Standard Area Diagram Research Articles

Assessing disease severity: accuracy and reliability of rater estimates in relation to number of diagrams in a standard area diagram set

Error in estimates of plant disease severity occur and standard area diagrams (SADs) help improve accuracy and reliability. The effects of diagram number in SADs are unknown. The objective of this study was to compare estimates of pecan scab severity made without SADs, and using three‐, five‐, seven‐ or 10‐diagram SADs. Disease severity was estimated to the nearest percent (NPE), or classified to the closest reference diagram value using a scale. Twelve raters assessed 20 images of scabbed pecan valves with and without the SADs using NPEs and the scale method (values were converted to midpoints prior to analysis). Increases in diagram number using NPEs did not necessarily result in more accurate or reliable estimates. Inter‐rater reliability was positively correlated with number of diagrams using NPEs (r = 0·3288 (P < 0·0001)) or a scale (r = 0·2803 (P < 0·0001)). The least accurate estimates improved the most using SADs with NPEs, but the gain did not relate to number of diagrams; as few as three diagrams reduced error of inaccurate estimates as much as five, seven or 10 diagrams. When used as a scale, only estimates made with 10‐diagram SADs had similar accuracy and inter‐rater reliability to estimates made using SADs with NPEs. Maximum disease severity and the disease severity range for the pathosystem and the SADs are important factors and will probably influence resulting accuracy and reliability. The ramifications of diagram numbers in SADs and assessment methods on accuracy and reliability of disease estimates are discussed.

Development and validation of a standard area diagram set to aid estimation of bacterial spot severity on tomato leaves

A standard area diagram (SAD) set to aid assessment of bacterial spot (caused by Xanthomonas spp.) severity on tomato (Solanum lycopersicum L.) leaves was prepared and evaluated for improving accuracy, precision and reliability of visual estimates. The proposed SAD set contains illustrations of leaves with 12 distinct disease severity values from 0.5 to 90 %, and was validated by 12 raters without experience of plant disease evaluation. The severity of bacterial spot on 150 images of diseased tomato leaves was first estimated without the SADs, and then assessed using the SADs by the same group of raters. Regression analysis and Lin’s concordance correlation (ρ c ) analysis of estimated versus true disease severity (based on image analysis) showed precision and accuracy improved for all raters using the SADs. The SAD set improved accuracy (correction factor, Cb = 0.91 and 1.00, without and with the SADs, respectively) and agreement (ρ c = 0.83 and 0.96, without and with the SADs, respectively) of the estimates of severity. Severity estimates were more reliable using the SADs (coefficient of determination, R 2 = 0.74–0.94 without the SADs, and R 2 = 0.82–0.95 with the SADs, and intra-class correlation ρ = 0.85 without the SADs and ρ = 0.91 using the SADs). The SADs proposed in this study will improve the accuracy and reliability of estimates of bacterial spot severity on tomato leaves for inexperienced raters, and could be used for assessing other bacterial disease severity on tomato leaves.

A set of standard area diagrams to assess severity of frogeye leaf spot on soybean

A set of standard area diagrams (SADs) was developed and validated to aid visual assessment of severity of frogeye leaf spot (FLS) caused by Cercospora sojina. The SAD has eight color images of diseased leaflets with severity values that ranged from 0.1 to 39.9 %. The SAD was validated by a group of 20 raters [10 experienced (ER) and 10 inexperienced (IR)], who assessed the same set of 50 images twice, the first without SADs and the second using SADs as an aid. The SADs significantly improved accuracy [coefficients of bias (C b ) were 0.64 and 0.99 for IR and 0.98 and 0.99 for ER, without and with SADs, respectively], precision [correlation coefficients (r) were 0.89 and 0.95 for IR and 0.94 and 0.97 for ER, without and with SADs, respectively] and overall agreement [Lin’s concordance correlation coefficients (ρ c ) were 0.57 and 0.94 for IR and 0.92 and 0.97 for ER without and with SADs, respectively]. The estimates of severity of FLS were more reliable when using SADs. Both the inter-rater reliability (coefficient of determination, R 2) and intra-class coefficient (ρ) were significantly increased by using SADs. Therefore, it is believed that the SADs proposed in the present study will be a useful tool to aid accurate, precise and reliable estimates of severity of FLS in experiments (e.g., fungicide screening, assessment of partial resistance of soybean genotypes to FLS) and to aid in decision-making purposes.

Development and Validation of Standard Area Diagrams as Assessment Aids for Estimating the Severity of Citrus Canker on Unripe Oranges.

