somayeh dariush

Rice grain discoloration disease is emerging as a major threat in improved and late ripening varieties in the north of Iran that deteriorates grain quality and physical properties. Breeding for disease resistance is the preferred strategy to manage grain discoloration. An important application of molecular markers in plant systems involves improvement in the efficiency of conventional plant breeding by carrying out indirect selection through molecular markers linked to the traits of interest. In this context, association analysis was used to identify molecular markers associated with grain discoloration resistance in rice genotypes in this study. 

In this study, resistance of 94 rice genotypes (Iranian landrace and improved varieties and exotic genotypes) to grain discoloration disease was evaluated during 2017 and 2018.Genotyping was done using 128 pairs of polymorphic microsatellite markers on 94 genotypes of rice. Then association analysis was conducted through general linear model (GLM) and mixed linear model (MLM) by using TASSEL software.

Phenotypic analysis showed that there is high diversity in the studied genotypes for resistance to grain discoloration disease. The model-based structure analysis classified rice genotypes into two sub-populations. In association analysis based on GLM and MLM models, 12 and 3 QTLs showed significant relations after considering Bonferroni correction with grain discoloration in two years, respectively.

Both GLM and MLM detected 3 markers (RM242, RM5709 and RM5955) associated with grain discoloration resistance in both 2017 and 2018, suggesting that these markers can be used in rice breeding programs like marker-assisted selection (MAS).

Brown spot, caused by Bipolaris oryzae, is a devastating disease of rice which can cause yield loss in most rice-growing regions of the world. Breeding for disease resistance is the preferred strategy of managing brown spot. Hence, identification and subsequent development of disease resistance in rice genotypes are crucial. The field resistance of 95 rice genotypes to brown spot was evaluated under water and fertilizer stress during 2017 and 2018. Partial resistance was assessed through reaction type (disease rating) and epidemiological parameters estimates i.e. final brown spot index, area under disease progress curve and apparent infection rate. Disease rating, brown spot index, and area under disease progress curve detected differences in the responses of rice genotypes to disease under field condition, which could be used to study brown spot resistance. Among the genotypes tested, 22 genotypes were resistant to moderately resistant (23.16%) while majority were moderately susceptible to susceptible (76.84%). A significant correlation between leaf angle and area under disease progress curve indicated positive influence of leaf erectness on severity of brown spot disease. Results showed that leaf infection did not significantly affect the number of filled grains per panicle or hundred seed weight, but caused yield decline by decreasing the number of productive tillers. Nevertheless, the infection of rice genotypes from flowering to ripening stages decreased the number of filled grains per panicle and grain weight. The resistant genotypes identified in this study can be exploited for future rice breeding programs to develop promising resistant lines in management of the brown spot disease.

The present investigation was carried out to examine virulence diversity among the 291isolates (seven populations) of Rhizoctonia solani AG-1 IA causing sheath blight of rice by sclerotia production and pathogencity studies on four rice cultivars representing different observed disease reactions, using detached leaf inoculation method in conjunction with mycelia growth measuring. Based on pathogencity test four tested cultivars were significantly differed from each other in response to each population of pathogen. In each population, all the test isolates exhibited varying degree of virulence on each four cultivars and were variable both in mycelial and sclerotial (number and weight) parameters. There was found one group in each population having more stable performance in virulence in a range of genetically different rice cultivars based on their virulence on each four cultivars and along with their coefficient of variation by cluster analysis method. According to these finding, six, two, five and one high-virulence isolates respectively from populations of Rasht, Tonekaboon, Amol and Golestan can be used to future greenhouse or field tests, selecting most suitable isolates for identification of accurate reaction of rice cultivars / lines for rice breeding programs in north of Iran.

Rice false smut disease caused by Ustilaginoidea virens (Cooke) Takahashi was known as one of the diseases of rice in some paddy fields of Guilan province in recent years. The present study was conducted to chemical control evaluation of rice false smut by application of tricyclazole (WP 75%), carbendazim+iprodion (Rovral-T-S® WP 52.5%), copper oxychloride (WP 35%), propiconazole (Tilt® EC 25%), and azoxystrobin+cyproconazol (Amistar Xtra® SC 28%) fungicides with dose rates of 0.5, 1, 3kg, 1 and 0.6 liter per hectare, respectively on a rice susceptible cultivar, namely Khazar at the mid boot and beginning of panicle emergence stages, in a field having multi-year infection in Abkenar location near the Anzali lagoon for two years. Data analysis showed that propiconazole, copper oxychloride and azoxystrobin+cyproconazol fungicides had significant effects on reducing of infected tillers (panicles) and pathogen sporeballs number per square meter and disease severity in first year (2011). Whereas the effects of tricyclazole and carbendazim+iprodion were not significantly differ from control treatment. In second year (2012) the disease severity was seriously developed in the field, but the application of propiconazole was more effective than other fungicides to reduce number of infected tillers (panicles) and pathogen sporeballs per square meter and disease severity in comparison with control treatment by percentage rates of 80.62, 83.92 and 83.01 respectively. So application of propiconazole at booting stage can be introduce to reduce disease incidence of false smut in paddy fields of northern Iran.

