Etiology, Epidemiology, and Management of Fusarium spp. Causing Cryptic Cankers in Cold-Stored, Bare-Root Propagated Almond Trees in California Nurseries

From 1997 to 2001, hundreds of thousands of bare-root stone fruit and nut trees were killed or destroyed in California due to a canker disease that appeared after cold storage at nurseries or soon after planting in orchards. The cankers would cause necrosis of the inner bark, cambium, and sapwood, which in severe cases girdled the trees. At that time, Fusarium acuminatum and Fusarium avenaceum were identified as the primary causal agents of the disease with Fusarium solani and a Cylindrocarpon species implicated as lesser causal agents. Mild desiccation stress leading to reduced bark turgidity corresponded with significantly increased susceptibility of almond branches to F. acuminatum..;In the winter of 2010-11, the disease reemerged in at least one nursery, resulting in substantial losses of cold-stored, bare-root almond trees. The incidence of pathogenic Fusarium spp. in the almond propagation system, multi-locus DNA sequencing, and vegetative compatibility tests were utilized to better understand the disease etiology and to evaluate and validate potential sources of inoculum. Pathogenic Fusarium spp. were isolated from throughout the production system including symptomatic and non-symptomatic almond trees, wheat rotation cover crops and residues in nursery fields, cold storage facility air and surfaces, and nursery equipment. Additionally, vegetative compatibility tests combined with multi-locus sequencing data confirmed that isolates of the same species from the various sources were highly genetically similar both to each other and to known pathogenic isolates, suggesting that many steps in the production system harbor potential inoculum. Host susceptibility and predisposing stresses were also investigated in this project. The effects of bark turgidity, desiccation stress, and season on host susceptibility were confirmed. There is a positive linear correlation between increasing days of desiccation and increasing lesion size caused by the pathogenic Fusarium species. The optimal bark turgidity for disease development is approximately 83%, and trees are most susceptible to this disease in the springtime. Fungicide assays were performed both in vitro and in planta to evaluate chemical treatments that could be used in conjunction with other management strategies. Understanding pathogen diversity, predisposing factors, and contributing cultural practices will help guide the development of an effective integrated approach to management of this disease.