Asiatic citrus canker (ACC) is an important disease of citrus in Brazil and elsewhere in the world. Infection with the causal pathogen, Xanthomonas citri subsp. citri, can cause severe disease on the fruit. Visual estimation of severity is the usual method used to quantify ACC on diseased fruit. The objective of this research was to construct and validate standard area diagram (SAD) sets as assessment aids for raters to improve the accuracy and reliability of visual estimates of ACC on unripe (green) fruit of sweet orange. Two SAD sets were constructed. A five-diagram SAD set had five severities depicted (0.5, 2.0, 8.0, 27.0, and 40.0%) and a six-diagram SAD set had six severities depicted (0.5, 1.0, 3.0, 9.0, 20.0, and 40.0%). Fifteen raters evaluated 40 images of cankered, unripe fruit. Both the five- and six-diagram SAD sets significantly improved the accuracy and reliability of estimates. Agreement, measured by Lin's concordance correlation coefficient, was 0.220 to 0.913 when not using SADs, 0.814 to 0.955 when using five-diagram SAD sets, and 0.863 to 0.925 when using six-diagram SAD sets. The five-diagram SAD set was significantly more accurate and reliable compared with the six-diagram set. Possible reasons for this are discussed. Based on the results, the five-diagram SAD set is preferable to use. Although the SAD set was developed for sweet orange, it doubtless has applicability to other citrus, including grapefruit. These SAD sets should be useful for research endeavors where accurate and reliable estimates of the severity of ACC are required.

A standard area diagram set to aid estimation of the severity of Asiatic citrus canker on ripe sweet orange fruit

Asiatic citrus canker (ACC) is a major disease of citrus in many tropical and subtropical citrus-growing regions. Severe disease can develop on the fruit, making it unmarketable. Research activities need accurate and reliable methods to estimate the severity of citrus canker, most often done visually by raters. The objective was to validate and compare two standard area diagram sets (SADs) as aids for raters to improve the accuracy and reliability of visual estimates of canker severity on ripe fruit of sweet orange (unripe fruit are green, and have slightly different symptoms compared to ripe fruit, warranting different SADs). A 5-diagram SAD set (0.7, 2.0, 7.0, 21.0 and 39.0 %) and a 6-diagram SAD set (0.7, 2.0, 4.0, 10.0, 21.0 and 39.0 %) were compared; the 6-diagram set had an additional diagram in the most common severity range. Fifteen raters evaluated 40 images of cankered, unripe fruit. Both the 5- and 6-diagram SADs improved the accuracy and reliability of rater estimates. Agreement, measured by Lin’s concordance correlation coefficient (ρ c ) was 0.735 to 0.906 when not using SADs, 0.931 to 0.985 when using 5-diagram SADs, and 0.879 to 0.979 when using 6-diagram SADs. Mean inter-rater reliability (R2) was 0.638 when not using SADs, 0.889 when using 5-diagram SADs, and 0.834 when using 6-diagram SADs. The 5-diagram SADs resulted in significantly more accurate and reliable estimates compared with the 6-diagram set, and is thus recommended for use. The SADs should have application for estimating ACC severity on ripe fruit of other citrus, including grapefruit.

Standard area diagrams for assessment of powdery mildew severity on tomato leaves and leaflets

Two standard area diagrams (SADs) to assess the severity of powdery mildew (Leveillula taurica) on tomato leaves and leaflets were developed and evaluated. The SADs are composed of two sets of images of leaves and leaflets with six distinct percentages of diseased areas (1, 5, 10, 20, 40 and 60%). Diagrams were evaluated by 16 raters divided into two groups, based on their former experience in assessing plant disease intensities, using the two sets of images, first without SADs and then using the proposed SADs. Lin's concordance correlation analysis of estimated vs. actual disease severities showed that precision and accuracy were clearly improved when raters were aided by the proposed diagrams, for both leaflets and leaves. Based on coefficients of determinations (R2) and on the intra-class correlations (ρ), the estimates of severity were consistently more reliable when using SADs. Furthermore, when the scales were adopted, the performance of the inexperienced group improved to the same level of that of the experienced group. The diagrams improved subjects' ability to accurately, precisely and reliably estimate tomato powdery mildew severity, and as such can be used to assess severity for studies in epidemiology, quantitative host resistance and in the evaluation of management practices in this important pathosystem.

