Relationship between histofoliar alterations and tolerance to Tetranychus urticae Koch in different strawberry cultivars

The two-spotted spider mite, Tetranychus urticae Koch (TSSM), is a key global pest of the strawberry crop, difficult to control, that threatens the Argentinian strawberry industry. The relationship between TSSM density in strawberry leaves and histofoliar characteristics of two cultivars: ‘Petaluma’ and ‘San Andreas’, chosen for their high and low tolerance to TSSM, respectively, from a previous cultivar screening, was analyzed. The study was carried out during three annual production cycles (2018, 2019 and 2020) at the INTA Famaillá (Tucumán, Argentina) experimental field. The incidence of TSSM in random samples of the central leaflet of six plants per cultivar was recorded under a stereoscopic microscope (40×), and then fixed in FAA (formaldehyde, acetic acid and 80% ethanol). Cross sections freehand made and stained with Safranin-Astra Blue were mounted in water-glycerin (1:1). The results indicated that the TSSM density in ‘Petaluma’ was lower than in ‘San Andreas’ (0.43 and 1.34 TSSM mm^‑2, respectively) and that ‘Petaluma’ showed a higher density of calcium oxalate crystals (65.89 versus 44.15 druses mm^‑2), thicker cuticle (2.37 versus 0.70 μm) and also presence of abundant starch and dark bodies that suggest phenols. These results show a potential relationship between the histofoliar characteristics of the strawberry and the incidence of TSSM (density), linked to a multiple metabolic response of the mesophyll cells against TSSM attack, which allows us to conclude that varietal tolerance to TSSM could be related to foliar alterations not reported before, such as higher crystal density and cuticle thickness, which could be potential selection characters in strawberry breeding programs.