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A standard area diagram (SAD) set to aid assessment of the severity of coffee leaf rust (Hemileia vastatrix) was prepared and validated, and the characteristics of the rust lesions on Coffea arabica and C. canephora leaves were compared. The results indicated that the lesions were similar, so one scale could be used to evaluate the severity of rust on both species. The proposed SAD set contains illustrations of leaves with six disease severities (2AE5, 5, 10, 20, 40 and 80%). The SAD set was validated by 10 raters with no previous experience of disease evaluation. The severity of rust was first estimated without using the SADs, and the same raters assessed the leaves again using the SADs. Regression analysis and Lin's concordance correlation (q c ) analysis of estimated against actual disease severity showed precision and accuracy was significantly better using the SADs, for most raters, with improvements in both the slope and intercept of the regressions, and the values of u, t (components of accuracy and precision of the q c ) and q c improved to 60, 70 and 90% of raters, respectively, when the SADs were used. The absolute error was £38% without the SADs, and £22% with them. Severity estimates were more reliable using the SADs (R 2 = 0AE70-1AE00 unaided, and R 2 = 0AE80-1AE00 using the SADs). The SAD set improved rater assessments for the estimation of arabica and conilon rust severity, and can be used for assessing rust severity for many purposes, including plant breeding, fungicide screening and pathotype characterization.

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Soybean rust (SBR), caused by Phakopsora pachyrhizi, is the most important yield‐damaging fungal disease of soybean due to severe reduction in healthy leaf area and acceleration of leaf fall. In experimental research, SBR severity is estimated visually aided/trained by a standard area diagram (SAD) developed and validated during the mid‐2000s (Old SAD). In this study, we propose a new SAD set for SBR with six true‐colour diagrams following linear increments (c. 15% increments) amended with four additional diagrams at low (<10%) severities, totalling 10 diagrams (0.2%, 1%, 3%, 5%, 10%, 25%, 40%, 55%, 70%, and 84%). For evaluation, 37 raters were split into two groups. Each assessed severity in a 50‐image sample (0.25%–84%), first unaided and then using either the Old SAD or the New SAD. Accuracy, precision, and reliability of estimates improved significantly relative to unaided estimates only when aided by the New SAD (accuracy >0.95). Low precision (<0.78) and a trend of underestimation with an increase in severity were the main issues with the Old SAD, which did not differ from unaided estimates. Simulation to evaluate the impact of the errors by different methods on hypothesis tests, showed that the new SAD was more powerful for detecting the smallest difference in mean control (e.g., 70% vs. 65% disease reduction) than the Old SAD; the latter required a 2‐fold increase in sample size to achieve the same power. There is a need to improve some SADs, taking advantage of new knowledge and technology to increase accuracy of the estimates, and to optimize both resource use efficiency and management decisions.

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Three field experiments were carried out to assess the level of resistance of several cultivars to early blight (EB) and to examine the association between host resistance and either foliage maturity or tuber skin types. A total of 26 cultivars were used in Exps. 1 and 2, and 24 in Exp. 3. Plants were inoculated with isolates of Alternaria grandis at 31 days after planting. EB severity was quantified in each plot every seven days. The approach to determine the resistance levels of potato cultivars was based on multivariate analysis techniques. The tested cultivars responded as either resistant, moderately resistant, moderately susceptible or susceptible to EB. Most of the cultivars were classified as susceptible or moderately susceptible to EB. Resistant cultivars were mid-season, mid-late or late maturity. None of the susceptible cultivars were later maturity (mid-late or late maturity). In most cases, susceptible cultivars were earlier maturity (early or mid-early maturity). Most resistant cultivars had rough, mid-rough or smooth skin. None of the susceptible cultivars had rough skin. In most cases, susceptible or moderately susceptible cultivars had smooth skin. Obtaining potato cultivars that are resistant to this destructive disease will help reduce production costs and the need for costly fungicides.