Identification and Characterization of Neofabraea kienholzii and Phlyctema vagabunda Causing Leaf and Shoot Lesions of Olive in California

Field Evaluation of Fungicides for the Management of Neofabraea Leaf Lesion of Olive in California

Field experiments were conducted during the fall-winter seasons of 2017 to 2018 and 2018 to 2019 to evaluate the efficacy of various fungicides to control Neofabraea leaf lesion of olive. Field trials were conducted in the highly susceptible cultivar Arbosana in a commercial, super-high-density orchard in San Joaquin County, California. Up to eight fungicidal products were applied using an air blast backpack sprayer, and their efficacy was compared with different application strategies. Results showed that most products were effective in reducing infection by the pathogens and limiting disease severity. Overall, best disease control was achieved by thiophanate-methyl, cyprodinil, difenoconazole + cyprodinil, and chlorothalonil, providing up to 75% reduction in disease severity. Copper hydroxide did not control the disease. In 2018 to 2019, the fungicides difenoconazole + cyprodinil and ziram were evaluated in additional field trials using different application strategies (single, dual, and combined applications) suitable for pathogen resistance management. Results showed that both products provided significant reduction in disease severity (∼50%), although no differences in efficacy were found between the two products nor between the different application strategies. Both products performed equally using one or two applications at 2-week intervals following harvest.

Identification and Characterization of Neofusicoccum stellenboschiana in Branch and Twig Dieback-Affected Olive Trees in Italy and Comparative Pathogenicity with N. mediterraneum

For about a decade, olive groves in Apulia (Southern Italy) have been progressively destroyed by Olive Quick Decline Syndrome (OQDS), a disease caused by the bacterium Xylella fastidiosa subsp. pauca (Xfp). Recently, we described an additional wilting syndrome affecting olive trees in that area. The botryosphaeriaceous fungus Neofusicoccum mediterraneum was found associated with the diseased trees, and its high virulence toward olive trees was demonstrated. Given the common features with Branch and Twig Dieback (BTD) of olive tree, occurring in Spain and California, we suggested that the observed syndrome was BTD. During our first survey, we also found a botryosphaeriaceous species other than N. mediterraneum. In the present article, we report the morphological and molecular characterization of this fungal species which we identified as Neofusicoccum stellenboschiana. In the study, we also included for comparison additional N. stellenboschiana isolates obtained from olive trees in Latium and Tuscany region (Central Italy). The occurrence of N. stellenboschiana in olive trees is reported here for the first time in the northern hemisphere. The pathogenicity and virulence were tested in nine inoculation trials, where the Apulian N. stellenboschiana isolate was compared with the isolate from Latium and with the Apulian isolate of N. mediterraneum. Both isolates of N. stellenboschiana proved pathogenic to olive trees. They caused evident bark canker and wood discolouration when inoculated at the base of the stem of two/three-year-old trees and on one-year-old twigs. However, virulence of N. stellenboschiana was significantly lower, though still remarkable, compared with N. mediterraneum in term of necrosis progression in the bark and the wood and capacity of wilting the twigs. Virulence of N. stellenboschiana and N. mediterraneum did not substantially change when inoculations were performed in spring/summer and in autumn, suggesting that these fungal species have the potential to infect and damage olive trees in all seasons. The high thermotolerance of N. stellenboschiana was also revealed with in vitro growth and survival tests. The high virulence of these Botryosphaeriaceae species highlights their contribution in BTD aetiology and the necessity to investigate right away their diffusion and, possibly, the role of additional factors other than Xfp in the general decline of olive groves in Apulia. Hence the importance of assessing the degree of overlap of BTD/Botryosphariaceae with OQDS/Xfp is discussed.

Phylogenetic Diversity and Phenotypic Characterization of Phlyctema vagabunda (syn. Neofabraea alba) and Neofabraea kienholzii Causing Postharvest Bull's Eye Rot of Apple in Northern Italy

Bull's eye rot, caused by Phlyctema vagabunda and Neofabraea species, is one of the most important postharvest diseases of apple. South Tyrol (northern Italy) is the largest continuous apple-producing area in Europe, with approximately 1 million tons being produced yearly and conserved in technologically advanced storage facilities for several months. Still, studies on the pathogen species causing postharvest bull's eye rot of apple, as well as their diversity and biology, are lacking for this region. Therefore, the main purpose of the present work was to identify and characterize fungal isolates obtained from decayed apple fruit with symptoms of bull's eye rot that were collected in 2018 and 2019 in different packinghouses in South Tyrol. Among more than 1,000 fungal isolates that were obtained, 419 could be assigned to the genera Phlyctema and/or Neofabraea based on rot symptoms on apple fruit and colony morphology on potato dextrose agar. A smaller subset of 101 representative isolates was further analyzed by DNA sequencing of the internal transcribed spacer region. Furthermore, partial segments of the β-tubulin gene, the translation elongation factor 1α gene, and the 16S mitochondrial ribosomal RNA gene were studied. The phylogenetic analyses, including sequences of reference species, showed that P. vagabunda is the dominant species associated with bull's eye rot of apple in the study area, whereas Neofabraea kienholzii was found only on a small number of apple fruit samples. The combination of multilocus sequence data revealed 11 unique genotypes that belonged to P. vagabunda and four to N. kienholzii. To the best of our knowledge, this study is the first to report N. kienholzii as a postharvest pathogen of apple in Italy. Finally, a pathogenicity test demonstrated different degrees of virulence among selected isolates of P. vagabunda and N. kienholzii on the cultivar Golden Delicious. The present study emphasizes the importance of accurate species identification, because different species may vary in their biological and pathogenic characteristics, and consequently require distinct disease management strategies, both in the field and during the postharvest stages.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Pleurostoma Decline of Olive Trees Caused by Pleurostoma richardsiae in California

