alterations in its taxonomic classification, host range, symptoms, geographic distribution, epidemiology, virulence, and strategies for effective management. This study also concentrates on comprehending the taxonomic relationships and intraspecific variations within N. dimidiatum, with a particular emphasis on N. oculus and N. hylocereum, proposing to consider these two species as synonymous with N. dimidiatum. Furthermore, this review identifies prospective research directions aimed at augmenting our fundamental understanding of host-N. dimidiatum interaction.

Evaluation of Biological Control Agents for the Protection of Almond Pruning Wounds Against Infection by Fungal Canker Pathogens

Fungal canker pathogens of almond initiate infection in trees primarily through pruning wounds. Biological control agents (BCAs) have the potential to provide long-term protection of pruning wounds by colonizing the wound surfaces and underlying tissues. Laboratory and field tests were performed to assess the efficacy of various commercial and experimental BCAs as wound protectants against almond canker pathogens. Four Trichoderma-based BCAs were evaluated using detached almond stems in the laboratory against the canker pathogens Cytospora plurivora, Eutypa lata, Neofusicoccum parvum, and Neoscytalidium dimidiatum. Results indicated that Trichoderma atroviride SC1 and T. paratroviride RTFT014 significantly reduced infections by all four pathogens. The abilities of these four BCAs to protect almond pruning wounds against E. lata and N. parvum were further evaluated in field trials using two almond cultivars and during two consecutive years. Both T. atroviride SC1 and T. paratroviride RTFT014 protected almond pruning wounds against E. lata and N. parvum as efficiently as thiophanate-methyl, the recommended fungicide for treatment of almond pruning wounds. Comparisons of different application timings of BCA in relation to pathogen inoculation revealed a significant improvement in wound protection when inoculations were conducted 7 days versus 24 h post-BCA application for N. parvum, but not for E. lata. T. atroviride SC1 and T. paratroviride RTFT014 are promising candidates for the preventive protection of almond pruning wounds and for inclusion in integrated pest management programs and organic almond production systems.

Duplex Real-Time PCR Assays for the Simultaneous Detection and Quantification of Botryosphaeriaceae Species Causing Canker Diseases in Woody Crops

Woody canker diseases caused by fungi of the Botryosphaeriaceae family are producing increasing losses in many economically important woody crops, including almond. To develop a molecular tool for the detection and quantification of the most aggressive and threatening species is of main importance. This will help to prevent the introduction of these pathogens in new orchards and to conveniently apply the appropriate control measures. Three reliable, sensitive and specific duplex qPCR assays using TaqMan probes have been designed for the detection and quantification of (a) Neofusicoccum parvum and the Neofusicoccum genus, (b) N. parvum and the Botryosphaeriaceae family and (c) Botryosphaeria dothidea and the Botryosphaeriaceae family. The multiplex qPCR protocols have been validated on artificially and naturally infected plants. Direct systems to process plant materials, without DNA purification, allowed high-throughput detection of Botryosphaeriaceae targets even in asymptomatic tissues. These results validate the qPCR using the direct sample preparation method as a valuable tool for Botryosphaeria dieback diagnosis allowing a large-scale analysis and the preventive detection of latent infection.