Orange Rust Effects on Leaf Photosynthesis and Related Characters of Sugarcane

Authors: Zhao, Duli; Glynn, Neil; Glaz, Barry; Comstock, Jack; Sood, Sushma

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2011
Publication Date: 5/24/2011
Citation: Zhao, D., Glynn, N.C., Glaz, B.S., Comstock, J.C., Sood, S.G. 2011. Orange Rust Effects on Leaf Photosynthesis and Related Characters of Sugarcane. Plant Dis. 96:640-647.

Interpretive Summary: Sugarcane orange rust was first found in Florida in 2007 and has caused substantial yield losses of susceptible sugarcane cultivars in the region and worldwide. Little is known about physiological mechanisms of orange rust impact on sugarcane yields and relationships between severity of this disease and leaf photosynthetic characteristics. The quantitative determination of the effects of orange rust on photosynthesis of individual sugarcane leaves is the first step toward a broader understanding of crop yield losses due to orange rust infection. The objective of this study was to determine physiological mechanisms of the orange rust-induced reductions in sugarcane growth and yield by quantifying effects of this disease on leaf greenness (SPAD index), net photosynthetic rate, dark respiration rate, photosynthetic radiation use, and the relationships between these leaf photosynthetic components and rust disease ratings. Plants growing in pots were inoculated with orange rust urediniospores using leaf whorl inoculation method. The disease rating was evaluated weekly after inoculation using a scale from 0 (uninfected) to 4 (most severely infected) with the intervals of 0.5. When the disease rating reached =2, the rust infected leaf portion of inoculated plants showed significant reductions in SPAD index, radiation use efficiency, carbon use efficiency, stomatal conductance, leaf transpiration rate, and leaf net photosynthetic rate; but the rusted portion of the infected leaves had increased intercellular CO2 concentration and leaf dark respiration rate. Although leaf SPAD index, net photosynthetic rate, stomatal conductance, and transpiration rate at the rust infected portion linearly declined with increased rust rating, the effect of orange rust on leaf photosynthetic rate was much greater than that on stomatal conductance and transpiration rate. Reduction in both green leaf area and leaf photosynthetic rate after orange rust infection resulted in poor growth and low yields in sugarcane. The results can improve understanding of orange rust disease etiology. Field scale experiments using natural infection conditions are required in order to further determine orange rust effects on the physiological traits that are related to sugarcane yield.

Technical Abstract: Orange rust of sugarcane (Saccharum spp. hybrids), caused by Puccinia kuehnii (W. Krüger) E.J. Butler, is a relatively new disease in the Western Hemisphere that substantially reduces yields in susceptible sugarcane genotypes. The objective of this study was to determine the physiological mechanisms of orange rust-induced reductions in sugarcane growth and yield by quantifying effects of the disease on leaf SPAD index (an indication of leaf chlorophyll content), net photosynthetic rate, dark respiration, maximum quantum yield of CO2 assimilation, carbon fixation efficiency, and the relationships between these leaf photosynthetic components and rust disease ratings. Plants growing in pots were inoculated with the orange rust pathogen using a leaf whorl inoculation method. A disease rating was assigned using a scale from 0 to 4 with intervals of 0.5. At disease ratings =2, the rust infected leaf portion of inoculated plants showed significant reductions in SPAD index, maximum quantum yield, carbon fixation efficiency, stomatal conductance, leaf transpiration rate, and net photosynthetic rate; but the rusted portion of the infected leaves had increased intercellular CO2 concentration and leaf dark respiration rate. Although leaf SPAD index, photosynthetic rate, stomatal conductance, and transpiration rate at the rust infected portion decreased linearly with increased rust rating, the effect of orange rust on photosynthetic rate was much greater than that on stomatal conductance and transpiration. Unlike earlier reports on other crops, reduction in leaf photosynthesis by orange rust under low light was greater than that under high light conditions. These results help improve the understanding of orange rust etiology and physiological bases of sugarcane yield loss caused by orange rust.