Emerging Plant Pathogenic Bacteria and Global Warming

Several bacteria, previously classified as non-fluorescent, oxidase positive pseudomonads, Ralstonia, Acidovorax, and Burkholderia have emerged as serious problems worldwide. Perhaps the most destructive is R. solanacearum (RS), a soilborne pathogen with a very wide host range. RS race 3, biovar 2 infects potato and geranium during cooler weather making it an additional threat. Acidovorax avenae subsp. avenae has emerged as a disease of upland rice in Southern Europe during periods of high temperatures and B. gladioli has emerged as a serious pathogen of orchids in Thailand. B. andropogonis has been identified for the first time on jojoba in eastern Australia; the plant grows under very high temperatures and the disease occurs in nursery stock grown under overhead watering. Burkholderia glumae is emerging as a problem on rice in the southern United States and the pathogen has become much more prevalent in southern South Korea during periods of high temperatures. Why the increase in these bacteria? One possible explanation is global warming. A common trait among them is an optimum growth tempera- ture of 32-36°C; most grow well up to 41°C, whereas most other plant patho- genicbacteria grow best at lower temperatures. An increase in extreme weather conditions, including extended heat waves, long periods of rain, and storms such as hurricanes appear to favor these high-temperature bacteria. There has been a gradual increase in the mean global temperature over the past century. Recent summer temperatures in the southern USA. have been 1-2°C higher than that which is optimum for many crops. Evidence is growing that the increase in the occurrence of extreme weather, including heat waves, continual rains, and hurricanes is caused by rising CO 2 levels from expanding world economies. Several heat waves have occurred in Europe and the US in recent years. The year 1995 was the warmest since global records began in 1856, and the 2003 heat wave killed hundreds in Europe. An increase in violent and extreme summer storms has occurred the past several years. Ice core studies, photographic recordings of retreating glaciers, and recorded temperatures provide solid evidence that global warming is occurring at