Application of antagonistic bacteria in management of crown gall disease (Agrobacterium tumefaciens) on roses in Kenya

Roses are among the three top flowers exported from Kenya. Thousands of people depend on rose flower production and handling for income and direct employment in different parts of the country. However, roses are highly vulnerable to infection by Agrobacterium tumefaciens, which causes crown gall disease. Infection can lead to losses of up to 60% if no control measures are implemented. Roses are typically grown in greenhouses in a perennial system lasting several years per planting, which provides ideal conditions for pathogen to build up in the soil. Application of copper based chemical products and cultural measures are some of the recommended disease management practices. However, chemical use pollutes the environment, the products are costly, and more importantly, they have limited effectiveness. There is therefore need to explore alternatives that are effective, environment friendly and less expensive. This study aimed to identify bacterial antagonists of Agrobacterium sp. that can be formulated into commercial products for crown gall control. Bacterial antagonists were isolated and screened against A. tumefaciens using agar disk-diffusion method. Four isolates identified as Lactobacillus brevis 2.28.11, Micrococcus luteus 2, Micrococcus luteus 1 and Arthrobacter sp 1 significantly suppressed A. tumefaciens growth in vitro. These isolates were further evaluated by application to the galls formed on naturally infected rose plants growing under greenhouse conditions. Data were recorded on number of galls, change of gall size, and shoot growth over a period of 10 weeks. Arthrobacter sp. One was most effective in vitro, while under field conditions, isolates L. brevis 2.28.11 and M. luteus 2 were most effective. Antagonists reduced gall size by up to 26% and in some cases caused complete gall degeneration. Effective isolates were therefore identified that can be developed into commercial products for biocontrol of crown gall disease.