IRANIAN JOURNAL OF PLANT PATHOLOGY
Year:2005 | Volume:41 | Issue:4|Papers Count:12

Powdery mildew, caused by Podosphaera fusca (syn. Sphaerotheca fuliginea), is considered to be an important disease of cucurbits, which is commonly controlled by fungicide application. The side effects of fungicides, however, necessitate searching for new and safe methods, including inducing disease resistance in the hosts. In this research, the effect of the following substances was investigated for inducing resistance in cucumber (cv. Super Dominus) against powdery mildew: Acibenzolar-S-methyl (ACI), b-amino butyric acid (BABA), di-potassium hydrogen phosphate (K2HPO4), nicotinic acid (NA), salicylic acid (SA) and extract of Reynoutria sachalinensis (F. Schmidt) Nakai (Rs). The inducers were sprayed onto the upper and lower sides of cucumber leaves. In another experiment systemic effects of these chemicals were tested. The inducers were sprayed onto the upper and tower sides of first true leaves only. Twenty-four hours later, the plants inoculated with the spore suspension of pathogen. Disease severity was determined 10 days after inoculation by measuring number of disease spots per cm2 of leaf surface. Results showed that all tested chemicals significantly reduced the disease severity on the treated leaves, but in varying degrees. ACI and Rs reduced disease severity more than 95% as compared to control. BABA and K2HPO4 reduced disease severity about 50-70% and showed relative effectiveness in disease control. SA and NA showed less than 30% control and considered to be the least effective compounds. From the systemic effect point of view, ACI was very effective; BABA and KzHP04 were relatively effective so that, disease control on the second leaves was 70- 80% that of the first leaves; and Rs, SA and NA did not show a considerable systemic effect (disease control on the second leaves was less than 30% that of the first leaves). These results indicated that ACI and Rs were the most effective compounds for inducing resistance to powdery mildew in cucumber.

The seedling damping-off and root rot diseases caused by Rhizoctonia solani lead to considerable damage on sugarbeet in Khorasan province. DNA polymorphism of some pathogenic isolates of R. solani was determined using Random Amplified Polymorphic DNA (RAPD). Seven random primers of UBC e.g. UBC6, UBC53, UBC82, UBCI96, UBCI99, UBC222, UBC228 revealed a high level of polymorphism among the isolates of this fungus.Out of 105 DNA bands produced, 78 bands (nearly 75%) were polymorphic. DNA bands were scored by assigning "0" for absence and "1" for presence and used in a matrix to compute genetic distance values among isolates. These values were much lower among members of an anastomosis group (AG) than those between AGs, resulting in classification of isolates compatible with clustering based on anastomosis characteristics. Comparison of RAPD classification with geographical and pathogenicity factors revealed that geographical distribution of isolates is much more consistent with RAPD classification than with the pathogenicity factors. These results suggested that RAPD is an effective marker for determination of diversity among isolates of R. solani.

Fifty four isolates of Rhizoctonia solani AG-I-IA the causal agent of rice sheath blight disease from rice fields of Mazandaran province were isolated and analyzed for probable polymorphism ofITS-5.8S rDNA region by PCR based RFLP.Genomic DNA was extracted from isolates and amplified by PCR, using internal transcribed spacer (ITS) primers. Primer pairs ITS4 and ITS5 were used and the PCR products were resolved on 8% polyacrylamide gel and stained with ethidium bromide.The isolates were differentiated into two groups on the basis of the size and number of fragments amplified. One group of isolates produced a 700 bp fragment and members of the other group displayed three bands, 700, 850 and 950 bp in size.To assess existence of any further polymorphism in ITS region, the PCR products were digested with restriction endonucleases BamHI, Hind Ill, XhoII, HaeIII, XhoI, EcoRI and Taq1.BamHI, HindIII and XhoI did not have recognition sequences on the amplified products EcoRI, HaeIII, TaqI and XhoII cut the amplified ITS regions into two or three fragments but the digested products were similar among the isolates in each of the two groups. Thus no further polymorphism was discernible.

Genetic diversity of Iranian isolates of R. solani associated with sugar beet root rot was studied using RAPD-PCR and ITS-rDNA analysis. Twelve of the 14 random oligo-primers used in RAPD-PCR yielded scorable polymorphism among the isolates. The DNA bands on agarose and polyacrylamide gels were scored using binary code and a similarity matrix based on simple matching coefficient, was generated using SPSS software. No correlation was found between the RAPD profiles and the geographic location and virulence level of, or the type of disease caused (host, tissue affected) by the isolates of R. salani. The isolates belonging to different anastomosis groups could be distinguished at 85% similarity level. A DNA fragment of 650-750 bp in size was amplified from DNA preparations of all isolates with the ITS4 and ITS5 primers. The fragments possessed no recognition site for endonuclease Pst1. The RFLP profiles generated through digestion of the amplified products with EcaRl and Tru91 were useful in differentiating the isolates of different anastomosis groups.

