One Thing After Another
Sometimes the California rice industry feels like its a member of the Disease of the year club. Our latest problem is bakanae, a fungal disease. Bakanae is one of the oldest known diseases to infect rice in Asia, but it was not observed in the U.S. or California until 1999. Since its introduction, bakanae has spread to most of Californias rice growing regions.
Dr. Robert Webster, UCD Pathologist, is confident that it was introduced by someone bringing it in on infected seed. It has complex reproduction and infection requirements that would not be met without an introduction. This is just a reminder that there is a quarantine procedure for any seed introductionand a good reason for it.
Current losses to bakanae in California are minor, but in Asia, losses of up to 70% have been reported in their transplanted culture. The extent to which bakanae could effect Cali-fornia rice production is currently unknown.
Bakanae is generally thought of as a seedling disease; however, it can be observed throughout the growing season. The earliest symptoms of bakanae are manifested roughly one month after planting. Infected seedlings appear to be taller, more slender and slightly chlorotic when compared to healthy seedlings. The rapid elongation of infected plants is due to the production of Gibberellin, a plant hormone, by the fungus. Bakanae plants are often visible arching above the healthy rice plants. As the season progresses, bakanae plants may die before reaching maturity, or if they do survive to heading, the panicles they produce are mostly empty. These classic symptoms, seedlings that rapidly elongate and then die without producing seed are where the disease gets its name, Bakanae, meaning foolish seedling.
As the diseased plants senesce and die, mycelium of the fungus may emerge from the nodes and sporulation of the pathogen may be easily observed above the water level. After the water is drained, the fungus sporulates profusely on the stems of diseased plants. This sporulation appears as a cottony mass and serves to contaminate the outside of healthy seeds during harvest.
The bakanae pathogen is known to over-winter as spores on the coat of infested seeds. Since infected plants produce only empty panicles, there is no evidence of internal infection of seeds. Unlike blast, which infects the interior of the seed, bakanae spores only infest the outside.
The fungus can over-winter in the soil and residue, and infection of non-infested seeds by spores in the soil has been demonstrated. The length of time that spores can survive in the soil is unknown. It is likely that infested seeds are the most important source of inoculum, leading to both infected plants in the field and also introduction of the disease to infested fields.
Bakanae Research for 2002
Your research dollars will be funding trials to find out what the best material and rate would be for a seed treatment. This work will involve both greenhouse and field trials. The extent of the distribution of the fungus will also be explored along with the variability of the pathogen population. This information will assist researchers in control efforts.
The primary means of infection is by spores on the outside of the seed. The extent that over-wintering soil inoculum contributes to the level of infection is not known and will be the subject of our research as well.