Effectiveness of Antagonistic Bacterial Isolates against Crown Gall Disease on Roses in Kiambu County, Kenya
Murero, Aprodisia Kavutu
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Crown gall is one of the major constraint in rose flower production in Kenya. The study aimed at developing a bacterial based biopesticide to manage crown gall disease on roses. A baseline survey on status of crown gall disease was conducted from July to October, 2017 using a questionnaire administered to the production managers in randomly selected flower farms in Kiambu, Nakuru and Laikipia Counties. Pathogen and the biocontrol isolates were isolated from galls and soil samples, respectively at Kenyatta University. The isolates were screened to determine their effectiveness against Agrobacterium tumefaciens. The most effective four isolates were identified using biochemical, physiological and molecular tests and evaluated in planta. At KU, rose stems were inoculated with A. tumefaciens and the effective isolates at the same time before planting. In another set up, plants were inoculated with A. tumefaciens and the test isolates applied after galls formed. For experiments conducted at the flower farms, test isolates were smeared on galls and bruised stems of naturally infected plants. The test isolates included Lactobacillus brevis 2.28.11, Micrococcus luteus 2, Micrococcus luteus 1 and Arthrobacter sp 1; other treatments included funguran®, infected plants (with galls treated only with distilled water) and uninfected control (without galls). Treatments were replicated 7 times and arranged in a Complete Randomized Design. The number of galls, change of gall size and number of shoots were recorded at 7 days interval for 10 weeks. Heights of shoots were recorded for 6 weeks. Survey and field (In vitro and in vivo) data were analyzed using Statistical Package for the Social Sciences and Statistical Analysis Software (SAS), respectively. Field data was subjected to analysis of variance and the difference between the treatments means separated using the Fisher's least significant difference test at 5% probability level. Majority of the farms in the surveyed Counties recorded a disease incidence and severity above 50%. The largest inhibition zone in vitro resulted from Arthrobacter sp 1 with a mean of 7.8mm. Lactobacillus brevis 2.28.11 and M. luteus 2 reduced gall size by 25.5% and 26.1% on Tropical Amazon® variety, respectively. On Upper class® variety, Lactobacillus brevis 2.28.11 and M. luteus 2 reduced gall size by 21.0% and 20.3%, respectively. Plants treated with L. brevis 2.28.11 and M. luteus 2 had all galls completely dry by week 10 in all the sites. When isolates tested for preventive use, galls did not form on plants treated with L. brevis 2.28.11 and M. luteus 2. On number of shoots, L. brevis 2.28.11 and M. luteus 2 produced significantly more and taller shoots in all the sites. The study revealed that crown gall remains a threat in production of roses and local environments hold promising antagonistic bacteria against A. tumefaciens. Lactobacillus brevis 2.28.11 and M. luteus were effective against crown gall. The study recommends that awareness on greenhouse hygiene and its impact on crown gall spread be increased to rose flower growers. Further exploitation and screening of bacterial antagonists from the local environment against diseases should be encouraged. Lactobacillus brevis 2.28.11 and M. luteus can be advanced to commercialisation and promoted for use in managing crown gall disease.