Bacterial brown blotch, caused by Pseudomonas tolaasii is a major disease of cultivated mushroom (Agaricus bisporus) in mushroom production facilities in Iran. Several control measures, including sanitation, precise control of the atmosphere in production houses and application of chemicals and biological control agents have been used for reduction of the disease incidence and the losses inflicted by the pathogen, throughout the world. In the present study the efficacy of some potentially antagonistic bacteria, isolated from diverse sources (Le. mushroom surface, peat moss, compost and soil microflora) in reducing the incidence and severity of the brown blotch disease was evaluated. Seven out of 406 bacterial strians, isolated from soil and peat moss displayed in vitro inhibitory activity against P. tolaasii and prevented pitting of mushroom blocks, when Co - inoculated with the pathogen at on equal or 10- fold excess population densities. Three of these potential antagonists which were identified as P.jluorescense (strains designated 114, 119 and 120) showed in situ inhibitory activity against P. tolaasii and markedly reduced the incidence and severity of brown blotch on mushrooms grown under commercial production conditions.

Seventy wild-type isolates of Verticillium dahliae were recovered from pistachio trees with Verticillium wilt symptoms in Kerman province. Seeds of ten commercial Pistacia vera cultivars including Qazvini, Badami Zarand, Badami Ravar, Sarakhs, Akbari, Ohadi, Sabz Peste Noogh, Khanjary Damghan, Kaleh Ghoochi and Ahmad Aghaee were established in virgin soils under greenhouse conditions. Microsclerotia were obtained in liquid medium. Six month-old seedlings were transplanted in soils containing 50 microsclerotia /g dried soil. Verticillium wilt symptoms were observed in all pistachio seedlings 67 to 95 days after inoculation. To study the reaction of pistachio cultivars to V dahliae, three distinct groups were identified. Cultivars Ohadi, Badami Ravar, Badami Zarand and Qazvini had the lowest and Sabz Peste Noogh, Khanjary Damghan and Ahmad Aghaee the highest disease severity index (DSI), browning index (BI), plant height reduction, stem colonization (SC) and death, which were regarded as tolerant and very susceptible group respectively. Cultivars Sarakhs, Akbari and Kaleh Ghoochi were between and considered as susceptible. Interaction between isolates of V dahliae (with different pathotypes) and pistachio cultivars that inoculated with conidial suspension (107/ml) of each isolate showed that DSI and SC of defoliating isolates were higher than non-defoliating type indicating that the isolates had different degrees of pathogenicity on pistachio cultivars.

During 2000- 2001, sixty three isolates of bacteria were isolated from infected tubers and stems of potato, infected roots of carrot, infected leaves of lettuce and cabbage with blackleg and soft rot symptoms from different regions of Khuzestan province.All of the isolates were rod-shaped, motile with peritrichous flagella, gram negative, facultative anaerobe, oxidase negative, catalase positive, and capable of rotting potato tuber slices, but negative in hypersensitive reaction in tobacco. The majority of isolates produced acid from glucose, glycerol, arabinose, lactose, maltose, ribose, mannitol, mannose, xylose and fructose. All of the isolates reduced nitrate and produced acetoin and H2S from cystein, but they were urease negative and were identified as Pectobacterium.The isolates were placed into four groups on the basis of numerical analysis of phenotypic features. On the basis of differential biochemical and physiological features, the isolates of the first, second and fourth groups were similar to P. chrysanthemi and the isolates of third group were similar to P. carotovorum subsp. carotovorum.Protein electrophoretic patterns of the isolates showed remarkable variations. In clustering of isolates on the basis of similarity ratio of electrophoretic patterns, the isolates were placed into three groups, and the results were similar to those of the numerical analysis of phenotypic and nutritional features.

Survival of Septoria trifid, the causal agent of Septoria leaf blotch of wheat was studied under natural and laboratory conditions. Infected leaves containing pycnidia were placed on soil surface and buried under the soil (10- 20- 30 and 40 cm). The germination of pycnidiospores'decreased after eight month on soil surface. Leaf pieces containing pycnidia buried under the soil were colonized by saprophytic agents resulted in total destruction of pycnidia at less than two month. Under laboratory conditions (4-5 QC), eighteen to twenty percent of pycnidiospores germinated from leaf samples stored over 25 months. To study the formation of sexual stage of the pathogen under natural conditions, infected wheat plant were placed in open air. Morphological study of ascospores formed under natural conditions, corresponded to Mycosphaerella graminicola reported by others. Attempts made to germinate ascospores were failed and no pathogenicity could be accomplished. Ascocarps could not be obtained under laboratory conditions. To study host range and the role of weeds for survival of the pathogen, various plant species as Avena fatua, A. ludvidana, Aegilopes crassa, Agropyron repens Bromus sterlis, Digitaria lunguinalis, Hordeum murinum, H. spantaneum, Lolium rigidum, Phalaris minor, Poa annua, Secale cereale, Setaria viridis, H. vulgar cultivar Rihaneh and Zea mays cultivar KSc704 were inoculated with suspension containing of 107 spores /ml under greenhouse and controlled growth chamber conditions. L. rigidum and S. cereale were infected and pycnidia were produced on dry leaves 40-45 days after inoculation. The fungus was reisolated and found to be pathogenic on wheat, L. rigidum is a new host for Iran. Under field conditions, no symptom of the disease was found on collected weeds. Attempts made to detect the pathogen on seeds collected from infested wheat fields, were unsuccessful.

