First Report of Acidovorax avenae subsp. avenae Causing Sugarcane Red Stripe in Gabon.

During a disease inspection at the sugarcane estate SUCAF near Franceville, Gabon, in March 2011, 1- to 3-mm wide and several dm long dark red stripes were observed on sugarcane (Saccharum spp.) leaves of many plants of cultivar R570. These plants were 5.5 months old in the first ratoon crop. Additionally, spindle leaves of several stalks were rotted and could be easily pulled out of the top of the stalk. Longitudinal sections of diseased stalks showed reddish-brown discoloration of the upper stem and the rotted spindle had an unpleasant odor. Circular, convex, smooth, yellow-cream pigmented bacterial colonies with 2 to 3 mm diameter were isolated after 3 days at 28°C from young leaf lesions on YPGA (yeast extract 7 g/L, peptone 7 g/L, glucose 7 g/L, agar 15 g/L, pH 6.8 to 7.0). The 16S-23S internal transcribed spacer (ITS) of two representative colonies was PCR amplified, and the nucleotide sequences were shown to be 99% identical to the 16S-23S ITS sequence from the genome of Acidovorax avenae subsp. avenae strain ATCC 19860 (GenBank: CP002521.1). One of these A. avenae subsp. avenae isolates from Gabon was inoculated into greenhouse grown plants of sugarcane cultivar R570. Plants were inoculated by injection into the sheath of spindle leaves above the meristem with the bacterial strain (12 plants) or with a water control (six plants). In this method, the bacteria (10 CFU/ml) were injected using a syringe through the leaf sheath until filling the leaf whorl. Three weeks post-inoculation, one to several cm long red-brown stripes were observed on leaves of 11 of 12 inoculated plants. Seven weeks post-inoculation, all plants exhibited symptoms, from red, brown, or black stripes to leaf necrosis, rotting, and death of the spindle leaves (six plants). All six control plants were symptomless. In a second experiment, 6 of 12 plants showed symptoms 3 weeks post inoculation, and the pathogen was successfully re-isolated from all six symptomatic plants with YPGA medium. The 16S-23S ITS of three single colonies obtained each from different symptomatic plants were PCR amplified and the nucleotide sequences were again found 99% identical to the 16S-23S ITS sequence from the genome of A. avenae subsp. avenae ATCC 19860. To our knowledge, this is the first report of A. avenae subsp. avenae, the causal agent of sugarcane red stripe (also reported as top rot), in Gabon. It is also the first description of the occurrence of the top rot form of the disease in R570, a cultivar that is grown in several locations of Africa, the Mascarene Islands, and the French West Indies. A large-scale survey needs to be undertaken to determine the distribution of red stripe in Gabon, a disease for which several outbreaks have been reported recently worldwide (1,2). References: (1) M. P. Grisham and R. M. Johnson. Phytopathology 101:564, 2011. (2) S. Zia-ul-Hussnain et al. Afr. J. Biotechnol. 10:7191, 2011.