Phoebe E. Gordon

California produces approximately 80% of the world's almonds [Prunus dulcis (Miller) D. A. Webb]. In May of 2024, yellow spots and orange to dark red-brown blotches, were detected in an almond orchard (Nonpareil, Monterey, and Fritz cvs.) in Merced County. Similar symptoms were observed during June of 2024 from 12 additional orchards in Fresno, Madera, Merced, San Joaquin, and Stanislaus counties. Disease incidence in orchards varied from a few infected leaves to nearly 100%. Symptoms were characteristic of red leaf blotch caused by the fungal pathogen Polystigma amygdalinum. Leaves from all orchards were collected and taken to the laboratory in sealed resealable bags to conduct morphological and DNA analyses to identify the pathogen involved. For molecular identification, we used a quantitative real-time PCR detection method, and a primer pair (PamyI2F4/PamyI2R2) previously developed for P. amygdalinum (Zúñiga et al. 2018). Following direct DNA extraction from necrotic leaf tissues, the assay confirmed the detection of P. amygdalinum in symptomatic leaves. In addition, four lesions on separate leaves from different orchards were selected for genomic DNA extraction and sequencing of the internal transcribed spacer region (ITS) using ITS1F/ITS4 primers (Gardes and Bruns 1993) and a segment of the 28S region of the large subunit (LSU) ribosomal RNA gene using LR5 and LR0R primers (Guterres et al. 2018). Resulting sequences (ITS: GenBank Accession PV491257-PV491260; LSU: GenBank Accession PV487836-PV487836) were compared to reference sequences (Habibi et al. 2015; Guterres et al. 2018) using BLAST analysis. The ITS sequences of the Californian isolates were 100% identical to at least one of P. amygdalinum reference strains MH205935 or KC756362; LSU sequences were at least 99.46% similar to that of P. amygdalinum reference strain (KM111540.1). Morphological analyses were done by mounting sections of necrotic leaves on microscopic slides for observation under a light microscope (Leica DM2000 LED Microscope, Wetzlar, Germany). Conidiomata consisted of globose pycnidia, 80-120 μm diam. Conidia were hyaline, aseptate, filiform, 22.1 to 36.9 × 1.6 to 2.5 μm (n = 60). Perithecia formed in fallen leaves during January and were flask-shaped and contained clavate asci with eight ascospores. Asci were 95.4 to 129.5 × 18.3 to 25.1 μm. Ascospores were aseptate, ovoid to cylindrical with one end round and one end slightly acute, sometimes slightly constricted in the middle, hyaline, and measured 15.2 to 20.7 × 6.1 to 8.4 μm. Pathogenicity tests were conducted on potted, 1-year-old almond trees (cv. Monterey and Nonpareil) under greenhouse conditions. An ascospore suspension of 1 × 104 ascospores·ml-1 was obtained by crushing fallen almond leaves that contained mature perithecia of P. amygdalinum. The emerging leaflets from 10 plants of each cultivar were spray-inoculated with the ascospore suspension and then covered with plastic bags for 48hrs. After 40 days, typical symptoms of red leaf blotch appeared on leaves of all inoculated plants for each cultivar and no symptoms occurred in the control plants. Koch's postulates were satisfied after confirming P. amygdalinum from symptomatic leaves through molecular identification as described above. This is the first detection of P. amygdalinum from almond in California, the United States, and North America. These findings improve our knowledge of the etiology and biology of red leaf blotch of almond in California.

Pistachio (Pistacia vera) is a drought- and salinity-tolerant perennial whose fruit features a fleshy exo-mesocarp, or ‘hull’, that protects the kernel. Hull development and degradation are key to kernel quality, yet the anatomy and mechanisms driving hull breakdown during late-stage development remain largely unknown. Here, we show that the hull contains anatomically distinct layers of hypodermal parenchyma and filler parenchyma. Using a combination of transcriptome analyses and immunohistochemistry, we show that changes in pectin-associated gene expression and modification of this polysaccharide are involved in hull cell size increase, loss of cell–cell adhesion, and hull softening. Anatomical analysis shows that filler parenchyma expands during late-stage hull development while hypodermal parenchyma remains constant in size. Field data suggest that irrigation and humidity affect pistachio hull split, implicating a role for water status in cell expansion. In summary, the complex interplay between molecular, cellular, and environmental changes suggests that cell layer-specific modifications of the cell wall are linked to exo-mesocarp splitting, forming a model for understanding the mechanism of fruit split during ripening in non-berry fruit crops.

Ganoderma adspersum, previously unreported in North America, was identified as the primary causal agent of root and butt rot–related failure in almond trees in California’s Central Valley. This disease has been observed over approximately 340 km in California and in orchards spanning 5 to more than 20 years old. The most common sign is reddish-brown basidiomata that develop at the base of the trees in the later stages of the disease. Ganoderma adspersum was isolated from the decayed wood of failed trees with the peach rootstock cultivar Nemaguard. Rootstocks were inoculated with G. adspersum in a greenhouse setting, and decay similar to naturally infected trees developed. The pathogen was reisolated from the inoculated plants, confirming pathogenicity.

