Differential Responses of Breeding Strains to Straighthead.

Straighthead disorder is the most important non-fungal disease for rice in the U.S. When a highly susceptible cultivar is grown under conditions favorable for straighthead, yield losses can be up to 100%. Improving straighthead tolerance will greatly secure grain yield and reduce rice production costs. All previous studies on straighthead have been conducted under induced conditions using arsenic chemicals. Evaluation of this disease on natural conditions is practically important for cultivar improvement. Based on 2008 results, 12 lines including three commercial cultivars and nine breeding strains of long grain rice were selected for this study in a natural field. These lines were planted on April 28, May 8, and May 18 in 2009, and repeated in 2010 and 2011, respectively in the farm of University of Arkansas Pine Bluff (UAPB). Grain yields of the cultivars and strains were greatly reduced because of the natural straighthead. Breeding strains PB-11 and PB-12 showed a high tolerance to straighthead consistently for the three years, indicated by a low score 1-2 for straighthead symptoms. Strain PB-2 showed a fair tolerance with the scores ranging 3 to 5 in the three years. As a result, PB-11, PB-12 and PB-2 yielded significantly greater than the control cultivar Francis in three years (P<0.001). Francis is a widely used commercial cultivar and fairly susceptible to straighthead. Straighthead performance of strain PB-13 was unstable among three years with a variation 1 – 7 for the scores. Consequently, its yield reduction increased along with an increase of its straighthead score each year. Our findings suggest that genetic variation of straighthead resistance is enough to cope with straighthead disease in rice production. Consistent performance of some strains on straighthead symptoms demonstrates a feasibility to establish a natural site at the UAPB to evaluate straighthead disease directly for cultivar improvement.