Species concept and concept below the species level in Puccinia recondita s. lat. Differs considerably. Some authors prefer a narrow species concept emphasizing biological specialization (host range) and minor differences as the main criteria for species delimitation. Other authors used a broad species concept with morphological characters as the main criteria for species delimitation. This broad concept made it easier to identity species using the light microscope but it also led to polyphyletic complex species. Both narrow and broad species concepts are still used and, consequently, many different and sometimes confusing scientific names occur in the literature. In this study 27 herbarium specimensre presenting P. recondita s. lat. collected mainly from Iran were used for morphological observations. The specimens were classified into three groups based on their morphology. For molecular phylogenetic analysis 10 specimens were selected from the specimens used in morphological studies. In the molecular studies we compared ribosomal DNA sequences of the variable internal transcribed spacer (ITS I+II) region and the more conservative 5.8S ribosomal RNA gene. The specimens of P. recondita s. lat. were separated into three groups by phylogenetic analysis of ITS regions as well. Based on the morphological and molecular results we recognized four taxa within this complex species as follows: - P. recondita S.str. on Secale segetale and Elymus sp. with aecial state on Cerin the minor from Boraginaceae.- P. bromina on Bromus spp. with unknown aecial state.- P. persistens subsp. triticina (wheat leaf rust) on Triticum aestivum and Aegilops tauschii with unknown aecial state.- P. persistens subsp. agropyrina on Elymus spp. with aecial state on Thalictrum minus from Ranunculaceae.Our phylogenetic analysis showed P. recondita S.str. is more closely related to P. hordei s.lat., than to P. persistens. ITS sequences of leaf rust from Triticum aestivum and leaf rust on Aegilops tauschii were identical, which supports the idea that leaf rust collections from wheat could infect Aegilops species.

Soil samples collected from alfalfa fields in 55 localities of Hamadan Province, during 2002-3 years, yielded 26 species of 17genera of order Tylenchida. Nematodes were extracted by the centrifugal-flotation method, killed, fixed and processed to anhydrous glycerin. The nematodes were mounted on slides and studied by light microscope. This study showed that Pratylenchus neglectus, Boleodorus thylactus, Helicotylenchus vulgaris, Mesocriconema antipolitanum and Geocenamus brevidens were found in 67.2, 63.6, 45.4, 38.1 and 30.9 percent of samples, respectively, and were the most abundant species in the alfalfa fields of the province. Twelve out of 26 identified species are new records of the nematode fauna of alfalfa in Iran. Pratylenchoides leiocauda can be separated from the other morphologically closely related species by bacilliform sperms, smooth tail terminal and relatively short body length (530-670 mm). It was found in very low population in one field in Bahar City, and is a new record for nematode fauna of Iran. Also in this study one specimen of G. brevidens has been found with anteriorly outstretched ovaries and post vulval uterine sac.

During growing seasons 2001 and 2002, chickpea fields were visited throughout Fars province (Shiraz, Marvdasht, Ghir and Karzin, Mamasani, Fasa, Khafr, Bavanat, Neyriz, Estahban, Zafarabad, Sepidan, Firouzabad, Kavar, Abadeh and Eghlid) and 154 isolates of the Fusarium spp collected from root, crown and stem of diseased chickpeas and root of weeds growing in chickpea fields. Isolates were identified and divided into six species and their frequency in descending order was F. oxysporum, F. solani, F. equiseti, F. proliferatum, F. sumbucinum and F. scirpi. Fusarium oxysporum (Fo) and F. solani (Fs) were highly pathogenic on chickpea and accounted for 45.64 and 39.6% of the isolates respectively.Pathogenicity tests conducted on all species with chickpea seedlings were as root dip method and infected wheat seeds around hypocotyl respectively for Fo and other Fusarium species. Fifty three isolates of Fs (causal agent of root rot in chickpea) and 38 isolates of Fo (causal agent of wilt in.chickpea) were identified. The results indicated that Fs and Fo isolates had significant differences in disease severity. Host range study with Fs, showed that the pathogen caused root rot only in chickpea and pea and was identified as F. solani f. sp. pisi (Fsp). F. oxysporum caused wilt only in chickpea which was considered to be F. oxysporum f. sp. cieeri (Foc). Under field condition Fsp was isolated from Allium seabriseapum, Chenopodium album, Amaranthus retroflexus, Capsella bursapastoris and Glyeyrrhiza glabra and Foe from Carthamus oxyaeantha, G. glabra, C. bursa-pastoris and A. seabriseapum. Of the several field crops and weeds artifitially inoculated with Fsp under green house conditions, only roots of A. seabriseapum, C. album, G.labra and Phaseolus vulgaris were colonized by the pathogen.