LEAF DISK SCREENING FOR RESISTANCE TO BOTRYTIS CINEREA IN STRAWBERRY GERMPLASM

Abstract

Several methods using detached leaf tissues were investigated as potential early screening techniques for identifying strawberry germplasm resistant to Botrytis cinerea, which causes gray mold fruit rot. The fungus penetrates young leaves, remaining latent and symptomless under the cuticle until the leaves die, when the pathogen resumes growth and sporulates, providing almost all the primary inoculum for fruit infections. The best leaf screening method involved spraying potted strawberry plants in a 20±5 C greenhouse with a suspension of B. cinerea conidia, enclosing the plants in a plastic tent with a humidifier to provide humidity >95% for 72 hours, then resuming normal greenhouse conditions for several days before sampling young leaves. Leaf disks 1 cm in diameter were then cut, surface sterilized in 70% ethanol for 15 sec and 1% sodium hypoclorite for 2 min, dipped in paraquat herbicide (20 mg/l a.i.) to kill the tissues, rinsed twice in sterile water, and plated on water agar containing .01 % streptomyacin to inhibit bacterial growth. Plates were incubated at 21±2 C under 12 hour daylengths for seven days, and incidence and density of B. cinerea conidiophores on the disks determined. Despite much variation between disks within genotypes, clear differences in susceptibility between genotypes emerged which often correlated well with field suscpetibility to gray mold fruit rot.