Background and literature

For estimating disease 

One of the percentage scales more commonly used to evaluate severity of potato late blight is the ‘modified blight rating system’ developed by W.E. Fry and co-workers (unpublished data) (Table 5). This rating system was based on data reported by Fry (7), James (10), and the British Mycological Society (4). The methodology to estimate late blight severity consists in dividing the plot (= experimental unit) in small quadrants, each containing 20 to 25 plants. The percentage of infected tissue is then estimated in each quadrant according to Table 5, and averaged to obtain the estimated severity in the plot. The number of quadrants to be evaluated varies according to plot size: in 4.5 x 4.5 m plots containing 100 plants, the number of quadrants is four (7), i.e., 80 to 100 plants are evaluated. In bigger plots, the number of quadrants could be lower depending on the variability in disease distribution: more variability, more quadrants. This scale was developed for epidemics in which the first symptoms appeared when plants had about 200 leaves (7) (approximately two-month old plants), but it has been used successfully with younger plants (1, 3). The modified blight rating system may be used just as a reference, because there is evidence suggesting that it is better to simply estimate the percentage of foliage (everything green: stems, leaves, etc.) which is affected by disease rather than using some kind of scale (6).

Table 5. Modified blight rating system (W.E. Fry and co-workers, unpublished data). See text for references.

For AUDPC and DPC

Severity evaluation should start before the initiation of disease. For certain type of experiments (validation of LATEBLIGHT, a potato late blight simulator [2]), it is critical to have an accurate estimation of the time when disease started and initial severity (3). Evaluations should be done at least once a week and continue until senescence of the plants.

The severity evaluations are used to graph the disease progress curve (DPC) and to calculate the area under the disease progress curve (AUDPC). The DPC has being called the ‘signature’ of the epidemic, because it is critical for analyzing, comparing, and understanding epidemics (5). The DPC is disease severity (or incidence, depending on the disease) on the y-axis plotted on a measure of time (e.g., days) on the x-axis. The AUDPC (14) is a key variable in the analysis of the epidemiology of many plant diseases. For example, AUDPC has proved to be a reliable measure to estimate the effect of fungicide, host resistance (8) or pathogen fitness (15) on potato late blight epidemics. It is calculated with Eq. [1] (5):

                                                                                                                                            [1]

where y_i = disease severity (% or proportion) at the i^th evaluation, t_i = time (days) at the i^th evaluation, and n = total number of evaluations. The units of AUDPC are percent-days or proportion-days (5).

We will use data from an experiment carried out in Peru in 1999 to learn how to graph DPCs and how to calculate AUDPCs. The experiment had two factors (potato cultivars and fungicide applications) each with two levels (cultivars: Amarilis and Tomasa; fungicide applications: no fungicide and fungicide every seven days) resulting in four treatments. The number of repetitions was four.

 All about disease scales

 Many phytopathological studies and routine activities in crop improvement depend on assessments of disease severity and/or incidence. Disease severity is defined as the ‘area of a sampling unit (plant surface) affected by disease, expressed as a percentage or proportion of the total area’, while disease incidence is defined as ‘number or plant units sampled that are diseased expressed as a percentage or proportion of the total number of units assessed, e.g., proportion (percentage) of plants diseased in a population’ (13). Severity has been quantified with sophisticated equipment (e.g. (12), but these procedures are normally too expensive or too labor-intensive for large-scale disease evaluations. For this reason, severity of foliar diseases is generally estimated visually.

To improve visual estimation of disease severity, scales have been developed for numerous diseases (see Campbell and Madden (5)). Different scales can be compared based on the accuracy and precision of their estimates. Accuracy is defined as the ‘measure of the closeness of an estimate (disease assessment) to the true value’, and precision is defined as a ‘measure of reliability and/or repeatability of disease assessments’ (13). Hau (cited in Campbell and Madden (5)) proposed the following method to determine the accuracy and precision of disease severity estimates: plant units with known severity are assessed by an evaluator with no previous knowledge of the actual severity. The estimates are then regressed on the actual values with a no-intercept model (regression through origin). The slope of the regression represents the accuracy: the closer to 1.0, the more accurate the estimation made by the evaluator. Precision is in turn directly related to the coefficient of determination (r²).

Severity of potato late blight is routinely evaluated using two scales: a Horsfall-Barratt scale (Table 1) and direct percentage (6). Horsfall and Barratt (9), through application of the Weber-Fechner law to visual assessment of disease severity, proposed that the human eye would estimate high and low disease severities with greater precision than mid range severities. To correct this problem, they suggested that scale increments should be logarithmic rather than linear. On the other hand, the percentage scale estimates disease severity directly. Forbes and Korva (6) compared a Horsfall-Barratt scale and direct percentage on potato late blight under field conditions and found that evaluators tended to linearize the Horsfall-Barratt scale and, therefore, direct percentage estimation was more accurate.

 Table 1. Scale formerly used in the International Potato Center to estimate severity of potato late blight (6). This is a typical Horsfall-Barratt scale.