This article summarizes and updates a recent Plant Disease feature article (Plant Disease 82: 356-367, see link at left) where the reader can access more detailed discussion and relevant references. References here are restricted to those not cited in the feature article.
Survival of C. africana in the Americas is being evaluated across diverse environments including the variable winter environments of the Northern Plains states of the U.S. The mechanism of survival and the source of initial inoculum in the current growing season will be of great interest because true sclerotia have been difficult to identify in the Americas. Johnsongrass may be one of the primary overwintering reservoirs for C. africana but there is uncertainty about the ability of the pathogen to survive the low temperatures and freeze-thaw cycles of the winters in the Plains states.
Claviceps africana was continually present as far North as Corpus Christi, Texas on ratoon and "volunteer" sorghum escaping frost during the mild winter of 1997-98. Similarly, in the humid region of Southern Tamaulipas, Mexico, ergot incidence was high in seed production fields that flowered during December, 1997 to January, 1998. Incidence of ergot was often high because cool temperatures induced sterility in johnsongrass and other self-fertile sorghums. A similar persistence of ergot is assured in many areas of Mexico where sorghum is produced during the winter months and there is usually no frost to kill johnsongrass and feral sorghums along roadsides and in abandoned fields. However, during the 1998 spring season, not a single sorghum flower has been reported with ergot in the important sorghum-producing region of Northern Tamaulipas, Mexico, where the growing season has been dry and hot. In the current U.S. crop that is past bloom (LRGV and South Texas) sorghum ergot has been detectable at low levels only in sorghums directly adjacent to areas of winter survival and virtually nondetectable in commercial sorghum fields. The extremely dry environments in Mexico and South Texas have severely limited ergot development and spread.
Claviceps africana (and other Claviceps spp.) attack only nonfertilized ovaries so sorghums most susceptible to ergot are male-sterile sorghums including those used as seed parents in hybrid seed production. Ergot appears to be a minor threat to commercial grain sorghum production except where environmental factors interfere with self-fertility that normally prevents sorghum ergot. Preliminary evidence suggests that some commercial hybrids are more prone to loss in self-fertility, and therefore are more susceptible, than others under similar environmental conditions. Although the potential for ergot in commercial grain sorghum fields is minimal the dependence on hybrid seed for planting those fields puts the producer and the entire sorghum industry at some risk. The higher potential for ergot in hybrid seed production fields may affect the quality, cost, and availability of some hybrid sorghum seed. Until host plant resistance is present in commercially useful male-sterile sorghums it will be necessary to utilize other integrated control methodologies including pollen management, chemicals, and other alternate control strategies.
In self-fertile sorghums, much of the identified host plant resistance to ergot may be an acquired type of resistance related to maintenance of high self fertility under environments in regions of adaptation and testing. This resistance is often lacking when these sorghums are evaluated at other geographic locations. Interaction between environment and various fertility factors of lines and hybrids are being investigated globally including cool temperature pollen sterility and flowering synchrony.
Resistance to ergot based on self-fertility factors would be non-functional in male-steriles. All male-sterile sorghums appear to be susceptible to C. africana but relative differences in susceptibility noted between some lines may be due to other factors such as length of female receptivity. Unique characteristics of fertilization for specific male:female inbred combinations are among several other factors being investigated to identify characteristics that may contribute to either susceptibility or resistance.
Biotechnological approaches to incorporate fungitoxic principles in sorghum pistils may provide new avenues to accomplish ergot resistance in agronomically desirable sorghums.
Any factor ensuring maximum pollination/fertilization of the male-sterile parent directly reduces the risk of sorghum ergot. Pollen management techniques such as reduction of female:male row ratios, multiple planting dates of male pollinators, and field capping cropping practices are some of the current methods being employed by the industry (figure 21). Commercial seed producers know that variable flowering and other characteristics of individual seed parent lines will become even more important now that ergot is an additional threat to seed set and the production of quality hybrid seed.
Triazole fungicides have been effectively used to control sorghum ergot in hybrid seed production fields in the Americas and Australia since their initial field use in Brazil. Systemic translocation within the host tissues is apparently necessary to stop infection by C. africana because contact fungicides are ineffective in field applications. The most effective application methods have combined ground application with a head-directed spray of the fungicides (figure 22). The vast acreage and wet soils of irrigated U.S. hybrid seed production fields indicate that aerial application may be the only method that will allow timely application of triazole fungicides to control ergot (6).
Following crisis exemptions and section 18 approval to apply Tilt (propiconazole) for ergot control in 1997, a similar section 18 was granted in 1998 primarily for use in hybrid seed production fields (http://www.agr.state.tx.us/pesticide/tilt98.htm). The label allows Tilt to be applied at 125 g a.i./ha (4 oz/ac of Tilt 3.6E [41.8% by wt], 0.113 lb a.i./ac) in each of three applications for a total of 375 g a.i./ha (12 oz product/ac)(6). A generally low and variable incidence of ergot in Texas hybrid seed production fields in 1997 has produced some industry uncertainty concerning efficacy of aerial applications especially at common application volumes of only 47 liters/ha (5 gal/ac).