Pistachio ( Pistacia vera ) is a drought and salinity-tolerant perennial whose fruit features a fleshy exo-mesocarp, or “hull,” that protects the kernel. Hull development and degradation are key to kernel quality, yet the anatomy and mechanisms driving hull breakdown during late-stage development remain largely unknown. Here, we show that the hull contains anatomically distinct layers of hypodermal parenchyma and filler parenchyma. Using a combination of transcriptome analyses and immunohistochemistry, we show that changes in pectin associated gene expression and modification of this polysaccharide are involved in hull cell size increase, loss of cell-cell adhesion, and softening. Anatomical analysis shows that filler parenchyma expands during late-stage hull development while hypodermal parenchyma remains constant in size. Field data suggest that irrigation and humidity affect pistachio hull split, implicating a role for water status in cell expansion. In summary, the complex interplay between molecular, cellular, and environmental changes suggests that cell layer–specific modifications of the cell wall are linked to exo-mesocarp splitting, forming a model for understanding the mechanism of fruit split during ripening in non-berry fruit crops. Highlight: Cell-layer specific modifications of the cell wall are associated with cell expansion and loss of cell-cell adhesion, leading to hull split during late-stage pistachio fruit development.

In California, the most significant insect pest of almonds and pistachios is the navel orangeworm, Amyelois transitella (Walker) and is a pest of walnuts. This pest attacks the nuts and infestation is associated with aflatoxin contamination. While there are multiple integrated pest management (IPM) practices available, anecdotal evidence suggests that not all tools are equally utilized. To understand current industry practices and barriers to adoption, the authors surveyed tree nut growers and industry professionals across 7 University of California Cooperative Extension meetings in California’s Central Valley. Findings indicate that as managed acres decreased, participants were less likely to report using as many IPM tools, and pest control advisers reported higher use of multiple practices compared to orchard owners and managers. Key barriers to adoption varied by practice and included high economic costs, low labor availability, environmental conditions, and/or timing constraints. Many respondents were skeptical of the efficacy of mating disruption, a relatively new strategy compared to other IPM tools. Finally, a cluster analysis identified thirteen aggregate grower profiles. Groups primarily differed in their use of monitoring practices, although some groups were also less likely to use sanitation or pesticides. Two groups were distinguished as “early adopters” through their use of mating disruption and/or ovibait traps. These findings document variability in adoption of IPM practices for A. transitella in tree nuts. Identification of the primary barriers and constraints for specific grower groups will inform research and extension efforts to further promote adoption of key IPM strategies.

The black fig fly, Silba adipata McAlpine (Diptera: Lonchaeidae), is a Mediterranean insect that exclusively reproduces on figs, Ficus carica L. (Rosales: Moraceae). Widely considered a pest of figs throughout southern Europe, North Africa, and the Middle East, S. adipata was recently discovered infesting figs throughout southern California, representing the first report of this pest in the United States. Here, we provide background information on the distribution, biology, ecology, monitoring, and management of this pest, as well as outline research and extension efforts that have been catalyzed by this recent invasion. California accounts for a majority of both fresh and dried fig production in the United States. To date, S. adipata infestations have been limited to orchards in coastal and southern California, whereas most commercial fig production takes place in the inland Central Valley region. Better knowledge of temperature thresholds is currently needed to estimate potential range expansion to these more inland regions of the state.

Almond canker diseases are destructive and can reduce the yield as well as the lifespan of almond orchards. These diseases may affect the trunk and branches of both young and mature trees and can result in tree death soon after orchard establishment in severe cases. Between 2015 and 2018, 70 almond orchards were visited throughout the Central Valley of California upon requests from farm advisors for canker disease diagnosis. Two major canker diseases were identified, including Botryosphaeriaceae cankers and Ceratocystis canker. In addition, five less prevalent canker diseases were identified, including Cytospora, Eutypa, Diaporthe, Collophorina, and Pallidophorina canker. Seventy-four fungal isolates were selected for multilocus phylogenetic analyses of internal transcribed spacer region ITS1-5.8S-ITS2 and part of the translation elongation factor 1-α, β-tubulin, and glyceraldehyde 3-phosphate dehydrogenase gene sequences; 27 species were identified, including 12 Botryosphaeriaceae species, Ceratocystis destructans, five Cytospora species, Collophorina hispanica, four Diaporthe species, two Diatrype species, Eutypa lata, and Pallidophorina paarla. The most frequently isolated species were Ceratocystis destructans, Neoscytalidium dimidiatum, and Cytospora californica. Pathogenicity experiments on almond cultivar Nonpareil revealed that Neofusicoccum parvum, Neofusicoccum arbuti, and Neofusicoccum mediterraneum were the most virulent. Botryosphaeriaceae cankers were predominantly found in young orchards and symptoms were most prevalent on the trunks of trees. Ceratocystis canker was most commonly found in mature orchards and associated with symptoms found on trunks or large scaffold branches. This study provides a thorough examination of the diversity and pathogenicity of fungal pathogens associated with branch and trunk cankers of almond in California.