عنوان مقاله [English]
Canker symptoms similar to those produced by Pseudomonas syringae pv. Syringae were observed on peach (Prunus persica) and nectarine (Prunus persicae var. nusipersica) trees in several fruit tree growing areas in Khorasan in 2011-2012.Symptomatic stem tissues were thoroughly washed under running tap water, surface disinfestated with sodium hypochlorite (2 min. in 0.5 %, ai, hypochlorite), washed in three changes of sterile distilled water (SDW) and the bark tissues were stripped-off and chopped in 1-2 ml of SDW with a sterile scalpel. Drops of the suspensions, following 10-20 min standing at room temperature, were streaked onto plates of sucrose nutrient agar (SNA) and the plates were incubated at 26-28 °C. The predominant isolates recovered had cream to pinkish, circular, glistening, pasty and convex colonies with entire margins. The isolates did not produce fluorescent pigments on medium B of King. A number of isolates induced a hypersensitive(HR)-like reaction in geranium (Pelargonium × hortorum) and in tobacco (Nicotiana tabacum) 4-5 and 11 days after injection of somewhat turbid suspensions, respectively. The necrotic areas or spots formed on tobacco were surrounded by halos and tended to enlarge, albeit, slowly. DNA of a representative from each group of isolates, differentiated by their colony morphology on SNA, was extracted and the ITS region of the rDNA operon was amplified in PCR and sequenced. Based on the identity of the nucleotide sequence of this region with those deposited in GenBank, the two colony types that were proved to be pathogenic to peach and induced a Hr-like reaction in tobacco and geranium were found to belong to the yeast species Cryptococcus adeliensis and C. magnus. Koch’s postulates were fulfilled with representative of both species on peach budlings(Borhani et al. 2012. 11 th European Conference on Fungal Genetics, Philipps-University, Marburg, Germanyand Borhani et al. 2012. 13 th International Congress on Yeasts, Univ. Wisconsin, Madison, USA).
A number of isolates displayed characteristics distinct from those of the two Cryptococcus species described. Colonies of the present isolates were glistening, pasty, milky white in color and very low convex. Blast analysis of the ITS sequence of a representative (NCBI # JQ039905) of this group of isolates, which produced a Hr-like reaction in geranium but appeared to be avirulent on peach in greenhouse tests, showed 100% identity with those of Meyerozyma guilliermondi isolates present in GenBank. The fatty acid methyl ester (FAME) compositions of a representative isolate and a reference of strain Meyerozyma guilliermondi,CBS # 8417, were determined by gas chromatography. The similarity of the two isolates was verified further by the identity of the types and the percentages of fatty acids. C18: 1 was determined to be the predominant fatty acid (40% of the total fatty acid compositions) in both isolates. The role of this third group of yeast isolates, if any, in the stem canker complex, could not be established in the preliminary pathogenicity trials carried out in the present study and remains to be determined in the future. Induction of hypersensitive reaction on geranium may be considered as the potential of this yeast species in causing disease in one or more plant species.