Estimating leaf blast intensity in advanced growth stages of rice based on initial lesion counts

Abstract

The leaf blast (Pyricularia oryzae) at panicle initiation stage of rice growth was estimated about 40 days in advance, considering the increase of disease as function of time and initial inoculum. Data from five disease progress curves representing field conditions and three cultivars (‘IAC 1246', 'IAC 47' and 'Montanha Liso') during the period 1975/77 were analysed. The percentage leaf area affected with blast at the end of the logarithmic phase of disease progress curve was directly proportional to the initial inoculum measured on the basis of lesion numbers per leaf. Linear regression equations were derived for different levels of initial inoculum using percentage area affected with leaf blast at different dates during the second phase of disease progress curve in which blast did not exceed 35%. The difference between estimated percentage leaf area affected with blast, computed from the equations and observed disease in field plots at the panicle initiation stage was less than 5%, whereas at the boot stage it was around 10%. While estimated blast intensities at the panicle initiation stage was about 10% for initial inoculum levels varying from eight to 34 lesions/100 leaves, the disease intensity increased to 29% for the initial level of 278 lesions/100 leaves. These results indicated applicability of this method for forecasting blast by developing general equations for different initial inoculum levels.