Early detection of Super-elongation disease in Manihot esculenta Crantz (cassava) using molecular markers for gibberellic acid biosynthesis

Cassava (Manihot esculenta Crantz) is an increasingly important tropical root crop. It is however affected by an endemic disease in the Caribbean called Super-elongation caused by the fungus Sphaceloma manihoticola. Traditional methods of detection and laboratory confirmation of S. manihoticola infection depend on the observation of seasonally variable stem elongation symptoms, and confirmatory fungal growth of cultures that can take up to four weeks, due to the slow-growing nature of the pathogen. The elongation symptoms observed are due to the production of large amounts of gibberellin GA(4) by the pathogen. In this study, specific PCR primers were developed to target the Smp450-2 gene (Gen Bank Acession AM 886290), in the gibberellin biosynthesis gene cluster of S. manihoticola. Shade-house plants were inoculated with a suspension of S. manihoticola and DNA extracted over a 28-day period, followed by PCR amplification using the Smp450-2 primers. Detection of S. manihoticola in infected samples was possible 7 days post-infection in at least one local cassava variety, whereas visible disease symptoms appeared after 21 days. This PCR based method was also able to detect the pathogen in leaves and cankers of diseased field tissues, but not in disease-free material from either the laboratory or cassava fields. A new approach for the detection of S. manihoticola causing Super-elongation disease in cassava is therefore presented, and its potential use as a diagnostic tool for early detection of this disease is explored.