History of Boysenberry and Youngberry in New Zealand in relation to their problems with Boysenberry decline, the association of a fungal pathogen, and possibly a phytoplasma, with this disease

The history of Boysenberry (Rubus ursinus Chamisso & Schlenhtendal) and You ngberry (R. ursinus derivative) were traced from their origins in the Unite d States, to their importation into New Zealand, and subsequent use as the major Rubus crops. The investigation into possible causes of Boysenberry de cline disease are described, and the probable association of the recently d etected Cercosporella rubi fungus with this decline is discussed, as is the presence of Phormium yellow leaf phytoplasma in some Boysenberry decline a ffected plants. A graft-transmissible agent in Boysenberries, which may be C. rubi, was shown to take several seasons to induce visible symptoms follo wing graft inoculation. The means by which C. rubi may have gained entry to New Zealand by the importation of infected blackberry (Rubus spp.) cultiva rs from the southern United States is discussed, as is the means by which C . rubi may have been spread within New Zealand.