Relationship of Meteorological Factors and Plant Growth with Infection Rate of Spot Blotch in Wheat^*

Mean apparent infection rates (‘r’) of spot blotch, caused by Cochliobolus sativus in wheat varieties HP 1633, HD 2285, Sonalika, A-9-30-1, HUW 234 and NW 1014 between jointing to booting, booting to soft dough and soft dough to pre-maturity stages were noted to be lower in the first year (0.0875) in comparison to second (0.0984), when minimum-maximum temperatures (14.08–31.5°C), relative humidity (45.6–87.9%) and rainfall (1.37 mm) were higher over that of first year (14.0–31.4°C, 38.1–72.8% and 0.0 mm). Apparent infection rates between jointing to booting and soft dough to prematurity stages were higher for the second year (0.0613 & 0.0924) over first (0.0331 & 0.0698) as against higher ‘r’ in the first year (0.1546) over second (0.1419) between the growth stages booting and soft dough, which related to higher maximum temperature, other factors being optimum. Apparent infection rates (varietal mean) between jointing and pre-maturity stages showed non-significantly positive correlation. Plant growth stage booting to soft dough showed maximum susceptibility as exemplified by ‘r’ of 0.1596 in the first and 0.1419 in the second year as against 0.0698 and 0.0924 for soft dough to pre-maturity and 0.0331 and 0.0613 for jointing to booting stages in the respective years. Based on partial regression coefficient, linear equations were derived to predict ‘r’ for spot blotch depending upon weekly meteorological factors prevailing at the time. The highest values of coefficient of determination of 98% in the first year and 99% in the second were for the highly spot blotch susceptible Sonalika and A-9-30-1, while lowest of 93% in the first and 94% in the second for least susceptible NW 1014.