The influence of long and short daylengths on twospotted spider mite (TSSM) (Tetranychus urticae Koch) resistance of strawberry (Fragaria ×ananassa Duch.) foliage was studied. Photoperiods of 8 hours (short daylength) and continuous light (long daylength) altered the seasonal change in susceptibility of `Redchief' strawberry foliage to TSSM. Plants exposed to continuous light rapidly became resistant, those exposed to short daylength remained relatively susceptible, and plants under natural daylength exhibited the seasonal change of slowly increasing resistance. Plants resistant to TSSM under long daylength became susceptible 19 days after being switched to a short daylength. Plants that were 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 harvest when greatest resistance was observed. These results suggest that TSSM resistance in strawberry is influenced by daylength and that this effect may be independent of daylength effects on strawberry reproductive development.
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