Onion thrips (Thrips tabaci) is a major insect pest of onion and it has been identified as a likely vector of Pantoea agglomerans (bacterial stalk and leaf necrosis), a relatively new pathogen to Michigan’s onion industry. Our objective was to develop an integrated insect and disease management program by examining the efficacy of bactericides and insecticides alone and in combination to limit bacterial stalk and leaf necrosis caused by P. agglomerans. We also examined the association of onion thrips and disease incidence in the field, because thrips are known to transmit this pathogen. In the pesticide trial, insecticides reduced both thrips abundance and bacterial stalk and leaf necrosis incidence whereas bactericides alone did not reduce disease severity. Positive correlations among thrips population density, numbers of thrips positive for P. agglomerans, and bacterial stalk and leaf necrosis incidence in onion fields were determined. This study suggests that onion thrips feeding can facilitate the development of bacterial stalk and leaf necrosis in Michigan’s commercial onion fields, and results from the pesticide trials indicate that thrips feeding damage is positively correlated with disease incidence. Therefore, in order to reduce bacterial stalk and leaf necrosis incidence in onion, management efforts should include reducing onion thrips populations through the use of insecticides and other cultural practices.
Bacterial stalk and leaf necrosis of onion, Allium cepa L., is a concern of Michigan producers. Plants with symptoms of bacterial stalk and leaf necrosis were collected in 2013 and 2014 from 17 fields in six Michigan counties. Bacterial isolates were identified using Biolog and confirmed by sequencing the 16s rDNA gene. From a total of 414 isolates, 10 bacterial species were identified. Pantoea agglomerans (42.5%), Pantoea ananatis (17.4%), and Enterobacter cowanii (7.5%) were most prevalent and were tested for pathogenicity on onion foliage and bulbs. More P. ananatis isolates (92%) were pathogenic on seedlings than P. agglomerans or E. cowanii isolates (approximately 50%). When 197 bacterial isolates were tested for sensitivity to copper hydroxide (200 µg/ml), 41% of P. agglomerans isolates were found to be tolerant to copper hydroxide, whereas 19 and 22% of P. ananatis and E. cowanii isolates were tolerant, respectively. Identifying the bacterial species associated with foliar disease symptoms on onions in Michigan, their pathogenicity and tolerance to copper is an important step in developing improved disease management strategies.
Foliar disease of onion in Michigan, caused by Pantoea agglomerans, Pantoea ananatis, or Enterobacter cowanii, has recently become a concern to producers. The objective of this study was to determine the effect of temperature, relative humidity (RH), and plant age in growth chamber and greenhouse experiments on onion plants inoculated with each pathogen. A significant level of disease resulted from each pathogen at 25 to 30°C, with strong positive associations detected using regression analysis between the area under the disease progress curve (AUDPC) and temperature. RH also significantly influenced symptom development. Foliar disease symptoms developed sooner and were more severe when RH was high (80 to 100%) but was limited at RH < 60%. Significant positive associations between RH and AUDPC, as described by linear regression, were also detected. When 6- to 14-week-old plants were inoculated with each bacterial pathogen, susceptibility increased significantly with age. These results provide insight into the epidemiology of P. agglomerans, P. ananatis, and E. cowanii bacterial pathogens of onions in Michigan and can assist in the development and timing of management strategies.
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