Document Type : Research Article
Authors
1 Department of Horticultural Science, Faculty of Agriculture, Jahrom University, Jahrom, Iran
2 Ph.D. Student of Horticultural Science and Engineering, College of Agriculture, Hormozgan University, Iran
Abstract
Introduction: Citrus trees are among the most important tropical and subtropical fruit trees in the world. In recent years, newfound disorders have become a serious danger to citrus growing. In the last two years in Jahrom city and surrounding towns, there have been numerous reports about the rapid drying of whole of citrus trees or parts of them. Following studies by expert groups, the problem was referred to as "Citrus decline". Most researchers have divided the main causes of decline into biotic and abiotic groups. In fact, the disorder of citrus decline refers to any type of disorder, disease or problem that can ultimately lead to tree weakness and loss. In fact, decline can be considered a synonym for death.
Materials and Methods: In this study, 64 orchards located in the cities of Jahrom, Juyom, Khafr and their suburbs were studied. Each orchard consisted of at least 100 trees of ‘Lisbon’ cultivars (one to ten years old). Evaluations were carried out periodically and with a two-month intervals from March 1976 to November 1977. On the one hand decline rate and on the other hand desiccating rate (tree with complete drying) were measured. Average monthly air temperature, sodium absorption ratio (SAR), irrigation water, soil texture, manure consumption, irrigation amount and methods, and spatial and temporal distribution of irrigation water were measured. Longitude, latitude and altitude were recorded using GPS. The meteorological data were obtained from weather station of Jahrom city. Soil sampling was performed using standard methods from all orchards. After determining the soil texture type, in order to data convert from qualitative to quantitative according to the amount of clay available, the numbers 1 to 10 were assigned to each sample. Number 5 was considered as medium and standard soil texture. If manure was applied, one unit was added to the soil texture score of less than 5 and one unit was subtracted from the numbers above 5. The presence or absence of shading on the studied trees (Green lace or palm tree) was assigned to zero and one numbers, respectively. Numbers zero and one were assigned to applied or non-applied soil sodium solute reducing fertilizers, respectively. The amount of clay, SAR (sodium absorption ratio) in irrigation water were obtained using the usual measurement methods in soil and water experiments. Data were analyzed using SPSS software (version 25) and Path analysis diagram was plotted using AMOS software (version 24). Multivariate statistical analyzes including factor analysis, correlation, cluster analysis and path analysis were performed to determine the relationships between variables and the percentage of decline.
Results and Discussion: According to the results of factor analysis, the first four factors accounted for 80.53% of the total variation. The first component (soil physical properties) including clay and soil texture accounted for 26.37% of the total variance. The second component (salt evaporation and accumulation) included temperature, mean age of the trees, and the rate of sodium absorption ratio (SAR) with accounting of 23.95% of the total variance. Significant correlations were observed between decline and mean tree age, decline and clay percentage, decline and presence of shading. Almost all of the declined trees reached the full drying stage, which is justified by the high correlation of the decline with desiccating (r=0.90 **). Percentage of decline had the highest correlation with tree age (r=0.67 **). Percentage of complete desiccating of the tree was also highly correlated with tree age (r=0.51 **). T-test to determine the correlation between two-level nominal variables such as application of shading (presence or absence of shading) and anti-salinity compounds (application or not application of salinity fertilizers) with decline occurrence was used. The mean percentage of decline was 29.66% in trees under shade and 57.40% in trees without shade. In general, the rate of decline in trees without shading is twice more than trees with shading. Path analysis identified the direct and indirect effects of variables on the rate of decline.
Conclusion: It was generally found that the decline is a multivariate physiological disorder that largely, can be controlled by some orchard management operations. In the meantime, temperature control and soil texture correction are important, especially in older trees. Increased temperature is one of the major causes of citrus decline that is also indirectly associated with other deteriorating factors. Soil quality changes gradually due to various factors so its correction is very important in reducing the incidence of this disorder.
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