IRANIAN JOURNAL OF PLANT PATHOLOGY
Year:2009 | Volume:45 | Issue:1 (177)|Papers Count:11

A specimen of Viola sp. with smut symptoms was collected and studied from Arasbaran protected region. Preliminary study revealed that the fungus is a member of the genus Urocystis Rabenh. ex Fuckel. The specimen characterized by sori usually in the petioles as fusiform loculate swellings up to several cm long which distort the attacked parts. Sori infrequently on leaf veins as blister like swellings. Sourus covering of host tissue persistent, eventually rupturing irregularly to expose the spore mass. Spore mass powdery, blackish-brown, composed of spore balls. Spore balls globose, ovoid to elongated, irregular 21-73x21-45 mm, reddish-brown, each composed of 2-15 (-18) spores, completely covered by sterile cells. Spores globose, ovoid, elongate or irregular, 11-18x9-13 mm, yellowish- to reddish-brown, smooth.

In recent years, bean common blight as a destructive disease has caused serious yield losses in Iran. Infected seed is known as the major source of primary inoculum and serves as a pathogen to disseminate elsewhere. Therefore, using pathogen-free seed is an important factor in disease management. In this study, capability of several detection methods including Indirect ELISA, Direct PCR, Bio-PCR and Ic-PCR were compared for monitoring Xanthomonas axonopodis pv. phaseoli in bean seeds collected from Markazi province fields. The extractions of seed samples soaked in washing buffer and distilled water separately were used for Indirect ELISA and PCR amplification with species specific pairs of primer X4c\X4e. In Ic-PCR, seed extracts were loaded in wells which were coated with immunoglobulin and the sample DNA after boiling was subjected to PCR. For Bio-PCR assay, aliquots of the extracts were plated onto semi-selective modified nutrient broth yeast extract agar. The growing colonies were suspended in sterile distilled water after boiling was applied for PCR amplification. The results indicated that sensitivity of Indirect ELISA was low and at least 105 cfu/ml was needed as a detection threshold. The results for direct PCR were not reproducible and Ic-PCR was found an expensive method. The Bio-PCR technique was considered as a reliable and specific method which was able to detect as little as 1 cfu/ml in seed extracts plated on semi-selective media. Comparing the results indicated that the Bio-PCR assay is suitable for sensitive and routine testing of seed samples of beans for the presence of X. axonopodis pv. phaseoli.

Fusarium oxysporum causing potato Fusarium wilt has become widespread in most potato fields of Iran and causes consideraabl yield loss. Using resistant cultivars is still the most effective method of management of this disease. However, for screening cultivars for resistance, knowledge of genetic diversity of a fungal population is necessary. In this study, genetic diversity of the pathogen in Tehran, Hamedan and Ardebil provinces was studied. Seventy isolates of F. oxysporum were isolated from infected plants. Pathogenic variability of the isolates in terms of degree of wilting and root-rotting was tested and their genetic diversity was studied using seven RAPD primers. Pathogenicity tests were based on completely randomized design (CRD) with four replicates. The result of pathogenicity tests showed that isolates caused potato wilt had lower root-rot potential and vice versa. Cluster analysis of RAPD data using UPGMA method and Dice’s coefficient distinguished 8 main groups at 71% similarity level. Also, 18 molecular phenotypes were detected among isolates. The low number of distinguished groups observed among the populations could suggest that these isolates have probably originated from a common ancestor and sexual reproduction has played a minor role in genetic variability of the fungus in its life cycle. Also no apparent correlation was found between RAPD groups and geographical region and it suggests that they are independent from each other.

To study morphological and virulence variability of Cryphonectria parasitica isolates in Guilan province, the only region with important chestnut (Castanea sativa) stands in Iran, several collections were made from chestnut stands located in Shaft, Resvanshahr and Lahijan regions during 2006-2007. One hundred twenty single spored isolates were obtained. After primary evaluation based on colony morphology and growth rate 15 isolates, which some of them were resemble to hypovirulent isolate, were selected for more studies. These isolates were investigated for colony morphology, laccase activity, apple fruit rot, pathogenecity on cut stems and natural sprouts in forest. Isolates showed differences for some characteristics. Growth rate was 30-87 mm after 7 days on PDAmt at 25oC in different isolates. Colony color ranged from yellowish orange to brownish orange to white. Laccase activity on Tannic Acid Malt Agar media at 22oC and dark condition was different. Virulance of isolates on apple fruit, cut stems and natural sprouts showed significant differences. According to these results three isolates were determined as hypovirulent races. This is the first record of probable occurrence of hypovirulent isolates of Cryphonectria parasitica in Iran. These isolates would be good candidate for biological control of the disease in this region.

