Heat and fungicide treatments reduce Peronospora sparsa systemic infection in boysenberry tissue culture

Abstract

Downy mildew, caused by Peronospora sparsa, is a major disease of boysenberry (Rubus sp.) in New Zealand. The use of systemically infected plants for propagation has resulted in young plants being infected. To limit infection of new boysenberry canes prior to use in tissue culture two treatments, heat (34 °C) and fungicide sprays (mancozeb and phosphorous acid) + heat treatment were applied. Survival of the tissue culture plants from heat only, fungicide + heat and untreated control treatments was 41, 48 and 74%, respectively. Those potted plants incubated in the shade-house under conditions conducive to expression of disease produced characteristic P. sparsa symptoms in 13, 17 and 100% of each treatment, respectively. Asymptomatic infection was verified by nested PCR on leaves from the 127 canes with two plants shown to be infected, both from the heat only treatment. The “clean” plants were propagated and random PCR testing of asymptomatic plants on five occasions over the 12 month growing season did not detect P. sparsa. Treatment with heat either alone or in combination with fungicides reduced systemic infection. This method, together with nested PCR to confirm uninfected status, provides a valuable tool for the production of boysenberry material free of P. sparsa infection.