Pattern of disease spread provides improved knowledge on how the pathogen introduces itself and interacts with environment in fields and expresses as a disease. It is especially significant when epidemiology of a disease, such as Rice False Smut (RFSm), is unclearly understood. Not reported before, this study attempted an analysis of spatial pattern of natural spread of RFSm in nine fields in an intensive rice ecosystem in Bangladesh. Both conventional and specialized statistical methods were applied in the analysis. Results show that the spread of the disease was not similar between and within the fields and even some fields were almost disease free. RFSm recorded aggregation in spaces in most of the fields, but the location of this aggregation differed between the fields. Symptom recorded on panicles in regenerated tillers from harvested main crop (otherwise known as ratoons). The disease tended to be prominent towards proximity of drainage channels. The probability of occurring one diseased tiller per hill was calculated as 73% and cumulative probability of four or less smut balls per diseased panicle as little over 60%. This study establishes soil as the absolute dominant source of initiation of the epidemic. The analysis did not find evidence of any long-or short-distance primary and/or secondary sources of infection. It is concluded that the disease management be directed specific to the fields at risk. It suggests development of a soil testing tool for quantifying inoculum potential in a field to ascertain the risk. With the discovery of symptom on ratoons, this study highlights the need for fresh thinking on identifying the pathway of entry of the pathogen into the plant.

Hybrid rice varieties have higher yield potential over inbred varieties. This improvement is not always translated to the grain yield and its physiological causes are still unclear. In order to clarify it, two field experiments were conducted including two popular indica hybrids (BRRI hybrid dhan2 and Heera2) and one elite inbred (BRRI dhan45) rice varieties. Leaf area index, chlorophyll status, and photosynthetic rate of flag leaf, postheading crop growth rate, shoot reserve translocation, source-sink relation and yield, and its attributes of each variety were comprehensively analyzed. Both hybrid varieties outyielded the inbred. However, the hybrids and inbred varieties exhibited statistically identical yield in late planting. Both hybrids accumulated higher amount of biomass before heading and exhibited greater remobilization of assimilates to the grain in early plantings compared to the inbred variety. Filled grain (%) declined significantly at delayed planting in the hybrids compared to elite inbred due to increased temperature impaired-inefficient transport of assimilates. Flag leaf photosynthesis parameters were higher in the hybrid varieties than those of the inbred variety. Results suggest that greater remobilization of shoot reserves to the grain rendered higher yield of hybrid rice varieties.

Rice False Smut (RFSm) is presently an internationally important fungal disease of rice. While the Yield Loss (YL) from this disease is reported in many countries, there exists no tool to instantly estimate the YL by visual field inspection. This study developed a simple model, FLYER, for this purpose. The model is run by two inputs: (i) fraction of productive but diseased tillers in a field and (ii) averaged number of smut balls present in the diseased panicles. FLYER was developed using data from Bangladesh, India and Japan. The driving algorithm of the model, the yield reduction in a diseased panicle as a function of number of smut balls present in the panicle, was validated with additional data from Bangladesh and Japan. When tested with independent data from fields infected naturally by RFSm, FLYER closely estimated the Yield Loss (YL, %) against observed datasets from Bangladesh (Root Mean Squared Deviation (RMSD) = 1.15% YL), Egypt (RMSD = 1.65% YL) and India (RMSD = 1.68% YL). This model could contribute to rapid assessment of regional and variety-specific yield loss and strategic management of the disease on a field-by-field basis.

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