Epidemiology & Management of Center Rot of Onion

Summary

A marker, namely ice nucleation activity, was found that identifies strains of Pantoea ananatis that are pathogenic to onion. Also, specific whole cell fatty acids that are associated with copper-tolerant strains of Pantoea ananatis were identified.

Situation

Center rot of onion, caused by the bacterium Pantoea ananatis, continues to be a problem in the production of Vidalia onions. The disease reduces both yield and quality of onion bulbs. Under certain conditions center rot infections will promote a soft rot of the bulb prior to harvest. Incidence and severity of center rot in Georgia in 2006 were the highest observed in the past 5 years. In some instances entire fields were abandoned and the disease was responsible for hundreds of thousands if not millions of dollars in lost revenue. This was in spite of the fact that conditions during the 2006 onion-growing season, when center rot was the most devastating, were some of the driest experienced over the past 5 years. Center rot is a very unusual bacterial disease, as the vast majority of plant diseases caused by bacteria are favored by wet conditions. Also, about the only labeled pesticide for plant bacterial diseases, namely fixed coppers, are relatively useless for managing center rot as most strains are resistant.

Response

The Department of Plant Pathology at the Coastal Plain Experiment Station in Tifton, GA has had a research program on center rot since 1999. During that time at least four research faculty members, two extension faculty members, two postdoc scientists, several county agents, technicians, and part-time student laborers have spent a portion of their time conducting research on center rot of onion. Experiments in the lab, greenhouse and field were conducted to investigate the ecology of Pantoea ananatis and the epidemiology of center rot. In addition to conducting research, personnel of the phytobacteriology lab once again served as a resource for the state of Georgia by providing detailed identification of bacterial species submitted by growers, extension specialists, county agents, Georgia Department of Agriculture inspectors, and private industry.

Impact

Research Finding: Previously, infested onion seed, weed hosts (~25) and thrips have been shown as potential sources of primary inocula for center rot epidemics. Currently, it was found that dry weather conditions correlated with increased thrips populations and center rot severity by season. Different strains of P. ananatis recovered from tobacco thrips, asymptomatic weeds, onion seed or diseased onions were compared. Only 54 % of all strains were pathogenic when inoculated on greenhouse-grown, onion seedlings, but of those that were pathogenic, 93 % were positive for ice nucleation activity (INA). Likewise, 91% of nonpathogenic strains were negative for INA. Only 4 % of all strains identified as P. ananatis were pathogenic to onion but negative for INA. Interestingly, 100% of the strains recovered from diseased onions were both pathogenic and positive for INA. Similarly, 100% of INA-positive and INA-negative strains recovered from thrips were pathogenic and nonpathogenic, respectively. Impact: The correlation of thrips populations with increased center rot incidence and severity along with previously observed disease gradients in field plots, implicate thrips as the most important source of inoculum for center rot epidemics. Therefore, the primary focus for managing center rot should be on controlling thrips populations. INA could be developed as a means of rapidly screening for pathogenic strains of P. ananatis in the lab. These findings have stimulated new research on the interaction of INA with the development of center rot, survival of P. ananatis and the cold hardiness of thrips, which harbor internal populations of INA-positive strains in their gut. There is the potential to develop a biocontrol for thrips using an INA-positive, nonpathogenic bacterium. In addition, there is the potential of developing a disease forecast model based on the percentage of INA-positive thrips prior to the time one would normally first expect center rot symptoms, total thrips populations and amount of rain. Research Finding: Previously in a survey conducted to evaluate differences in fatty acid methyl ester (FAME) profiles, differences were found among P. ananatis strains recovered from different onion cultivars from three different sites in Georgia. Principal components (PC) calculated for those strains capable of growing on media amended with copper sulfate pentahydrate (200 µg/ml) indicated that copper-tolerant strains grouped into clusters separate from those of wild-type, copper-sensitive strains. Likewise, when PC1 was plotted against PC2 for generated, copper-tolerant clones and their wild-type parents, copper-tolerant clones again formed separate clusters distinct from copper-sensitive parents. Current research was conducted to determine what was responsible for the differences between copper-tolerant and copper-sensitive strains. Eigenvalues indicated that PC1 and PC2 accounted for 96 - 98% of the standardized variance. Furthermore, either the fatty acids cis-9-hexadecenoic acid / 2-hydroxy-13-methyltetradecanoic acid or cis-9/ trans-12/ cis-7-octadecenoic acid produced the highest or second highest absolute values for PC1 in all three strains of the parents vs copper-tolerant clones and hexadecanoic acid was the highest absolute value for PC2 in all cases. Impact: The identification of the fatty acids cis-9-hexadecenoic acid / 2-hydroxy-13-methyltetradecanoic acid, cis-9/ trans-12/ cis-7-octadecenoic acid and hexadecanoic acid as being involved in changes in copper-tolerant P. ananatis strains may help in identifying the mode of action of copper tolerance in this bacterial species. One possibility is that changes in these fatty acids may reflect changes in cell membrane structure. Eventually, such findings could lead to a better formulation for one of the only pesticides labeled for control of bacterial diseases.

State Issue

Agricultural Profitability and Sustainability

Details

• Year: 2006
• Geographic Scope: State
• County: Tift
• Program Areas:
  • Agriculture & Natural Resources