Steven Linscombe | 11/16/2012 8:42:15 PM
Rice blast disease, caused by a fungal pathogen (Pyricularia oryzae), can be a major disease on rice in most areas of the world. The disease can be manifested in several different ways in the plant, beginning with leaf blast, which normally first shows up during the tilling stages of growth. While very severe leaf blast can actually cause plant death, we seldom see symptoms this severe in Louisiana. Typically, the most severe damage from blast disease occurs when it infects the panicle on which the rice grains are poised to develop. The disease can infect at the base of the panicle (rotten neck blast) or on individual panicle branches (panicle blast). Regardless, beyond the point of infection in the panicle, all the florets will be blanked out and not produce grain. Another factor that makes this a confounding disease is that it has numerous races, or genetic variants. This is problematic because a rice variety can have good resistance to several races but be highly susceptible to others. However, we typically have two major races in southwest Louisiana, and this has not changed much over the years.
Each year we see some level of blast disease in Louisiana rice production. The level of severity can vary from very light pressure to heavy pressure. However, blast disease in south Louisiana rice production during the 2012 growing season was exceptionally high. I have been working with rice for the LSU AgCenter since 1982, and during the previous 31 growing seasons, I have never witnessed the amount of blast disease present this year.
Why so much blast disease this year? While we will probably never know the definitive answer, we think it is related to the very mild winter in the region last year (2011-2012). We never had temperatures low enough to kill summer annual vegetation, including rice. Therefore, much of the regrowth rice in crawfish ponds, as well as many summer annual grasses, continued to grow through the winter. We certainly know that the blast organism can infect rice in the crawfish ponds, and we speculate that the same organism can infect a number of summer annual grasses in the region. So, without a killing freeze, many of these plants probably provided a favorable environment for the blast organisms to survive and actually increase over the entire winter.
In contrast, in a typical winter with several killing freezes, while the organism will survive the winteras mycelium and spores on infected straw and seed, its population numbers are greatly reduced by springtime. However, as we were planting our rice crop this past spring, we had substantial populations of the organism present to infect susceptible rice plants as they reached the tillering stage. Southwest Louisiana also experienced a warm, wet spring, which was conducive to development of this disease. The fact that the organism is highly windborne also facilitated development and spread of the disease.
During the rice blast epidemic of 2012, we learned a great deal about several of our rice varieties’ resistance and susceptibility to the disease. We learned that a major variety, CL151, was more susceptible to the disease than we expected. However, several of our varieties were more resistant than expected. CL111 and Catahoula are good examples.
Dr. Don Groth does an outstanding job working with rice breeders in evaluating potential new varieties for susceptibility to all of our major rice diseases, including blast. To create high levels of blast disease, Dr. Groth plants very susceptible varieties (called spreaders) around his blast disease evaluation nursery. He typically uses M-201 and M-202, which are California-developed varieties with known high levels of susceptibility to the disease. With a favorable environment, the spreaders will develop high levels of blast disease early in the growing cycle, which the wind moves to all of the experimental lines under evaluation. Ideally, all of the experimental lines will be subjected to the blast disease organism, and those with good resistance will show little disease development; those with poor resistance will show a great deal of disease development.
Though Dr. Groth’s evaluation system is good, a nearly perfect system for evaluating lines for resistance or susceptibility was provided by this year’s massive blast pressure. This really allowed us to evaluate resistance or susceptibility of all the breeding populations planted on the Rice Station this summer. Based on these evaluations, we eliminated many lines based on their susceptibility to the disease. We also have a great deal of confidence that those lines that showed resistance were in fact very highly resistant, at least to the races present this summer.
Thus, while the blast epidemic caused significant issues in southwest Louisiana rice production in 2012, it also provided an excellent opportunity for breeders to screen for resistance to the disease. Ultimately, we feel this will allow us to deliver more varieties with high levels of resistance to the disease in the future.
Permission granted November 15, 2012 by B. Leonards (LA Farm & Ranch) to republish article on www.lsuagcenter.com