Silicon enhances the constitutive defence pathway in strawberries against strawberry powdery mildew and two-spotted spider mites

Strawberry powdery mildew (Podosphaera aphanis) is a major fungal disease affecting strawberry production. The work reported here assessed the use of silicon as a nutrient (Sirius) delivered via the fertigation system in enhancing the constitutive defence pathway in strawberries, contributing to reduced susceptibility to P. aphanis and two-spotted spider mites (Tetranychus urticae Koch) in 2014 and 2015. The field trials were set up on a commercial strawberry farm in polythene tunnels at Wisbech UK between May and September in both years. Treatments were untreated control, routine commercial fungicides for P. aphanis control, 0.017% Silicon nutrient without fungicides and 0.017% Silicon nutrient plus fungicides. Percentage leaf mycelium cover and the number of T. urticae per leaf were assessed fortnightly. In both years, crops that received silicon nutrient alone had both lower rate of epidemic (r) of P. aphanis and lower value of Area Under the Disease Progress Curve (AUDPC) (r=0.0036, AUDPC=475 in 2014; r=0.001, AUDPC=267 in 2015) compared with untreated control (r=0.0042, AUDPC=662 in 2014; r=0.0011, AUDPC=281 in 2015). Silicon also delayed the epidemic build-up in the silicon nutrient alone treatment for approximately 14 days compared with untreated control. The presence of T. urticae on the leaf surface was significantly lower (P < 0.001) compared to untreated control. Other work at UH shows that the silicon nutrient enhances the constitutive defence pathway by increasing the leaf wax and cuticle thickness, and that silicon is preferentially deposited in the epidermal, palisade cells and the vascular tissue of the leaves and peduncles.