Towards the development of a rapid detection method for bull’s eye rot in apples

Abstract: Neofabraea alba, one of four Neofabraea species causing bull’s eye rot (BER), occurssporadically in South Africa on late maturing apple varieties. Infection in the orchard can occur atany time during the growing season and incipient infections remain latent until symptomdevelopment after three to five months in storage. Developing a rapid detection method for bull’seye rot would allow for early marketing of grower-lots with high disease incidence (DI). Thestudy assessed both a conventional detection method and the use of paraquat to detect latency.For the conventional method, fruit was stored at 21°C under high relative humidity for 30 daysand evaluated for DI. For the paraquat method, asymptomatic fruit were surface sterilised andimmersed in a 1% paraquat solution. Fruit were incubated at 21°C in dew chambers andmonitored for symptoms at 7 and 14 days post treatment (dpt). Control fruit were stored for 4months at -0.5°C. Disease incidence was recorded at 67% for the control treatment, 63% for theparaquat treatment (14 dpt), 42% for the conventional treatment and 20% for the paraquattreatment (7 dpt). Only the paraquat treatment (14 dpt) was comparable to the control treatment.Asymptomatic ‘Cripps Pink’ fruit were harvested monthly from November 2010 to April 2011and using genus-specific primers, we determined whether N. alba could be detected on orchardfruit throughout the growing season. Inoculum was detected on orchard fruit as early as fourmonths before harvest. Rapid detection techniques, including molecular technologies for BER,will allow for timely and effective disease management strategies.