There is no apparent need to restrict or quarantine seed movement between regions where ergot has been previously observed. Ergot probably can be introduced on nontreated, contaminated seed but fungicide seed treatments with captan or thiram on hybrid seed should prevent dissemination (http://www.ars-grin.gov/ars/SoAtlantic/Mayaguez/seedtmt.html) (2). In those rare instances when fungicide-treated seed still produces some aerial inoculum of C. africana, a susceptible host in the bloom stage must simultaneously be present before infection can occur. Although this method could potentially spread the pathogen to new regions reintroduction will likely be insignificant because other local inoculum sources will probably be more consistently available. If local inoculum sources are low due to reduced survival of C. africana in northern sorghum growing regions, aerial spread from southern regions may still be a more important source of inoculum than reintroduction through seed.
Previous animal feeding studies demonstrated minimal toxicity of both the sclerotia of C. africana and dihydroergosine, the predominant, unique alkaloid within sclerotia. However, recent information from Australia indicated toxicity to poultry and swine that were fed grain contaminated with up to 5% sclerotia of C. africana (http://www.ars-grin.gov/ars/SoAtlantic/Mayaguez/feed.html). Varying the amounts of ergot sclerotia produced feed which had alkaloid contents of 0.002-0.005% of which approximately 90% was dihydroergosine. The high levels of ergot were restricted to a few very late-planted commercial grain sorghum fields in Australia that bloomed during cool temperature environments. Dry, cool environments after infection apparently allowed production of true sclerotia. Similar poultry and other animal feeding trials are being conducted or planned in the U.S. using tailings (screenings or discarded material) of conditioned grain from a few hybrid seed production fields heavily infected by ergot (figure 18). Dihydroergosine has been easily detected in sphacelia/sclerotia from these materials (Richard Shelby, Auburn University and James Porter, USDA-ARS, Athens, GA, Personal communication, http://www.ars-grin.gov/ars/SoAtlantic/Mayaguez/akaloid.html).
As C. africana became a global threat over the previous three years there was an intensification of research efforts that culminated in the Global Conference on Ergot of Sorghum held June 1 to 8, 1997 in Sete Lagoas, Brazil. In addition to information exchange, the conference developed recommendations in the following research and related areas: 1) epidemiology (natural and human-aided dispersal) and disease predictive models; 2) alternate hosts for C. africana and other sorghum ergots; 3) survival mechanisms and structures and sources of initial and secondary inoculum; 4) identification of genetic sources of resistance and mechanisms of resistance; 5) integrated control including fungicides, pollen management, and other methods; 6) genetic characterization of variability in C. africana; 7) toxicity to animals of grain and stover from ergot-affected fields; and 8) utilization of molecular approaches to differentiate Claviceps species, characterize populations within species, and develop sorghum host plant resistance. A U.S. Conference on Sorghum Ergot held in Amarillo, TX on June 11, 1997 further delineated our current knowledge and projected areas for ergot research in the U.S. (5). Another conference, Status of Sorghum Ergot in North America, is scheduled for June 24-26, 1998 at the Omni Hotel in Corpus Christi, Texas. Details on the conference can be obtained at http://www.ars-grin.gov/ars/SoAtlantic/Mayaguez/program.html.
Many informal linkages were established during the global spread of ergot to provide for the rapid exchange of ergot information via email, websites, and other methods of communication. Electronic communication was invaluable to global efforts in tracking the pathogen and in the timely exchange of scientific information for immediate application by scientists everywhere. This communication has fostered a cooperative attitude and approach that involves nearly every aspect of the sorghum industry including research scientists and others representing public and private institutions from various countries, national and state regulatory agencies, commodity or trade organizations and scientific societies like APS. This broad-based approach encourages dissemination of research information and is necessary because many objectives are regional or global in scope. An NCR project, NCR-501, was established in 1997 to develop collaborative sorghum research efforts among states most affected by sorghum ergot and among scientists who will be conducting this research.
The primary research perspective is to provide control methods for those areas of the sorghum industry most immediately threatened by ergot. Those efforts involve development of integrated controls for C. africana in hybrid seed production fields with emphasis on chemical and agronomic controls including pollen management. These controls will help reduce the immediate impact of sorghum ergot on the hybrid seed production industry until adequate host plant resistance and other appropriate control methodologies are developed and more definitive biological information about C. africana is available.
Others making contributions to the information presented in this article and colleagues collaborating on sorghum ergot from other states and institutions: J. Stack, S. Jensen (University of Nebraska), D. Jardine, L. Claflin, K. Kofoid, M. Tunistra, J. Leslie (Kansas State University), B. Rooney , C. Rush, J. Krausz (Texas A&M University), M. Ryley (Queensland Department of Primary Industries, Australia), N. McLaren (Grain Crops Research Institute, South Africa), P. Tooley (USDA-Frederick, MD), J. Porter (USDA-ARS, Athens, GA), R. Shelby (Auburn University), and T. Lust (National Grain Sorghum Producers Association, Abernathy, TX).
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