A Trial of Estimating Number of Leaf Blast Lesions on Rice Plants on the Basis of the Number of Trapped Spores and Wetting Period of Leaves

Abstract

Experiments in order to know the possibility of leaf blast forecasting by investigation of the number of trapped spores and the wetting period of leaves were conducted. The number of spores which was presumed to penetrate into leaves was calculated from the following equation by investigating the number of trapped spores, wetting period of leaves, leaf area, plant height and wind velocity in the experimental field.
PSN=DSN×RA×MIP×RI
DSN=CSN×C×LA/GA
PSN: The number of spores which was presumed to penetrate into leaves
DSN: The number of spores which was presumed to deposit on leaves
RA: The rate of spores which formed appressoria
MIP: The maximum rate of appressoria which succeeded in penetration under the most favourable condition
RI: The ratio of penetrated appressoria to MIP in each day which was calculated by using the wetting period of leaves and the mean temperature during the wetting period
CSN: The number of trapped spores was corrected corresponding to wind velocity and plant height
C: Correction coefficient
LA: Leaf area
GA: Cover glass area
As a result of calculation, the largest number of spores which was presumed to penetrate into leaves per plant was 3.95 on July 26 and the next was 3.70 on June 29. On July 23 the largest number of spores trapped in the period of investigation, however, the number of spores which presumed to succeed in penetration was only 0.85.
The numbers of lesions which were estimated by adding PSN in order of date almost coincided with the numbers of lesions occurred actually in the experimental field.
These facts indicate that the degree of leaf blast severity can be forecasted one week before lesion appearance by investigating the number of trapped spores and the wetting period of leaves.