Development and validation of a standard area diagram set to aid assessment of severity of loquat scab on fruit

A standard area diagram set (SAD) to aid visual assessment of loquat scab (caused by Fusicladium eriobotryae) severity on fruit was developed and evaluated for improving accuracy, precision and reliability of visual estimates. The SAD set contains eight black and white diagrams of diseased fruit with severity values from 2 % to 98 %. To evaluate the SADs, a group of 20 raters (comprising 10 ‘experienced’ and 10 ‘inexperienced’ raters) assessed the same set of 50 images three times, the first without SADs and the second and third using the SADs as an aid. Only for the group of inexperienced raters did SADs significantly improve accuracy (bias correction factor, C b = 0.93 without SADs and 0.98 with SADs), precision (correlation coefficient, r = 0.88 without SADs and r = 0.96 with SADs) and overall agreement (Lin’s concordance correlation coefficient, ρ c = 0.82 without SADs and ρc with SADs = 0.95) of the estimates. Accuracy and precision of the estimates by inexperienced raters were significantly higher than those obtained by the experienced raters, especially for the second assessment with SADs. Inter-rater reliability was improved when SADs were used by inexperienced raters, whereas a high degree of intra-rater reliability was obtained by both experienced and inexperienced raters when using SADs. The SADs developed in this study were useful for obtaining more accurate, precise and reliable assessments of loquat scab for inexperienced raters, and should be used as an aid for assessing scab in epidemiological studies or monitoring for decision-making purposes.

Development and validation of a set of standard area diagrams to aid in estimation of spot blotch severity on wheat leaves

This study aimed to develop and validate a standard area diagram (SAD) set to quantify the severity of spot blotch, caused by Bipolaris sorokiniana, on wheat leaves. The proposed SAD set includes images of leaves with 11 distinct disease severities (0·1, 1, 5, 10, 20, 30, 40, 50, 60, 70 and 83%). The SAD set was validated by 12 raters without experience in evaluating plant disease. Lin's concordance correlation analysis of estimated versus actual disease severity (based on image analysis) showed that precision and accuracy improved for all raters using the SAD set in contrast to assessments made without it. The SAD set improved accuracy (coefficient of bias, Cb = 0·88 and 0·99, without and with the SAD set, respectively) and agreement (Lin's concordance correlation coefficient, ρc = 0·81 and 0·96 without and with the SAD set, respectively) of the estimates of severity. The severity estimates were also more reliable when using the SAD set (coefficient of determination, R2 = 0·76 unaided and R2 = 0·92 with the SAD set, and intra‐class correlation ρ = 0·79 without the SAD set and ρ = 0·95 using the SAD set). The SAD set proposed in this study will improve the accuracy and reliability of estimates of spot blotch severity on wheat leaves.

Development and validation of a set of standard area diagrams to estimate severity of potato early blight

Standard area diagrams (SADs) to assess the severity of potato early blight (Alternaria grandis) on leaves of potato (Solanum tuberosum L.) were developed and validated. The proposed SADs include images of leaves with 10 distinct disease severities (0.1, 1, 3, 5, 10, 20, 40, 60, 80 and 100 %). The SADs were validated by 12 raters who had no previous experience in evaluating plant disease. Lin’s concordance correlation analysis of estimated vs. actual disease severity (based on image analysis) showed that precision and accuracy improved for most raters using the SADs, compared to assessments made without the SADs. The SADs improved accuracy (coefficient of bias, C b = 0.97 and 0.99, without and with SADs, respectively) and agreement (Lin’s concordance correlation coefficient, ρ c = 0.91 and 0.98 without and with SADs, respectively) of the estimates of severity. Severity estimates were more reliable when using SADs (coefficient of determination, R 2 = 0.80 unaided and R 2 = 0.95 with SADs, and the intra-class correlation ρ = 0.86 without SADs and ρ = 0.97 using the SADs). The SADs improved raters’ ability to accurately, precisely and reliably estimate potato early blight severity, and as such can be used to assess severity for several purposes, including breeding for resistance, fungicide screening, and pathotype characterization.

Development and validation of a standard area diagram set to assess blast severity on wheat leaves

This study aimed to develop and validate a standard area diagram set (SADS) to quantify the severity of blast, caused by Pyricularia oryzae, on wheat leaves. The SADs has ten levels: 0.1, 1, 5, 10, 22, 32, 42, 52, 62 and 72 % blast severity. To validate the SADs, 12 inexperienced raters estimated disease severity on 50 images of leaves from cultivars BR-18 (susceptible) and BRS-229 (partially resistant). Blast severity was first estimated without the use of the SADs on 50 leaves with a range of blast severity. The same raters evaluated the same 50 leaves using the SADs as an aid. The SADs improved accuracy (coefficient of bias, C b = 0.88 and 0.99, without and with SADs, respectively) and agreement (Lin’s concordance correlation coefficient, ρ c = 0.84 and 0.96 without and with SADs, respectively) of the estimates of severity. The absolute error was (-) 52 % without the SADs and (-) 24 % when using SADs as an aid. Severity estimates were more reliable when using SADs (R2 = 0.87 unaided and R2 = 0.92 with SAD). The SADs proposed in this study will improve accuracy and reliability of estimates of blast severity on wheat leaves.

