Robert M. Harveson, Extension Plant Pathologist Panhandle REC, Scottsbluff
Goss’ wilt and leaf blight is a destructive bacterial disease of corn, caused by Clavibacter michiganensis subsp. Nebraskense. It was first identified in 1969 from Dawson County near Lexington making it a true Nebraska native. Over the next decade, it was identified from at least 53 other counties within Nebraska and additionally spread into the surrounding states of Iowa, South Dakota, Kansas, and Colorado. The disease was eventually identified from additional U.S. corn-growing states, before disappearing just as suddenly as it first appeared.
Bacterial wilt of dry beans, caused by Curtobacterium flaccumfaciens pv. flaccumfaciens, has been a sporadic but often serious production problem since first being reported from South Dakota in 1922. By the 1940s it became one of the most problematic bacterial diseases in the United States, particularly throughout the irrigated high plains and Midwest. Wilt then persisted through the early 1970s as an endemic and economically important production constraint in Nebraska, presumably through seedborne transmission and dissemination, before essentially disappearing by the early 1980s.
The two pathogens share many similar characteristics, the most obvious being bacterial in nature. They are closely related, and in fact were once classified in the same genus (Corynebacterium) primarily due to a superficially similar particle shape, referred to “coryneform”. This term today is used to informally describe members of a heterogeneous group of Gram positive (this group of bacteria stain purple after a laboratory diagnostic procedure) bacteria with cell morphological features consisting of irregular, short rods that may be slightly curved, bent, or club-shaped (Figure 1). This characteristic gives the genus its name from the Greek noun “koruna” or “club”. The Goss’ wilt pathogen, Clavibacter, produces rods that are slightly thinner and longer (Figure 1, right) than the shorter, fatter rods characteristic of Curtobacterium (dry bean wilt – Figure 1, left). In addition to particle morphology differences, these two pathogens were also demonstrated to differ in biochemical and DNA characteristics and cell wall composition, further justifying the separation of these bacteria into different groups.
Favorable Conditions for Disease Development and Pathogen Survival
Disease for both pathogens, in general, is enhanced by injury or stress placed on the plant. In this region, these conditions are often provided by storms consisting of hail, wind damage, and sandblasting. The pathogens can also become systemic, moving into the vascular system of plants, resulting in wilting. Both pathogens also readily survive in infected residue on the soil surface, which serves as the primary inoculum source for future crops. Therefore, infection and spread throughout fields is enhanced by reduced tillage practices, continuous cropping, tissue injury on leaves, and sprinkler irrigation or rainfall; but insects or any other potential biological vector are not thought to be involved with disease spread or development for either disease. Once infection occurs, optimal temperature for disease development is 80 to 90 F. The pathogens are also carried in seeds, both on the outside of the seed coat, and internally, which may also explain its movement and distribution throughout the growing regions of the U. S.
Within the past decade, both pathogens have re-emerged to cause serious production problems across the Central High Plains wherever corn and beans are grown. The resurrection of this disease in Nebraska and other areas of the high plains after a long absence has been a puzzling development. However, after studying both diseases since 2003-2004, we have arrived at several hypotheses as to why these diseases seemingly came back with the same general time frame.
Factors Responsible for Disease Outbreaks?
One of the primary factors influencing the new epidemics is the changes in cultural practices that have been adopted in this region over the last 15 to 20 years. This perhaps may also be the most influential factor in this mystery. The majority of area producers have introduced some form of reduced tillage into their systems. Combining this practice with the simultaneous, region-wide increases in center pivot irrigation systems would reputably provide better conditions for enhancing pathogen survival, infection, and dispersal within fields. Furthermore, for Goss’ wilt, most growers quit using disease resistant varieties. This practice largely resolved the problem years ago, but a strong correlation has been observed between the disease’s reappearance and fewer varieties with resistance being available and utilized. For bean wilt, this is not as much of a factor. Only the great northern variety, Emerson, had resistance and it was a specialty variety, and not grown over the entire region.
Another factor in this re-emergence story may be explained conjecturally 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 could have easily contributed to better conditions for pathogen survival and increased plant stresses, which are well recognized to favor disease development and increased severity for both diseases.
Our studies suggest that it is no coincidence that these pathogens have re-emerged widely throughout this region at roughly the same time. It seems likely that neither disease completely went away, but perhaps survived at low levels as saprophytes on weed species or crop residues. We theorize that neither disease was noticed until the mid-2000s because in the past most fields were plowed, thereby removing a major mechanism for pathogen survival.
Over the last 10-15 years, reduced tillage has become a widespread practice, as has the addition of higher numbers of center pivots added in production fields. Both practices appear to enhance the conditions favoring survival and spread of both pathogens within fields. Since both diseases are more problematic under elevated levels of plant stress, it is also possible that the drought in the mid- 2000’s also played a role. In summary, we hypothesize that the combination of environmental stress and changes in cultural practices, all contributed to the high visibility and incidence of these diseases in the Central High Plains within the last decade.