Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/16/2010
Publication Date: 8/12/2010
Citation: Eizenga, G.C., Grunden, Q.P., Lee, F.N. 2010. Method for evaluating rice wild relatives (Oryza spp.) reaction to blast disease. p. 109 In: Proc. 5th Int. Rice Blast Conf. 12-14 Aug. 2010. Little Rock, Arkansas USA.
Technical Abstract: Rice wild relatives (Oryza spp.) are an important source of novel pest resistance genes, as well as, tolerance to abiotic stress and yield-enhancing traits. Resistance to rice leaf blast caused by Magnaporthe oryzae B. Couch has been reported in Oryza spp. accessions. Most of the Oryza spp. accessions have low seed set, poor germination, and shatter seed, thus it is advisable to grow the Oryza spp. accessions in the greenhouse. In order to determine the reaction of these accessions to U.S. races of M. oryzae, it is necessary to adapt the procedures used for evaluating reaction to blast disease for use with the Oryza spp. accessions and their progenies. Our objective is to describe the procedure used to screen the Oryza spp. for reaction to M. oryzae. To ensure the M. oryzae isolate of the specified blast race being used for inoculation was virulent, the culture was started from a stock culture of the isolate stored at -2°C by placing the dried culture on an oatmeal agar medium or an agar medium containing alfalfa pellets and rice hulls. After the initial culture grew, it was sub-cultured to obtain the number of spores needed for inoculation and if necessary, subcultures were started that could be stored for future use. Seeds of the Oryza spp. accessions or progenies to be inoculated were dehulled, surface sterilized, and placed on an orchid medium in a lighted incubator set at 27°C with 12 hr light/12 hr dark for about 10 days. After the seedlings were approximately 12 cm tall, 4-6 plants were transferred to 6.5 cm square pots containing a soil mixture (1 part soil: 2 parts RediEarth). When the seedlings were at the 3 to 4 leaf stage, they were drought stressed in preparation for inoculation. Drought stress was induced by dumping the excess water from the flooded pots/trays and allowing the soil to dry down until no moisture was evident (~24-48 hours at room temperature). The M. oryzae spores were placed in a xanthene gum mixture (0.1 g xanthene gum per 400 ml water), counted using a Hemacytometer, and diluted to 100,000 to 200,000 spores per ml. The spore solution was applied to the leaves of the drought stressed seedlings with an airbrush, and the seedlings subsequently were placed in a tight plastic bag for 24 hr at room temperature. After the seedlings were removed from the plastic bag, they were watered with a 20-20-20 (N-P-K) liquid fertilizer solution and rated approximately one week later for blast symptoms on a 0 to 9 scale (0=no disease to 9=dead). Initially, a collection of Oryza spp. was screened using a slight variation of this method. Currently, this method is being used to evaluate mapping populations for reaction to blast disease.