Resistance of strawberry plants against the two-spotted spider mite, Tetranychus urticae (Acari: Tetranychidae)

Strawberry plant resistance to infestation with the two-spotted spider mite, Tetranychus urticae Koch was investigated, using two cultivars: Sweet Charlie (Sw-cv) and Camarosa (Ca-cv). Ca-cv is more susceptible to infestation with T. urticae than Sw-cv. Chemical analysis of leaves revealed that total phenols and amino acids were higher in Sw-cv than in Ca-cv in, whereas the opposite was recorded for total sugars. An increase in total phenol content likely suppressed mite infestation, whereas increase of total sugars may have a stimulatory effect on spider mites. Leaf trichomes of Sw-cv have a higher density and are longer and sharper pointed than those of Ca-cv. Such dense and long-hairy leaves of Sw-cv were not conducive to mite infestation. Also when the two cultivars were fertilized with CaSO4 and K2SO4, mite infestation was lower than on unfertilized plants. These treatments lead to an increase in total phenols and amino acids in both cultivars. Increased potassium levels in strawberry plants lead to an increase in plant resistance to T. urticae infestation. Moreover, fertilization with CaSO4 and K2SO4 gave an increased strawberry yield, of 7-17% for Sw-cv and 6-22% for Ca-cv.