Development and validation of standard area diagrams to aid assessment of pecan scab symptoms on fruit

Pecan scab (Fusicladium effusum) causes losses of pecan nutmeat yield and quality in the southeastern United States. Disease assessment relies on visual rating, which can be inaccurate and imprecise, with poor inter‐rater reliability. A standard area diagram (SAD) set for pecan scab on fruit valves was developed. A set of 40 images of diseased fruit valves with known severity was assessed twice by 23 raters. The first assessment was conducted without SADs, and the second assessment was made using the SADs as an aid. SADs improved rater accuracy (correction factor, Cb = 0·86 and 0·97, without and with SADs, respectively) and agreement (Lin’s concordance correlation coefficient, ρc = 0·79 and 0·89 without and with SADs, respectively) with true values. SADs improved inter‐rater reliability (intra‐class correlation coefficient, ρ = 0·77 and 0·96 without and with SADs, respectively). The least accurate and precise raters without SADs improved more using SADs compared to the most accurate and precise raters. Experienced raters had significantly higher accuracy and precision compared to inexperienced raters, but only when unaided by the SAD set. There was no significant difference in time to assess images without SADs, but experienced raters using SADs were faster compared to inexperienced raters. There was a slight tendency for faster raters to assess more slowly, and slower raters to assess faster when using SADs. SADs improve rater estimates of pecan scab severity on fruit, and this SAD set should be useful for assessment where greater precision, accuracy and inter‐rater reliability are required.

Development and validation of a standard area diagram set to estimate severity of leaf rust in Coffea arabica and C. canephora

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 (2·5, 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 (ρc) 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, υ (components of accuracy and precision of the ρc) and ρc 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 (R2 = 0·70–1·00 unaided, and R2 = 0·80–1·00 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.

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

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.

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

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

Improving sooty blotch and flyspeck severity estimation on apple fruit with the aid of standard area diagrams

Sooty blotch and flyspeck is caused by numerous species of fungi that colonize the surface of apple fruit and thereby lower its market value. Although this disease poses a substantial threat to apple growers’ profitability in some regions, reliable and cost-effective methods for epidemiological and disease control studies have not been validated, nor are they widely available. We modified a standard area diagram to aid sooty blotch and flyspeck severity assessments and quantified its impact on accuracy and precision of visual estimates. Samples of ‘Fuji’ and ‘Mutsu’ fruit were photographed both from the top and laterally. Severity was assessed from a sub-sample of 160 images using image analysis software. Validation of the diagram was performed by eight raters who independently assessed severity in two series of selected images representing the lateral view and the top view, initially unaided and subsequently with the aid of the scale. Severity estimates ranged from 0.4% to 98% (most fruit had <10% severity). Accuracy and precision of the estimates were significantly improved when using the diagrammatic scale; concordance correlation coefficient values increased from 0.81 to 0.95. A strong tendency to underestimate severity for the mid-range to high levels was minimized when using the aid, which also improved reproducibility of the estimates among raters. In addition to strengthening evidence that a standard area diagram can be used reliably in sooty blotch and flyspeck studies, we expanded its application to disease assessment in the peduncle region, which enhances the usefulness of the method for evaluating efficacy of management practices.

Frogeye Leaf Spot of Soybean Caused by Cercospora sojina in Northwestern Argentina

