ROTEM, J., Division of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan, 50250, Israel
BASHI, E., Division of Plant Pathology, Agricultural Research Organization, The Volcani Center, Bet Dagan, 50250, Israel
KRANZ, J., Tropeninstitut, University of Giessen, Giessen, 6300, Germany

Relationships between healthy haulm area (HHA) and yield, and between losses of HHA and yield in the potato late blight system, were evaluated in several seasons and fields variously affected by the disease in Israel. The results showed that because of the different yield potential of potatoes cultivated in different conditions, evaluation of crop losses cannot be separated from evaluation of yield potentials of the crop as influenced by environmental and cultural factors. In one calculation the yield and the HHA in the healthiest plot in each field were equated to 100% and losses in other plots were derived from comparisons with this plot. In such a calculation the relationships of yield loss to HHA loss in all fields and seasons fitted one regression equation, and a high correlation was obtained between both parameters. However, the actual haulm areas and yield weights differed widely from one field to another, even in the healthiest plots. Also, a similar percentage of destroyed haulms sometimes corresponded to different areas of the remaining haulms and to different yields. So no correlation was found between HHA and yields over all fields and seasons. Such a correlation was found only for several plots, variously affected by late blight, in the same field and season and in different fields and seasons yields responded in a different way to increases in HHA. Copyright © 1983, Wiley Blackwell. All rights reserved