Temperature Affects the Development of Brown Spots on the Leaves of Oncidium Sharry Baby

The development of brown spots has been recognized as a significant problem which occurs on the mature leaves of Oncidium Sharry Baby. The cause of brown spots is still not clear although it appears to be related to environmental conditions and cultural practices during production. This study was conducted to investigate how temperature influences the development of brown spots in Onc. Sharry Baby ‘Red Fantasy’.  Propagules of Onc. Sharry Baby ‘Red Fantasy’ were obtained from a commercial orchid nursery at two developmental stages: 6-month-old plugs and 3-month-old plugs. Plants were planted and placed in a growth chamber maintained at day/night temperatures of 30/25 °C, 25/20 °C, or 20/15 °C with a 12h day, 60% RH, and 200 μmol·m^-2·s^-1 photosynthetically active radiation (PAR). Regardless of temperature and plant age, all plants developed brown spots during the 5-month growing period, however, the degree of brown spot development varied among the treatments. The 6-month-old plants rapidly developed brown spots within a month in all temperature schemes, and additional brown spots developed with higher growing temperatures and continued appearing over time. Meanwhile, the 3-month-old plants developed brown spots 3 months later when grown at lower temperatures, and developed symptoms earlier when grown at higher temperatures. Our results suggest that the development of brown spots in Onc. Sharry Baby ‘Red Fantasy’ is strongly associated with plant developmental stage, and that higher growing temperatures induce the formation of brown spots, possibly by stimulating rapid vegetative growth.