The geographic distribution, incidence and severity of rice blast have been much greater in 2010 than in recent years. There are several factors that may be contributing to this situation. For any plant disease you must have a susceptible host, favorable environment, and the presence of the pathogen for disease to occur. All of these factors seem to have come together in 2010. At season’s end, I will evaluate the climatic data to tease out any conditions or trends that may have favored rice blast this year. Questions always arise as to why rice blast may be found in one field and not another. There are several factors that influence the susceptibility or tolerance of rice plants to rice blast disease.

Plant resistance and rice blast

First and foremost is the inherent resistance of a specific rice variety. Currently, M208 is the only variety commercially available in California with a specific resistance gene to the race of the pathogen identified here. Unless a new race of the pathogen is introduced into California or the current race mutates and evolves to overcome the resistance gene, M208 will maintain its resistance to rice blast in California. None of the other rice varieties grown in California have specific resistance to rice blast. These varieties do however differ in their tolerance to infection by the pathogen. M104 and M205 appear to be the least tolerant of the most widely grown commercial varieties, while M202 and M206 are somewhat more tolerant. It is not unusual to see areas where M104 and M205 plants have been killed entirely by leaf blast.

There are also differences in tolerance due to plant tissue age. Younger, more succulent rice tissue is much more susceptible to rice blast than is older tissue. For example, panicle neck nodes are much more susceptible to infection at emergence when tissue is green than they are once the panicle has tipped and started to ripen. One of my concerns in 2010 is that delayed planting coupled with a mild growing season may have combined with favorable environmental conditions to present a wider window for leaf blast infections to occur and produce copious amounts of infectious spores.