Rice bacterial leaf blight: field experiments, systems analysis and damage coefficients.

Bacterial leaf blight (BLB) is an important vascular disease in irrigated rice, that is difficult to manage and that can cause up to 50% yield reduction. A systems analytical approach was adopted to increase understanding of damage and to develop a tool that generates disease management recommendations. Field experiments were conducted at three locations in India and The Philippines during the dry and wet seasons from 1991 to 1995, to assess the consequences of various epidemics for growth and production. On the whole, higher and longer epidemics reduced amounts of green leaf area, leading to reduced total above-ground dry matter production and grain yield. Higher nitrogen application rates caused greater grain yields, but also greater grain yield reduction due to BLB. The BLIGHT crop simulation model, incorporating the effects of BLB on area and photosynthetic rate of green leaf area at three canopy levels, was successfully tested against three field experiments, and used in scenario studies to quantify the grain yield reduction at harvest due to a certain amount of disease spread earlier in the season. This reduction, defined as damage coefficient (DC), was determined for various crop development stages and for a wide range of environments, epidemics and leaf nitrogen contents. The 216 data sets resulting from the scenario studies were structured by approximation by the ‘hat’-curve, which is characterized by the maximum DC for a certain scenario, the crop development stage at which this occurs, and a scale parameter that determines the width of the curve. On the average, the DC reached a maximum of 266 kg grain ha1 leaf abou leaf about mid-way through grain filling. It is concluded that in addition to traditional disease management options such as use of tolerant cultivars and optimizing transplanting dates, careful nitrogen management is a possible tool to limit spread of the disease