What are two ways that bacteria offect plants

Bacteria may affect plants in a number of ways. Diseases have been mentioned, but most bacteria co-exist with plants quite easily. Several bacteria are involved with the transitions of nutrients and make nutrients such as nitrogen plant available (the plant is only able to absorb some nutrients in certain molecular forms). While other bacteria convert nutrients and toxins into forms unavailable to the plant. 

Other bacteria make Some bacteria, Rhizobia in particular, develop symbiotic relationships with some plants. Rhizobia convert atmospheric nitrogen into ammonium which is plant available. In exchange, the plant provides Rhizobia with shelter (a root nodule) and food (photosynthates).