RESUMO
Taxonomically diverse Pseudomonas species induce bacterial blotch of edible mushrooms around the world. Pseudomonas tolaasii, [Pseudomonas gingeri], and Pseudomonas agarici are dominant mycopathogenic pseudomonads in mushroom production farms. In this study, among 216 mycopathogenic bacterial strains isolated from edible mushrooms in Iran, 96 strains were identified as Pseudomonas spp., while only three strains were preliminarily identified as P. agarici. Multilocus sequence analysis showed that only one strain (FH2) authentically belonged to P. agarici, while the other two strains either belonged to [P. gingeri] or represented a unique phylogenetic clade. The three strains also differed from each other in phenotypic characteristics e.g., production of fluorescent pigment and the reaction to tolaasin produced by P. tolaasii. Pathogenicity assays under controlled environment showed that the symptoms induced by authentic P. agarici were far less severe than those caused by the predominant species P. tolaasii. Furthermore, co-inoculation of P. agarici with three bacterial pathogens that are prevalent in Iran on mushroom caps i.e., P. tolaasii, Ewingella americana and Mycetocola sp. resulted in the development of combined symptoms representing characteristics of both pathogens. Antibiosis assay showed that tolaasin-producing strains of P. tolaasii could inhibit the growth of P. agarici, while tolaasin-negative strains of the same species were unable to do so. This led us to the hypothesis that the inhibitory effect of P. tolaasii on P. agarici is driven by tolaasin production in the former species. This inhibitory effect also associated with the rarity of P. agarici in natural conditions.
RESUMO
Bacterial blight of arugula (Eruca vesicaria subsp. sativa cv. Standard) was observed in a commercial crop being grown in a high tunnel under overhead irrigation in Argyle, NY in January 2021. Approximately 80-100% of the plants were affected. Symptoms started as small, angular, water-soaked lesions visible on both sides of the leaves, then expanded, coalesced and later dried and turned tan. Fluorescent pseudomonads from five different plants were isolated on King's medium B agar amended with boric acid, chloramphenicol, and cycloheximide (Schaad et al. 2001) from surface disinfested symptomatic leaf tissues macerated in phosphate buffer (10mM, pH 7.0). Five representative isolates from each of the five plants produced levan and were negative for arginine dihydrolase and oxidase. They did not rot potatoes but were able to induce a hypersensitive reaction on tobacco (Nicotiana tabacum L. cv Glurk) within 24 h, thus demonstrating that the isolates belonged to LOPAT group 1 (Lelliott et al. 1966). DNA fragment banding patterns of these isolates generated by repetitive extragenic palindromic sequence PCR (repPCR) with BOXA1R primers were compared to the pathotype strains of Pseudomonas cannabina pv. alisalensis and Pseudomonas syringae pv. maculicola, known fluorescent pathogens of the Brassicaceae. The repPCR banding pattern of all isolates matched the pattern of Pseudomonas cannabina pv. alisalensis. Bacterial inoculum for pathogenicity experiments was prepared from 48 h KBBC agar cultures suspended in phosphate buffer (10mM, pH 7.0) and adjusted to 0.6 optical density at 600nm yielding approximately 108 CFU/ml. Five-week-old arugula plants were sprayed until run-off with one of the three isolates from arugula, a negative control (sterile buffer), or a positive control (P. cannabina pv. alisalensis) that is pathogenic to crucifers and also reported to cause disease on arugula in California (Bull et al. 2004). Experiments consisted of three replications of each treatment and two independent experiments were conducted. Small water-soaked spots resembling the original symptoms developed on all plants inoculated with the three representative isolates and P. cannabina pv. alisalensis. Moreover, reisolates from the symptomatic tissues were fluorescent on KBBC and had identical repPCR banding pattern as inoculated strains, demonstrating Koch's postulates. To our knowledge, this is the first report of bacterial blight on arugula caused by Pseudomonas cannabina pv. alisalensis in the Northeastern US. It was previously reported in California, Nevada, and Minnesota (Bull and du Toit 2009; Bull et al. 2004). This report may have significance for all brassica leafy green growers in the Northeast as P. cannabina pv. alisalensis has a broad host range including members of the Brassicaceae and oats which are commonly used as cover crops in mixed vegetable production.
