Development and validation of a standard area diagram set for assessment of alternaria spot on the cladodes of the prickly pear cactus.

Abstract

Alternaria spot, caused by Alternaria tenuis, is an important disease of cladodes of the prickly pear cactus (Nopalea cochenillifera) in Brazil. There are no standard methods for assessment of the disease, so a standard area diagram set (SAD) was developed based on disease severity encompassing 2, 4, 7, 12, 22 and 40% of the cladode area spotted. SAD was validated by 10 raters, who analysed 50 cladodes with a range of disease severity, without and with SAD as an assessment aid. The accuracy, precision, reproducibility and repeatability of the estimates with and without the scale were determined using simple linear regression analysis. Without SAD the accuracy and precision of raters’ estimates were poor, with significantly constant and systematic bias, and determination coefficients (R2) ranging from 0.56 to 0.78. When raters employed SAD, significant reductions were observed in both constant and systematic bias, with R2 ranging from 0.91 to 0.96. In fact, raters achieved the highest repeatability (mean R2 = 0.95) and reproducibility (R2 =0.80 in all cases) of estimates when using SAD. The proposed SAD set is suitable for assessment of Alternaria spot of prickly pear cladodes and improves the accuracy and precision of assessments.