EFFECT OF INTENSITY AND DURATION OF 2-SPOTTED SPIDER-MITE (ACARI, TETRANYCHIDAE) INFESTATION AND WATER-STRESS ON LEAF SCORCH DAMAGE OF BARTLETT PEAR

A field experiment was conducted in 1990-1992 to investigate the relat ionship between the level and duration of twospotted spider mite, Tetr anychus urticae Koch, infestation and the development of leaf scorch d amage on 'Bartlett' ('William Bon Chretien') pear trees. The effect of water stress on the development of T. urticae leaf scorch damage was also investigated. The development of leaf scorch damage was strongly related to indices that combine the level and duration of mite infesta tion. Water stress was an important determinant of the severity of lea f scorch damage that developed. A strong correlation (P < 0.001) occur red between the mean number of mites per leaf and the proportion of le aves infested with mites. Mean mites per leaf counts and the percentag e of leaves infested were converted into cumulative mite days (CMD) an d cumulative percentage leaf infested days (CLID), respectively. The l evel of both CMD (P < 0.01) and CLID (P < 0.01) and the development of leaf scorch damage on individual 'Bartlett' pear trees were closely r elated. Water stress significantly increased the level of leaf scorch damage that developed at given levels of mite days (P < 0.05) and leaf infested days (P < 0.01). In one season only, there was an slight inc rease (P < 0.05) in mite populations on water stressed trees compared with well irrigated trees. A simple damage threshold of 5-10% scorched leaf area would correspond to levels of approximate to 100-150 mite d ays or 1,500-2,500 leaf infested days accumulated in a season.