Hop stunt viroid (HSVd) isolates have been reported as a causal agent of citrus cachexia in Mazandaran province and recently shown to be associated with yellow corky vein disease of sweet orange and split bark disorder of sweet lime in the Fars province. These observations indicate the importance of this viroid in citrus cultivations in Iran. In this study a number of symptomless citrus trees were sampled and studied for viroid infection. After RNA extraction, reverse transcription-polymerase chain reaction was conducted and the full length genomes of viroids were cloned and sequenced. Three HSVd isolates from Fars were selected and used for comparison with another two HSVd variants from Fars, two HSVd isolates from Mazandaran, and a number of other sequences from GenBank. On the basis of homology, Mazandaran isolates clustered in a single group whereas Fars isolates were placed in different groups. HSVd-cit1 from Fars was different from all other variants and formed a separate cluster. This indicates the exictence of more variation in the genome of the Fars HSVd isolates than Mazandaran isolates.

The Iranian isolate of tomato leaf curl virus (TLCV-Ir; GenBank Acc. No. AY297924) is a whitefly (Bemisia tabaci) transmitted begomovirus causing leaf curl disease of tomato in Iranshahr (Sistan and Baloochestan province, Iran). To test the infectivity of a full-length 2763 nucleotide DNA of this virus, a partial dimer (head-to-tail) was constructed in the binary vector pBin19 and transferred into Agrobacterium tumefaciens strain C58. A range of potential hosts including Lycopersicon esculentum, Nicotiana tabacum, N. benthamiana, Datura stramonium and some other indicator plants were agroinoculated with a bacterial culture of this construct. Mild curling and yellowing symptoms were observed on newly developed leaves of tomato plants within 30 to 45 days post-inoculation (dpi). However, the next phase of symptom development on tomato occurred 2-3 months after inoculation and produced elongated spindly shoots, smaller leaves and severe leaf cupping on the newly developed leaves. N. benthamiana plants agroinoculated with TLCV-Ir also showed severe symptoms including leaf curling and leaf cupping within 21 to 30 dpi. No symptoms were observed on other agroinoculated plants but dot blot and PCR analysis using TLCV-Ir-specific probes and primers, respectively, confirmed the presence of viral DNA in Datura stramonium and N. tabacum plants indicating that these plants are symptomless hosts of the TLCV-Ir. On the basis of host range, symptom severity, timing of symptom appearance and the level of replication of the virus in host plants in comparison with a severe isolate of TLCV, i.e., the Australian isolate of TLCV, TLCV-Ir is considered as a mild isolate. Transmission of TLCV-Ir from agro-infected tomato plants to healthy tomato seedling was achieved with one adult of B. tabaci and the inoculated plants showed typical symptoms of the disease. The infectious clone of TLCV-Ir can be used as a source of this virus in various studies including screening of resistance cultivars.

The phenotypic characteristics, electrophoretic pattern of cell proteins and rep- PCR generated DNA fingerprints of Pseudomonas syringae pv. syringae (Pss) strains causing sugarcane red streak disease were compared with those inciting canker on stone fruit trees, blight of wheat and leaf spot in sleepy mallow. The Pss strains from sugarcane were very similar in phenotypic characteristics but varied in the rate or level of formation of levan, syringomycin and in hydrolysis of Tween 80. The electrophoretic pattern of cell proteins of the strains causing red streak were similar although not identical to each other but differed markedly from those of the other groups. The protein patterns of the strains from stone fruits and wheat were indistinguishable. The groups of strains were best differentiated by their rep-PCR fingerprintins. The ERIC- and BOX-PCR fingerprints of the strains from sugarcane were distinct from those of the other groups. The results indicate that the Pss strains isolated from sugarcane with red streak symptom constitute a group genotypically distinct from those inciting canker on stone fruit trees, blight of wheat and leaf spot on sleepy mallow plants.