From July to September 2018, symptoms recalling those of bacterial infections were observed on different Prunus dulcis cultivars in several orchards of the Bari and Foggia provinces (southern Italy). Sunken and corked lesions oozing abundant gum occurred on the mesocarp of about 70 to 80% of the fruits, and dark circular spots were observed on the endocarp. Small, angular water-soaked spots surrounded by chlorotic tissue were initially observed along leaf midribs and on margins, turning light brown. The necrotic area was shed, giving rise to “shot-hole.” Sometimes the spots coalesced, resulting in large areas of necrotic foliar tissue. Isolations from diseased leaves and fruits were carried out on yeast extract–dextrose–calcium carbonate agar plates on which, after 4 days at 27°C, convex, smooth, mucoid, yellow, and glistening Xanthomonas-like colonies were observed. Three representative isolates were characterized as gram negative, oxidase negative, and catalase positive and able to hydrolyze esculin, gelatin, and Tween 80 but not starch (Schaad et al. 2001). Furthermore, these isolates utilized citrate, glycerol, and propionate, were hypersensitive in tobacco, digested casein, produced hydrogen sulfide from cysteine, and exhibited alkalinity from litmus milk but not indol, in accordance with the reactions of the reference strain NCPPB 416T of Xanthomonas arboricola pv. pruni (Xap). The identity of the isolates was confirmed by polymerase chain reaction (PCR) assays, sequencing, and pathogenicity tests. Genomic DNA, extracted from 24-h-old Luria-Bertani broth cultures of the representative bacterial isolates Xap-53, Xap-54, and Xap-55 and the reference Xap strain NCPPB 416T was successfully amplified in duplex-PCR with Xap-specific primers (XarbQ-F/XarbQ-R and XapY17-F/XapY17-R) (Pothier et al. 2011). Furthermore, the partial 16S rDNA genomic region was amplified using the primers fD1/rD1 (Weisburg et al. 1991) and custom sequenced (Genewiz, Takeley, U.K.). The sequences had 100% identity (e-value = 0; coverage 100%) with those of the same genomic region of other Xap strains, including the reference strain NCPPB 416 (GenBank accession nos. MK156160, DiSSPA_Xap_53; MK156161, DiSSPA_Xap_54; MK156162, DiSSPA _Xap_55; and MK156163, NCPPB 416). Pathogenicity was confirmed by infiltrating 107 CFU/ml of a bacterial suspension on detached leaves and leaves of 1-year-old potted plants of the almond cultivar Genco (Randhawa and Civerolo 1985). Control leaves were infiltrated with sterile distilled water. All leaves inoculated with the bacterial isolates and the reference strain developed confluent water-soaking spots 7 days postinoculation and incubation at 25°C and 16:8-h photoperiod. No symptoms were observed on controls. Bacteria reisolated from symptomatic leaf tissues exhibited the same morphological, biochemical, and molecular characteristics of the inoculated strains. In Europe, Xap was previously reported on almond in Montenegro (Panić et al. 1998) and Spain (Palacio-Bielsa et al. 2010). Although in Italy Xap has been considered endemic since the 1970s (Battilani et al. 1999), to our knowledge, this is the first report on almond. Xap is one of the most important diseases of stone fruits included in the EPPO A2 list of pests recommended for regulation for European Union member countries. Almond growing is expanding in Italy, and new technologies in crop management are being adopted. In this new scenario, together with climate changes, the pathogen could have a significant impact on almond production.
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|Titolo:||First Report of Bacterial Spot caused by Xanthomonas arboricola pv. pruni on Almond in Italy|
|Data di pubblicazione:||2019|
|Appare nelle tipologie:||1.1 Articolo in rivista|