Involvement of Calcium and Boron in 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 leaves of Oncidium Sharry Baby. The causes of brown spots are still not clear, yet some evidence suggests that it may be related to environmental conditions and cultural practices during production. Our previous results showed that the occurrence of brown spots depends on the size of plants, and is strongly associated with high growing temperature. This study was conducted to determine physiological and nutritional differences in plants grown at different temperature schemes. The 6-month-old plugs of Onc. Sharry Baby ‘Red Fantasy’ were obtained from a commercial orchid nursery, and planted in a pot filled with two parts coarse perlite, one part coconut husk chip and one part coarse peat. All plants were placed in a growth chamber maintained at day/night temperatures of 30/25, 25/20, or 20/15^oC with a 12h day, 60% RH, and 200 μmol·m^-2·s^-1 photosynthetically active radiation (PAR), and fertigated based on current farm practices. Our results showed that higher temperatures induced an initial vigorous growth, and therefore, plants were taller and produced more leaves, and continued to grow faster than the ones grown at the low temperature. The development of high-temperature-induced brown spots occurred within a month, and the number continued to increase, especially at the highest temperature. Once formed, the size of brown spots gradually increased over time. Leaf discs were taken from the lower and upper part of the leaves of the plants at the end of the experimental period, and nutrient analysis was conducted. Plants grown at the highest temperature have significantly lower calcium (Ca) content but significantly higher boron (B) content in the upper leaves, while the Ca and B contents were not different in the lower leaves. Potassium content was maintained lower in the upper leaves regardless of growing temperature.  There were no significant differences in other nutrient elements. Our results suggest that the contents of Ca and B in the upper leaves are associated with the high-temperature-induced brown spot development in Oncidium Sharry Baby. The interaction between calcium and boron may play a key role in the development of brown spots.