Submitted to: American Society of Sugar Cane Technologists
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 22, 1998
Publication Date: N/A
Interpretive Summary: The bacterium, Xanthomonas albilineans, causes sugarcane leaf scald disease that may result in serious loss in sugar yield and quality. The bacterium can be spread to a new field when infected stalks are cut and planted. A major problem is that the bacteria can be in stalks showing no symptoms. This is called a latent infection. The challenge is to have a method that can detect the leaf scald bacterium in the stalks having a latent infection. Methods are available to test for leaf scald infection, but the problem is knowing what part of the plant to test. Distribution of the leaf scald bacterium among internodes of infected sugarcane stalks was determined using two methods, polymerase chain reaction and isolation on semi-selective agar medium. The leaf scald bacterium was detected by both methods in every internode in stalks showing leaf scald symptoms except the top-most 1 to 6 internodes, where the bacterium was occasionally not detectable. In contrast, the leaf scald bacterium was found in only some of the internodes of infected stalks not showing disease symptoms and where the bacterium was found did not follow a pattern. Information is provided on how to sample sugarcane stalks with latent leaf scald infection to have the highest chance of detecting the bacterium. This information is essential for establishing sampling procedures to detect leaf scald in the commercial fields and to provide farmers with clean seed cane.
Technical Abstract: Polymerase chain reaction (PCR) and isolation on semi-selective medium were used to study the within-stalk distribution of the leaf scald bacterium, Xanthomonas albilineans (Xa). Xylem sap samples from 17 to 25 internodes per stalk were examined in 11 pairs of infected symptomatic vs. asymptomatic stalks. Xa was detected by both methods in every internode in the symptomatic stalks except the top-most 1 to 6 internodes where Xa was occasionally undetectable. Distribution of Xa infection among internodes of asymptomatic stalks did not follow a pattern. Among 462 internodes examined, results by both detection methods were the same for 395 (85%), of which 264 tested positive for Xa infection. However, 39 internodes (about 8%) tested positive only by PCR but not by isolation, and 28 internodes (6%) tested positive only by isolation but not by PCR. ln 76 isolation tests (16%), PCR had to be used to distinguish between Xa and other unidentified bacterial species. Although PCR was more sensitive than isolation, inhibitors may occasionally prevent successful PCR reactions. It is recommended that a mixture of xylem sap samples from every testable internode be prepared from asymptomatic stalks for the diagnosis of leaf scald pathogen.