Bacterial epidemics generally have bad press, but there might be a good news story in the case of Bradyrhizobium. Like other Rhizobium bacteria, these bacteria inoculate roots of legumes and usually forms symbiotic relationships with legumes. The bacteria stimulate the plant roots to form nodules in which nitrogen fixation occurs. This essential ecological process enables legumes to convert nitrogen from the air into a form that plants can use. Bradyrhizobium inoculates specifically birds’-foot trefoil, soybeans, peanuts, adzuki beans and cowpeas.
There are many species of Bradyrhizobium and different strains of each. Not all strains form symbiotic relationships.
Rhizobial bacteria usually don’t travel far. They can’t live for many years without inoculating a legume and many species can only inoculate one specific type of plant. However, strains of Bradyrhizobium have been spreading across California, either by windstorms or transportation of soil (perhaps on truck tires, with transplanted seedlings). Researchers found that many plants may be infected by as many six species at one time, and several subspecies of these. The good news is that the symbiotic strains (the ones that help the plants fix nitrogen) tend to spread more than non-symbiotic strains.
Farmers can welcome an epidemic of symbiotic bacteria into their soils. With more wild symbiotic bacteria in the soil, legumes (including weeds) may be able to fix more nitrogen. This can lead to more nitrogen in the soil for other plants to use and possibly lead to greater yields.
Source: Epidemic spread of symbiotic and non-symbiotic Bradyrhizobium genotypes across California. Amanda C. Hollowell et al. Microbial Ecology. Volume 71. Number 3. Pages: 700-710. 2016.
- Janet Wallace