Symptoms of spot blotch caused by Bipolaris sorokiniana in wheat (Triticum aestivum) first appear as small, dark lesions onthe upper side of the leaf. As the lesions expand, they become oval with a dark brown center and a yellow hallo. Under favorable weather conditions, leaf lesions coalesce and kill large areas of the leaf surface. The effect of temperature on the lesion expansion rate of B. sorokiniana was examined in eight wheat cultivars. Digital images of lesions of B. sorokiniana for each temperature were captured in each cultivar and stored in an electronic format. A specific computer program, called AreaScan, was used to measure the area of individual lesions. Lesion growth ratewas not uniform among cultivars.The wheat cultivar BR 18 showed the slowest lesion growth rate. The cultivars Embrapa 15 and Embrapa 16 showed the fastest lesion growth rates, while BR 15, BR 23, BR 35, Embrapa 40, andEmbrapa 49 had a moderate rate. The relationship between temperature and lesion expansion rate was described by a mathematical function, such as y=(a+bx)/(1+cx+dx²), where y is the response, x the temperature, and a,b,c, and d the parameters. Nocturnal temperature of 23 ºC and diurnal of 30 ºC proved to be the most favorable for lesion expansion. Selecting wheat cultivars with partial resistance to spot blotch is strategic for growing wheat in tropical and subtropical regions. Finally, the methodological approach used in this study for determining lesion growth rate may play an important role for selecting for this genetic trait.
