Reflections on Bacterial Wilt and a Summary of Studies Conducted in Nebraska
Robert M Harveson, Extension Plant Pathologist, Carlos A. Urrea, Dry Bean Breeder, and C. Dean Yonts, Extension Irrigation Engineer University of Nebraska, Panhandle REC, Scottsbluff
Introduction and Background
Bacterial wilt, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens (Cff), was first reported from South Dakota in 1922, and then became one of the most problematic bacterial diseases in the United States, particularly throughout the irrigated high plains and Midwest. It was an endemic problem in western Nebraska dry bean production during the 1960’s and early 1970s, but essentially disappeared (i.e. would only sporadically appear in seed and had no detectable effect on yield) with implementation of seed sanitation and crop rotation.
The resistant Great Northern cultivar ‘Emerson’ was developed during that period by the University of Nebraska specifically for controlling bacterial wilt, which demonstrates the importance that this disease once held. Breeding for resistance to this disease was later discontinued when it was determined that using seed sanitation, increasing disease-free seed in dry climates, and using proper crop rotations effectively stopped the introduction of the pathogen while eventually eliminating it from fields where it had been present.
New Reports and Results of Studies
The pathogen was again identified in 2003 for the first time in this area for almost 25 years. Over the last 7-8 years it has fully re-emerged in the Central High Plains (Nebraska, Colorado, and Wyoming) and has now been identified from more than 400 fields. Affected fields were planted with beans from multiple market classes and seed sources, including yellows, Great Northern, pintos, kidneys, cranberries, blacks, navies, pinks, and small reds, and disease incidence varied from trace levels to >90%.
Losses in both yield and quality have been demonstrated. Some heavily infested fields in Nebraska during 2004-2005 were estimated to yield up to two thirds less than would be expected of a typical crop. Other fields have been abandoned or plowed under without being harvested. Numbers of discolored seeds collected from surviving, but infected plants were found to average 25%, while the incidence of discolored Great Northern seeds harvested from entire fields approached 10% in some instances.
The pathogen is very heterogeneous and apparently varies tremendously. We have observed that strains collected in 2004-2005 were more aggressive and virulent than older reference strains that have been stored for 30+ years. Years ago different pathogen color variants were identified that were unique to Nebraska, including yellow, orange and purple. The purple variant was considered to be extremely rare, but we have continued to find new purple variants again, and additionally have found an extremely virulent pink variant that has never been reported from anywhere else in the world.
We have also found that wilt isolates can survive and remain pathogenic in soil for at least two years. However, the primary mechanism for survival is in crop residues. A comprehensive survey over the last 4 years has further revealed the presence of bacterial wilt isolates occurring with other crops grown in rotation with dry beans, including soybeans, corn, wheat, sunflower, and alfalfa. These isolates were found in association with other bacterial diseases, suggesting survival in those crops residues.
When the pathogen was found causing very severe infection (more than 90% incidence) in a sub-surface drip irrigation research field in 2004 we were presented with an opportunity to compare different irrigation methods and their influence on disease and dry bean yields. It was further determined that the pathogen remained viable in this field, providing a consistent source of infection for treatment comparisons. Based on our observations from this field, it was also thought that the reason for high infestation in this field was the drip irrigation may contribute to greater disease spread plant to plant through the soil. However, we have now completed an extensive four-year irrigation study in collaboration with Dean Yonts in this field that indicates that sprinkler irrigation produces more severe disease levels and yield losses than furrow or sub-surface drip irrigation. The pathogen apparently did not move through soil to a great extent in the field, but we did demonstrate that the pathogen was capable of root to root spread in enclosed pots under greenhouse conditions.
In summary, our studies suggest that the pathogen has re-emerged widely throughout the dry bean production areas due, in part, to our recent changes in cultural practices. It likely never went away, but we now think it survived at low levels as a saprophyte on weed species or crop residues. Thus it was not necessarily noticed until recently because in the past most fields were plowed (removing a source of survival), and the fact that the symptoms of wilt are additionally reminiscent of and likely confused with common blight. Over the last 10-15 years, reduced tillage has become a widespread practice, as has the addition of higher numbers of center pivots in production fields. Both practices enhance conditions for the survival and spread of the pathogen within dry bean fields. This disease is also more problematic under elevated levels of plant stress. During the mid- 2000’s, we had some very hot and dry growing seasons, which apparently contributed to more severe disease incidence and severity. At this point we hypothesize that the combination of environmental stress, changes in cultural practices, and unfamiliarity with the disease, all contributed to the high visibility and incidence of this disease in the Central High Plains within the last 8 years.
New Resistance Development
Bacterial diseases of dry beans, including wilt, are very difficult to manage due to the lack of adequate chemical products. Therefore, genetic resistance is generally considered to be the most effective means of disease management.
While the wilt-resistant cultivar Emerson is still available today, it is grown on a limited basis as a specialized variety for targeted markets in Europe and cannot be produced on every field where the disease has recently been identified. Thus we are faced with an emerging problem that needs addressing utilizing newly developed resistant cultivars.
Working with Carlos Urrea and funding from the Nebraska Dry Bean Commission, we have now screened the North American Dry Bean Core Collection (424 accessions) for resistance to bacterial wilt. Only one wild bean, with very small seed size (11,217 seeds/pound), was identified as a source of bacterial wilt resistance. It will take at least five to six generations to recover the normal seed size if the wild bean is used as a source of resistance.
The International Center for Tropical Agriculture (CIAT) common bean core collection, composed of 1,700 accessions, has been screened with the most virulent NE bacterial wilt isolate from Great Northern beans. Of the 1,700 accessions planted, 1,603 (94.3%) were susceptible to bacterial wilt, 73 did not germinate and 24 (1.4%) are showing resistance to bacterial wilt. The 73 that didn’t germinate are being replanted to inoculate, and the 24 accessions showing resistance will be planted and inoculated again with seven additional isolates to confirm resistance and try to account for the high degree of variability observed within pathogen populations in Nebraska.
Once sources of resistance are found, they can be used to produce new resistant cultivars for use in Nebraska for bacterial wilt, which are currently unavailable. Until new cultivars are developed for use in Nebraska, management practices must consist of planting certified seed treated with antibiotics such as streptomycin, removing volunteer beans, and avoiding practices that induce stress or transfer the bacterial pathogens and infected residue between and within fields.