Observers' bias in the assessment of pest and disease symptoms in leek

In the vegetable crop leek, Allium porrum L., the performance of observers was compared to the real situation of pest and disease infestation. In two experiments the infestation of plants in a number of rows was meticulously investigated thus reflecting the real situation as contrasted with the results of one or more observers. Included were the symptoms of leek moth (Acrolepiopsis assectella Zeller) feeding, both fresh and old, of onion thrips (Thrips tabaci Lindeman) feeding and of infestation with rust (Puccinia allii Rud.). The method of scouting was similar to growers' practice in commercial leek growing. The results show that even trained scouts vary considerably in accuracy when assessing infestation in rows at the same field. A group of observers show individual patterns of estimation, a wide variation in ability to make a correct assessment and a strong density dependencey of deviations of the real situation with the perceived density of the symptoms in the crop.