By Bob Harveson and Tamra Jackson-Ziems
Two closely related plant diseases — one in corn and the other in dry beans — have followed similar but somewhat perplexing patterns of appearing and disappearing and then resurging as a serious threat to crop yield. What factors affected this cycle, and could the cycle be predicted? This pattern called for a second look.
Goss's wilt and leaf blight is a destructive bacterial disease of corn, caused by Clavibacter michiganensis. It was first identified in 1969 near Lexington in Dawson County, making it a true Nebraska native.
Over the next decade, it was identified in corn samples from at least 53 other counties in Nebraska and also in Iowa, South Dakota, Kansas and Colorado. The disease eventually was identified in additional U.S. corn-growing states before disappearing in the mid-1980s just as suddenly as it first appeared.
Bacterial wilt of dry beans, caused by Curtobacterium flaccumfaciens, has been a serious but irregular disease problem since its initial discovery in 1922 in South Dakota. During the 1940s, it expanded in incidence throughout the bean-producing states, most notably in the High Plains.
The disease persisted into the early 1970s, becoming an endemic and economically important production constraint in Nebraska, before disappearing by the early 1980s, similar to Goss's wilt.
The two pathogens share similar characteristics. They are related and once were classified in the same genus (Corynebacterium), primarily because of a superficially similar particle shape, referred to as "coryneform." This term today is used to informally describe members of a variable group of bacteria consisting of irregular, short rods that may be slightly curved, bent or club-shaped. This characteristic trait gives the genus its name from the Greek noun "koruna" or "club."
They now are taxonomically divided into two genera, Clavibacter and Curtobacterium. The Goss's wilt pathogen, Clavibacter, produces rods that are slightly thinner and longer than the shorter, fatter rods characteristic of Curtobacterium dry bean wilt.
In addition to particle morphology differences, these two pathogens also differ in biochemical and DNA characteristics and cell wall composition, further justifying their separation into different groups.
Favorable conditions for disease development
Disease development from both pathogens is enhanced by injury or some form of stress placed on plants. In this region, these conditions often are provided by storms consisting of driving rain, hail, high winds and sandblasting.
Both bacterial pathogens also can become systemic, moving into and blocking water translocation within the vascular system of plants, resulting in the wilting symptoms. Both pathogens also readily survive in infected residue on the soil surface, serving as the primary inoculum source for future crops.
Infection and spread throughout fields is increased by reduced-tillage practices, continuous cropping, tissue injury on leaves, and sprinkler irrigation or rainfall.
Once infection occurs, the optimal temperature for disease development is 80 to 90 degrees F. The pathogens also are carried in seeds, both on the outside of the seed coat and internally, which also may explain its movement and distribution throughout the growing regions of the U.S.
During the mid-2000s, both pathogens reemerged, causing serious production problems wherever corn or beans were grown across the High Plains region. The resurrection of both diseases after a long absence was puzzling. However, after a substantial study of both diseases since 2003-04, we now have developed several hypotheses as to why these diseases seemingly reemerged within the same general time frame.
Possible factors responsible
Perhaps the most influential factor responsible for the new epidemics was the change in cultural practices adopted in this region. Within the past 20-plus years, the majority of area producers introduced some form of reduced tillage into their systems. Combining this practice with the simultaneous, regionwide increase in center-pivot irrigation systems also would improve survival, infection and dispersal of the pathogen within fields.
Furthermore, for Goss's wilt, most growers quit using the available disease-resistant varieties. This practice largely resolved the problem in the 1980s, and our investigations in the mid-2000s revealed a strong correlation between the reappearance of the disease and the reduced availability and utilization of resistant varieties.
For the bean wilt, this was not as much of a factor. Only the great northern variety Emerson had disease resistance, but it was a specialty variety — grown for a target market in southern Europe — and its production was limited.
Another factor in this reemergence story could be conjecturally explained by the climatic patterns observed throughout the region over this same period. The mid-2000s were characterized by warmer winters, and an extended drought with increasingly higher summer temperatures during the growing season. These factors definitely favor both diseases.
Nebraska studies suggest that it is no coincidence that these pathogens reemerged widely throughout this region at about the same time. It seems likely that neither disease completely disappeared but survived at low levels on weed species or infected crop residues.
We theorized that neither disease was noticed until the mid-2000s because in previous years, plowing had been a common practice, removing a major mechanism for pathogen survival.
Since then, reduced tillage became widespread and more center pivots were added in production fields, both of which enhance survival and spread within fields. Since both diseases are more problematic in periods of high plant stress, it is possible that the drought and higher summer temperatures in the mid-2000s also played a pivotal role.
Nebraska researchers have hypothesized that the combination of environmental stress and changes in cultural practices were responsible for the return of these diseases in the Central High Plains in the mid-2000s.
Harveson and Jackson-Ziems are Nebraska Extension plant pathologists.