Blast disease of rice caused by Pyricularia oryzae is a potentially damaging disease in northern due to planting of susceptible local cultivars and favorable conditions. To assess the effects of blast on rice yield components and yield loss, one field experiment was conducted at the lowland culture conditions. Analysis of blast effects on rice indicated that the maximum yield losses due to unprotected treatments at vegetative, reproductive and both stages were 4.72, 6.39 and 14.41% respectively. Node blast had more influence than neck blast in decreasing the 1000-grain weight and number of filed grains and in increasing the number of unfiled grains per panicle. Yield losses due to blast were correlated either to percentage of leaf blast (LB) or percentage of neck, node and panicle blast (NNPB) in unprotected treatments at vegetative stage in each two years of 2009 (for LB, R= 0.868, P. value= 0.00517; for NNPB, R= 0.848, P. value= 0.00775) and 2011(for LB, R= 0.785, P. value= 0.0211; for NNPB, R= 0.801, P. value= 0.0169). Also two positive significant correlations were observed between LB % and NNPB% in 2009 (R= 0.882, P. value= 0.00376) and 2011 (R= 0.812, P. value= 0.0143) in this treatment. Whereas, in unprotected treatment at reproductive stage yield loss was correlated only to percentage of NNPB (R= 0.918, P. value= 0.0013).

Blast disease caused by Pyricularia oryzae is the principal disease of local rice cultivars innorthern Iran. Because of this, its chemical control is being in possession of special priority.Application of various fungicides and fungicides mixture is an important strategy on the chemical management of this disease. In this study efficacy of Vista®, as a fungicide mixture (tricyclazole + thiophanate methyl WP 72.5%) was evaluated in controlling of rice leaf, panicle, neck and node blast disease with dose rates of 400, 500 and 600 g/ha, in comparison with recommended fungicides, tricyclazole (WP 75%), carpropamid (SC300 50%) and edifenphos (EC 50%) with dose rates of 500g, 400 ml and 1000 ml per hectare respectively, in randomized complete block design under field condition in 2010 and 2011. In Rasht county, the results revealed that the effects of fungicides Vista® with all three dosages, carpropamid and tricycalazole were more significant to control of leaf blast on Hashemi cultivar in comparison with control treatment, at both time ofevaluation. All fungicides had significant effect to reduce panicle and node blast (exceptedifenphos) in first year and neck blast (except edifenphos) in second year. In second year node blast was seriously developed and application of tested fungicides was not significantly effective to reduce of node blast in comparison with control treatment. In Amol county, edifenphos was the least effective of all the fungicides in controlling of leaf blast disease as similarly found in Rasht county.

Rice sheath blight disease caused by Rhizoctonia solani AG1- IA is the second economically important disease of the crop in most rice- growing regions of the world، including Northern of Iran. The disease is mainly controlled through application of fungicides. The present study was performed to identify the candidate cultivar (cultivars) as source of resistance to the pathogen in rice field conditions in Northern Iran. Thirty seven vs twenty three rice cultivars respectively from Iranian Local (IL) and Iranian Improved (II) cultivars along with ten Foreign Resistant (FR) cultivars were field grown at the Rice Research Institute of Iran. They were inoculated with the mycelial mass of a virulent isolate (G309) of pathogen، 45 days after being transplanted. To supply favorable humidity for disease development، the plants were kept wet until the evaluation time through 2-5 times of daily water spraying، while applying a overhead water sprayer system available on the field. Assessment of cultivars، reaction was done by recording the Sheath Blight Severity as appeared on the sheath (SSS)، number of Lesions on the Sheath (LS)، Relative Lesion Height (RLH)، percentage of Infected Tiller (IT)، number of Dead and Green Leaves (DL and GL)، Number of tillers (Nt)، Tiller Angle (TA)، Stem Width (SW)، and Plant Height (PH). Results indicated that، the group of FR cultivars significantly differed from groups (IL) and (II) cultivars by exhibiting less SSS، LS، DL vs more GL. Cluster analysis of the cultivars as based on RLH، SSS and DL، using UPGMA method revealed 4 groups. One of the grpups comprised of: Jasmine 85، Teqing، Pecos and Tetep was observed as resistant، whereas، another one containing Swarna (Foreign susceptible)، Moosatarom، Hassansaraei، Jamshidjo، Khazar، Shiroodi، D4 (IL and II)، and Fujiminori was recorded as susceptible. Analysis of the relationship between plant traits and disease response indicated that RLH and SSS were not significantly correlated with the assessed morphological characteristics as regarded FR cultivars، whereas a positive significant correlation was observed between SSS and TA (r = 0. 20، P. value= 0. 033)، and a negative one between RLH and PH (r = -0. 29، P. value= 0. 002) as detected on local cultivars.