DAYLENGTH AND RESISTANCE OF STRAWBERRY FOLIAGE TO THE 2-SPOTTED SPIDER-MITE

The influence of long and short daylengths on twospotted spider mite ( TSSM) (Tetranychus urticae Koch) resistance of strawberry (Fragaria x ananassa Duch.) foliage was studied. Photoperiods of 8 hours (short da ylength) and continuous light (long daylength) altered the seasonal ch ange in susceptibility of 'Redchief' strawberry foliage to TSSM. Plant s exposed to continuous light rapidly became resistant, those exposed to short daylength remained relatively susceptible, and plants under n atural daylength exhibited the seasonal change of slowly increasing re sistance. Plants resistant to TSSM under long daylength became suscept ible 19 days after being switched to a short daylength. Plants that we re switched from short to long daylength changed from TSSM susceptible to resistant. Field-grown plants of 'Redchief', a short-day sensitive cultivar, and 'Tribute', a day-neutral cultivar, exhibited increasing resistance to TSSM from 2 weeks before bloom until 2 weeks into harve st when greatest resistance was observed. These results suggest that T SSM resistance in strawberry is influenced by daylength and that this effect may be independent of daylength effects on strawberry reproduct ive development.