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 super-high-density orchards in the Sacramento and San Joaquin Valleys of California. Morphological characters of the pathogen included two distinct types of conidia: 1) thick-walled, dark-brown, and globose and 2) thin-walled, hyaline, 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 on 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, which is for the first time reported to cause 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 wide spread in agricultural systems and should be considered an emerging pathogen of fruit and nut crops in California.