Methods for assessing resistance to bacterial blight
1989
Koch, M.
Infection by Xanthomonas campestris pv. oryzae can lead to three forms of disease symptoms, of which bacterial blight lesions are the most common. Three types of resistance can be discerned: (1) seedling resistance is expressed at all growth stages of the plant; (2) adult plant resistance is expressed only in adult plants; and (3) quantitative resistance is expressed as reduced susceptibility. The degree of symptom expression is assumed to reflect the degree of resistance. Symptom expression is assessed by estimating the percentage of diseased leaf area (% DLA), sometimes expressed as a class within a discrete scale. Two scales are often used - a 0-7 disease index scale and the 0-9 Standard Evaluation System scale. It is also possible to measure the length or area of the bacterial lesions. In the field, the frequency of hills, plants, or tiller with disease symptoms can be assessed. Proper assessment requires uniform exposure to the pathogen. If the whole plant or plot is exposed to the inoculum, as in the field or with spray inoculation, the % DLA assessment procedures are adequate. For more accurate comparisons, inoculation of specified leaves at specified plant stages is carried out. In such cases, % DLA assessment is inadequate, because it measures the ratio between diseased leaf area and total leaf area. Entries with large leaves would give a lower % DLA than entries with small leaves and so may be erroneously classified as more resistant. In this case (pin prick and clip inoculation methods), the lesion length or lesion area is a better assessment parameter.
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