A single fungal pathogen was consistently isolated from symptomatic wood of olive trees (Olea europaea) displaying branch and trunk cankers in superhigh-density orchards in the Sacramento and San Joaquin Valleys of California. Morphological characters of the pathogen included two distinct types of conidia (thick-walled, dark brown, and globose and thin-walled, hyaline, and oblong to ellipsoid) and three types of phialides, indicating a pleurostoma-like fungus. Phylogenetic results of four nuclear loci including the internal transcribed spacer (ITS) region (ITS1-5.8S-ITS2) and partial sequences of the actin, beta-tubulin, and translation elongation factor 1-α genes confirmed the isolates as Pleurostoma richardsiae. Pathogenicity trials conducted in the field involving 2- to 3-year-old branches of three widely planted oil olive cultivars (Arbequina, Arbosana, and Koroneiki) satisfied Koch's postulates and confirmed the pathogenic nature of this species to cause the decline of olive trees in California. All three cultivars were equally susceptible to Pl. richardsiae, indicating no detectable resistance to the pathogen. Additional isolations from symptomatic hosts including almond, peach, pistachio, and plum, also confirmed this species, suggesting that Pl. richardsiae is widespread in agricultural systems and should be considered an emerging pathogen of fruit and nut crops in California.

Endophytic Fungi of Olive Tree

In addition to the general interest connected with investigations on biodiversity in natural contexts, more recently the scientific community has started considering occurrence of endophytic fungi in crops in the awareness of the fundamental role played by these microorganisms on plant growth and protection. Crops such as olive tree, whose management is more and more frequently based on the paradigm of sustainable agriculture, are particularly interested in the perspective of a possible applicative employment, considering that the multi-year crop cycle implies a likely higher impact of these symbiotic interactions. Aspects concerning occurrence and effects of endophytic fungi associated with olive tree (Olea europaea) are revised in the present paper.

Olive Twig and Branch Dieback in California Caused by Cytospora oleicola and the Newly Described Species Cytospora olivarum sp. nov

Field surveys conducted throughout California olive-growing regions in 2008 and 2009 resulted in a collection of 101 Cytospora-like isolates from olive twig and branch dieback symptoms. Cytospora isolates were isolated from multiple cvs. in different olive orchards in Fresno, Madera, Merced, Napa, Riverside, Santa Barbara, Sonoma, Tulare, and Ventura counties. Taxonomic studies of macro- and microscopic structures along with multigene phylogenetic analyses of the internal transcribed spacer region, including the 5.8S rDNA (ITS1-5.8S-ITS2), and fragments of the translation elongation factor 1-α, beta-tubulin, and actin genes identified two species, Cytospora oleicola and C. olivarum sp. nov. Pathogenicity studies conducted in mature olive trees cvs. Manzanillo and Sevillano showed both species to be pathogenic and able to cause vascular necrosis and cankers in olive branches. This study adds to the current knowledge on the etiology of olive twig and branch dieback and provides new important information for the development of effective control strategies against canker diseases affecting olive in California.

Identification of Arthrinium marii as Causal Agent of Olive Tree Dieback in Apulia (Southern Italy)

Olive (Olea europaea L. var. sativa) is one of the most economically important tree crops grown in the Mediterranean basin. Arthrinium Kunze ex Fr. (teleomorph: Apiospora Sacc.) is a widespread fungal genus, and Arthrinium marii Larrondo & Calvo is a ubiquitous species, found in algae, soil, plants, and agricultural communities. A. marii was isolated from olive trees showing dieback from orchards located in Andria and in Fasano, Brindisi (Apulia, southern Italy) and identified based on morphological features and molecular analysis of four genomic regions (ITS, TUB2, TEF1, and LSU). Two-year-old olive plants artificially inoculated with three representative A. marii isolates showed complete dieback within 6 months, and the fungus was reisolated, satisfying Koch's postulates. This is the first report of A. marii causing dieback on olive trees that could represent an important threat for olive cultivation.