Study the Effect of Thiobencarb Method of Application on Causing Dwarfism in Rice

In order to investigate the effect of thiobencarb (TB) method of application and dosage on causing dwarfism in rice this study was carried out in 2007 and 2008 in research fields of Rice Research Institute of Iran-Rasht. The experimental plots had the precedence of dwarfism occurrence. Experimental design was a randomized complete block design with 3 replications in factorial split arrangement of treatments. Main plot included factorial arrangement of time of TB application (before and after transplanting)، and time of flooding (before and after transplanting) and sub plots were TB dosage in 6 levels (0، 1. 5، 3، 4. 5 and 6 kg. ai. ha-1). Visual observation showed that TB causes dwarfism in paddy fields and its very distinctive symptoms were dark greenish color of leaves، short stature of seedlings، excessive unfertile tillering، fishhooking curvature leaves، brittle leaves and stems، delay in flowering and malformed panicle. These symptoms appeared 4-6 weeks after rice transplanting and lasted up to harvesting. Data analysis showed that the main effect and interaction of treatments affected dwarfism، days to 50% flowering، height، infertile grain، harvest index، economic and biologic yield. TB application “before transplanting and flooding” and “after transplanting and flooding” showed the highest and least negative effect on the above mentioned traits respectively. TB application “before transplanting and flooding” showed seven times more dwarfism compared with its application “after transplanting and flooding”. Average grain and biological yield loss of rice affected by dwarfism if TB is applied “before transplanting and flooding” was 50% and if it is applied after transplanting and flooding was about 23%. In addition results of this research showed that in treatments with high dwarfism the average grain yield loss was two times more than biological yield loss. More decrease of grain yield (reproductive stage) compared with biologic yield (vegetative phase) indicates the role of a physiological disorder in addition to visual phytotoxicity. Researchers have reported that the toxic derivatives released from biological degradation of thiobencarb causes dwarfism in rice. Based on this results it seems that the microbial degradation of thiobencarb in paddy field of Guilan province has occurred from two decade ago base on prevalence of dwarfism in paddy field of this province.