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.

Water management and rice blast

Any practice which leads to aerobic conditions in the soil predisposes rice plants to rice blast disease. Drill seeding and draining for stand establishment or herbicide applications in water seeded systems increase the risk of infection and susceptibility to rice blast. Rice plants grown in deeper water exhibit increased resistance to the disease over those grown in shallower water depths. This is apparent when we often see localized increased disease severities associated with high spots within a field. From an irrigation standpoint, maintaining a deep continuous flood is the best option for minimizing the risk associated with rice blast disease.

Nitrogen fertility management and rice blast

The impact of fertility management on rice blast disease susceptibility may be easily seen in affected fields where excessive levels of nitrogen have been applied. Excessive nitrogen fertilizer rates lead to increased host susceptibility, disease incidence, and disease severity. Areas where leaf blast is so severe that plants are actually killed often occur in aqua overlaps in fields planted to less tolerant varieties such as M104 and M205.

Plant stress and rice blast

Any form of stress may predispose rice plants to rice blast disease. Even slight stress may significantly alter the plant’s ability to tolerate infection by the pathogen. Nutrient deficiencies such as potassium and silica have been shown to significantly increase rice blast disease incidence and severity around the world. Other common stresses that may impact disease tolerance in California may include salinity, extreme temperatures, and herbicide injury. Managing the rice crop to avoid plant stress is a significant and often overlooked tool in minimizing risk associated with rice blast disease.

Converging factors and rice blast

As you can see there are many factors that influence the incidence and severity of rice blast in a specific field. I have only scratched the surface and there are many other factors that play a role. Rice blast is a very complicated disease that has the ability to increase in incidence and severity very rapidly under the right conditions. Growers need to manage for this disease in a holistic approach rather than just relying on fungicide applications for effective management. Little changes here and there in cultural practices can add significantly to the management of this disease.