In vitro selection of blackberry (Rubus fruticosus 'Tupy') plants resistant to Botrytis cinerea using gamma ray-irradiated shoot tips

Blackberry is an economically important crop in Mexico, and its yield is substantially reduced by gray mold, a disease caused by Botrytis cinerea. One of the means to obtain B. cinerea-resistant plants is gamma irradiation. Shoot tips of in vitro-micropropagated blackberry plants (Rubus fruticosus 'Tupy') were irradiated with five doses of Cobalt-60 gamma radiation (0, 15, 30, 45, and 60 Gy) and cultured on Murashige and Skoog basal medium containing 1.0 mg l(-1) benzylaminopurine and 0.06 mg l(-1) indole-3-butyric acid (MSB medium). After 28 days of culture, survival was evaluated to determine mean lethal dose (LD50), and 200 shoots were further irradiated at the determined LD50 (30.8 Gy). After 28 days, the surviving shoots were micropropagated on MSB medium for 60 days. Non-irradiated shoots were screened for the in vitro selection of resistant B. cinerea, exposing them to different concentrations of sterile culture filtrate of B. cinerea (0, 2, 4, 6, 8, and 10 g l(-1)) for 28 days to determine mean lethal concentration (LC50), and the irradiated surviving shoots were further exposed to the determined LC50 (4.6 g l(-1)). Three surviving lines (rfgum5, rfgum6, and rfgum17) that did not present changes compared with the control shoots were micropropagated to obtain plantlets, which were further subjected to in vitro resistance assays using detached leaves inoculated with B. cinerea (1x10(3) spores ml-1). Plants of rfgum5 and rfgum6 mutant lines were highly resistant and presented similar growth to control plants. Therefore, this methodology is useful to obtain B. cinerea-resistant blackberry plants.