The below mentioned article provides notes on diseases severity in wheat.
Several diseases rating scales has been proposed by various workers for evaluation of resistance in wheat and barley. Adlakha (1984) gave 0-5 disease severity scale which is less precise to differentiate the available wheat genotypes to foliar blight. Per cent leaf area scale of Conner (1990) and Hetzler (1992) is easy as well as precise but unable to integrate the severity at different growth stages of wheat.
Disease scoring at different interval for calculating area under disease progress curve is a better measure to define the genetic difference against spot blotch. The double digit scale of Sarri and Presscot (1975) is good to demonstrate the movement of the disease in a plant as well as to record the percent leaf area infected by the foliar blight.
This scale is widely adopted in many countries as such or with modification. This scale is good for the quick disposal of large numbers of germplasm. In many countries of south East Asia, foliar blight is crucial problem on flag and penultimate leaves.
For ease most of the disease data are collected on these two leaves in double digit system where first numerical value indicate the disease severity on flag leaf while the second numerical value show the disease intensity on penultimate leaf. However, this scale does not account the disease severity on other part of the plant i.e., sheath, nodes and spikes.
The rate of disease spread (r) is a good measure of resistance also based on % diseased severity is influenced by the initial disease scoring. It will not give good information if the disease severity on first day of scoring. Under this condition apparent infection rate will be always be low. Chaurasia (1999) evaluated 1387 wheat genotypes under field conditions by using AUDPC measurement.
They found low AUDPC with most of the resistance genotype. Sarma (2004) used double digit scale (00-99) of Saari and Prescott (1975) to assess the wheat foliar diseases. In double digits scale, first digit (D1) indicates the vertical progress of disease on the plant and the second digit (D2) refers to disease severity measured as diseased leaf area.
They further calculated the per cent disease severity that was based on the following formula.
% Severity = ((D1/9) x (D2/9) x 100
The Area under Disease Progress Curve (AUDPC) was calculated using the percentage disease severity estimates corresponding to three diseases rating as outlined by Shaner and Finney (1977) and Roelfs (1992):