Frogeye leaf spot of soybean (Glycine max (L.) Merr.), caused by Cercospora sojina Hara, was first detected during the 1997-98 growing season at low incidence and severity (<1% of the leaf diseased) levels in the provinces of Tucumán, Salta, Jujuy, Catamarca, and Santiago del Estero in northwestern Argentina. During the 1998-1999 growing season, disease incidence increased and disease severity grew to 10% of the leaf surface diseased on highly susceptible cultivars in a few locations. An outbreak of frogeye leaf spot occurred throughout northwestern Argentina during the 1999-2000 growing season. Frogeye leaf spot was severe on susceptible cultivars in the provinces of Salta, Santiago del Estero and Catamarca with the greatest intensity in the northeastern part of the Province of Tucumán. Symptoms on leaves were circular lesions that ranged in size from 1 to 5 mm, were reddish-brown to gray or tan, and were bordered by a narrow, reddish-brown to purple margin. Conidiophores and conidia of C. sojina developed on the abaxial leaf surface (1,2). Severely diseased leaves were desiccated and dropped during the R6 stage of growth. Lesions also developed on stems, pods, and seeds. Field surveys indicated that this disease reduced the yields of the highly susceptible cultivars Anta 82 RR, Coker 6738, and A 6445 RG by 48, 34, and 25%, respectively. C. sojina was cultured from diseased tissue on PDA acidified with 0.2% lactic acid and maintained on V-8 juice agar amended with streptomycin sulfate (100 mg/l). Conidia were elongated, dark, 38 to 62 × 5 to 9 μm, with 2 to 6 septa, and borne on dark conidiophores with 1 to 4 septa. Pathogenicity tests were conducted on seedlings of the susceptible cultivars A 6445 RG and Coker 6738 and on the resistant cultivars A 8000 RG and Shulka. Seedlings were inoculated at the V3 growth stage by spraying the leaves with a conidial suspension (4 × 104 conidia/ml) using a hand-held atomizer. Control plants were sprayed with sterile distilled water. Plants were placed in a moist chamber at 26°C for 2 days and then transferred to a greenhouse bench where they were kept at 25 to 30°C. Symptoms identical to those observed in the field became visible after 7 to 10 days. Ratings were made 14 days after inoculation by estimating the percentage of leaf area affected using a standard area diagram. Lesions covered 60 to 65% of the leaf area of susceptible cultivars, but less than 2% on resistant cultivars. Control plants remained healthy. C. sojina was reisolated from lesions on leaves of susceptible plants. Above-average rainfall and high relative humidity in northwestern Argentina during the first three months of 2000 may have encouraged the severe outbreak of frogeye leaf spot of soybean. The outbreak was aggravated by the widespread use of notillage systems in the region and the large hectarage planted with susceptible cultivars.

A Pictorial Series of Disease Assessment Keys for Bacterial Leaf Streak of Cereals

A series of disease assessment keys based on the percent affected leaf area at flowering stage is proposed as a tool to better evaluate bacterial leaf streak symptoms in barley, bread wheat, durum wheat, triticale, and rye. Streak symptoms are more distinct in barley, triticale, and rye than in wheat. The series of standard area diagrams is based on disease patterns on leaves from naturally infected fields and shows that symptoms progress from the central part of the leaf, where free water remains longer in the morning, and not only from the tips of the leaves downward, as suggested previously. Also, the study of bread wheat genotypes that vary in resistance to bacterial leaf streak indicates that stripes are a bit more conspicuous in susceptible genotypes than in resistant ones

Impact of Cercospora Leaf Spot on Root Weight, Sugar Yield, and Purity ofBeta vulgaris

The impact of Cercospora leaf spot, caused by Cercospora beticola, on the yield and quality of sugar beets (Beta vulgaris) was studied in one field trial in 1982 and two trials in 1983. Fungicides were applied to cultivars of various disease susceptibilities to obtain different levels of disease severity. Plants were rated for disease severity with a spot-percentage scale according to the number of lesions per leaf at intensities <3% and with standard area diagrams at intensities ≥3% (.)

Studies of the incidence, severity and distribution of false rust (Synchytrium psophocarpi), leaf spot (Pseudocercospora psophocarpi) and powdery mildew (Oidium sp.) on winged bean (Psophocarpus tetragonolobus) in Papua New Guinea

SUMMARYThe incidence, severity and distribution of false rust, (Synchytrium psophocarpi), leaf spot (Pseudocercospora psophocarpi) and powdery mildew (Oidium sp.) on winged beans (Psophocarpus tetragonolobus) were evaluated on pure lines and mixed stands grown in Papua New Guinea between 1976 and 1978. Almost all of the 148 pure lines were susceptible to false rust and leaf spot and the incidence of these diseases increased with plant age. Standard area diagrams were superior to disease keys in evaluating disease severity. Levels of false rust on 10 promising agronomic lines differed according to the locality where the lines were grown. Two Thai lines showed resistance to false rust but P. scandens was immune to false rust and leaf spot.Surveys of 51 sites containing mixed stands of winged beans in the Eastern Highlands, Western Highlands, East New Britain, East Sepik, Madang and Morobe Provinces of Papua New Guinea indicated that leaf spot and false rust were present in both the wet and dry seasons and the apparent absence of disease was probably due to time and size of samples. The diseases appeared to be present in all the traditional winged bean growing areas of Papua New Guinea. It was difficult to obtain evidence of differences in palatability of tissues infected with false rust.The value of surveys and disease assessment in relation to evaluation of crop losses of winged bean caused by these pathogens and selection for resistance is discussed.