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Grain yield loss in rice (Oryza sativa L.) caused by blast disease, Magnaporthe grisea (Hebert) Barr, is a major concern of rice growers worldwide. Blast is considered as the most injurious disease of rice in Iran, resulting in severe loss especially to susceptible rice cultivars. In order to assess yield loss caused by blast pathogen and develop an appropriate model, different disease onsets and levels were simulated in the experimental field in a split-plot experimental design. Independent variables including early diseased leaf area (X1), final diseased leaf area (X2), early neck blast index (N1), final neck blast index (N2), area under leaf blast disease progress curve (AUDPC1) and area under neck blast disease progress curve (AUDPC2) were taken as predictors and regressed to the loss in yield. Statistics as coefficient of correlation (r), coefficient of determination (R2), adjusted coefficient of determination (aR2), standard error (SE), F and Durbin-Watson were considered in evaluating the resulting models. The most appropriate model was the one which predicts rice yield loss based on final diseased leaf area and final neck blast index.

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The performance of different yield loss models from an exponential family was evaluated in safflower-redroot pigweed systems in two field experiments conducted during 2007 and 2008 growing seasons at the research field of Agricultural College of Shiraz University, Iran. The yield loss of safflower was recorded as relative yield loss in experimental plots laid out in split plot design with three replicates. Three different irrigation treatments were allocated to the main plots and consisted of full irrigation or 100% field capacity (FC), 75% FC, and 50% FC, while five weed densities (0, 3, 6, 9, and 12 weeds m-2) were assigned to the sub-plots The Logistic and Gompertz models and a user defined Power-Exponential model were fitted to the data to relate crop yield loss to the weed densities under different water stress conditions. The Power-Exponential model was chosen as the best fit to the data with statistically acceptable model diagnostics. Logistic and Gompertz models showed good fit to the observed data, but underestimated the yield loss under three levels of irrigation. Model performance in all cases was influenced by water stress as models generally showed greater constant and systematic biases under severe water stress (50% FC). Model parameters were used to explain the impact of water stress in crop/weed system. The exponential family models globally performed better over common empirical models such as Spitters, Kropff and Lotz and Cousens models.

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To examine the effect of Citrus Red Mite (CRM) damage on quantitative and qualitative characteristics of Thomson navel orange Citrus sinensis (L.) Osbeck, paired-treatment experiment, multiple treatment experiment, and regression/correlation methods were used. There were significant differences among the treatments in paired-treatment and multiple treatment experiments in terms of the average population per leaf of CRM of different life ages and concentration of chlorophyll in the leaves. Also, there were significant differences among the treatments in multiple treatment experiment in terms of the Total Soluble Solids (TSSs) of the fruit extract. In control treatment of the paired-treatment experiment, the highest concentration of chlorophyll (79.13±1.06) was observed when the population density of CRM was 0.48±0.09 per leaf. With increasing the mean number of CRM to 10.59±1.09 per leaf, chlorophyll content was decreased to 62±1.15. Furthermore, the results showed that the storage life of the fruits in treatments with mean number of 6.33±1.8 mites per leaf was significantly reduced. The results of regression and correlation analysis on chlorophyll showed significant and negative relationship between these parameters and CRM population density. The results indicated that increasing the population density of CRM to 10.59±1.09 per leaf at the beginning of the season caused fruit drop and dry twigs. In addition, by increasing population density of CRM to 5.72±0.43 per leaf, the storage capability of the fruits was significantly reduced.
 

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The present work was aimed to investigate the life cycle and the effect of different initial population densities of Globodera rostochiensis on potato yield and growth on the susceptible potato cultivar, Spunta, in Algerian environmental conditions. The length of the life cycle of G. rostochiensis differed among growing seasons and was 72 days or 699 DD_6.2 for potatoes planted in mid-February and 66 days or 496 DD_6.2 for potatoes planted in early November. A significant reduction in growth and potato yields was observed. The increase of initial Population densities (Pi) of G. rostochiensis were associated with a significant reduction in plant growth and potato yields. The maximum yield and plant height reductions were 85 and 75%, respectively, at the highest Pi of 512 eggs g^-1 soil. The final nematode Population density (Pf) increased with the increase of the initial population densities. The Reproductive factor (Rf) initially increased with the increase of Pi up to 4 eggs g^-1 soil and then decreased for Pi≥ 4, suggesting an intraspecies competition. Results contribute to the knowledge of G. rostochiensis thermal time requirements in Algerian environmental conditions and can be a valuable tool to develop appropriate potato cyst nematode control strategies considering that the nematode can also cause severe damage and yield losses at very low Pi densities.