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A specific analysis was therefore carried out to:

• compare four methods of crown canker assessment;
• test the among and within assessor repeatability of one of the methods compared;
• characterise the spatial pattern of the disease;
• define the sample size required to achieve a given level of disease assessment precision.

The methods compared examined the symptoms with different procedures and graded the plants observed into six severity classes. A disease index (DI) summarised the severity distribution observed. Examination of crown cross-sections was the most precise method for assessing crown cankers. The method was repeatable, though an 'assessor effect' was apparent. The disease generally had a random pattern although significant spatial correlations were detected for four out of the fifteen plots studied at the scales examined. A relationship between the coefficient of variation of the DI and the sample size was established, evaluated with experimental field data and exemplified for typical severity distributions.

Authors: Aubertot JN., Schott JJ., Penaud A., Brun H., Doré T.

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