Rice Straighthead Decreasing and Seed Set Increasing Associated with the Application of Calcium on a Silt Loam Soil.

Even though large efforts have been made to study straighgthead disease for improving cultivar resistance since early 1900s in the US, its causal factors are still not exactly known. We conducted this experiment in greenhouse conditions to address the relationship of straighthead with application of Ca into a silt loam soil.

US cultivar Cocodrie and three breeding lines, 8-18, 8-9 and 12-38, ranging from susceptible, medium resistant and high resistant to straighthead, were used for this study. Cocodrie is known to be susceptible to straighthead, and then it has been widely used as susceptible check in studies for straighthead. The breeding lines 8-18 and 12-38 are the medium resistant to straighthead, and 8-9 are relatively high resistant to straighthead. Three plants were grown in each pot (22 cm tall and 22 cm diameter) filled up with the silt loam soil collected from the University of Arkansas at Pine Bluff farm where straighthead naturally occurs. Four levels of treatments, 0g Ca (CK), 297g Ca at vegetative stage (V_Ca treatment), 235g Ca at booting stage (B_Ca treatment), and 297g Ca at vegetative stage plus 235g Ca at booting stage (V+B_Ca treatment), were applied to each of the varieties with three replications. The pots were completely randomly placed in the greenhouse. During the growing season, a variation of straighthead symptoms including sterile and deformed grains and panicles were observed among the Ca treatments and varieties. Seed set rate (%) was analyzed using SAS program version 12.0. Analysis of Variance of the seed set rate showed that the variations due to varieties, Ca treatments and their interactions were all highly significant (p<0.000). The difference among the varieties showed that the most susceptible cultivar Cocodrie had almost no seed set (1%) for CK (No Ca), while V+B-Ca treatment recovered Cocodrie to 52% seed set rate; the medium resistance varieties 8-18 and 12-38 had seed set rates 9% and 61% for treatment CK (no Ca), respectively, and increased to 62% and 74% under the treatment of V+B_Ca treatment, respectively; the relative high resistant cultivar 8-9 had 78% seed set rate under the treatment of CK (No Ca), and increased to 90% under the treatment of V+B_Ca. Average over all treatments, Cocodrie had  seed set rate of 21%, while line 8-9, the mostly resistant, had a seed set of 80%. There were also big differences among the Ca treatments. Over all varieties, CK (no Ca treatment) had the lowest seed set rate of 37%, while the V+B-Ca treatment resulted in the highest average seed set of 70%, which was also significantly higher than the 47% average seed set rate of 4 varieties for V-Ca and B-Ca treatments.