Vector-borne Maize Pathogens: Lessons Learned
In the first half of the 20th Century, the US Corn Belt escaped serious damage from vector-borne maize pathogens. This was not the case in other parts of the world, i.e. the Mediterranean region (maize rough dwarf), Africa (maize streak), or Latin America (corn stunt). This situation changed when new and rapidly spreading virus-like diseases appeared in the southern fringe of the Corn Belt in the early 1960s.
The region was ill prepared to deal with the problem; few scientists in the Corn Belt were experienced with vector borne plant pathogens. The Ohio State University (OSU) took the lead, hiring faculty with expertise in electron microscopy (1964), plant virology (1966) and arthropod transmission of plant pathogens (1966). Faculty members were stationed at the Ohio Agricultural Research and Development Center, OSU’s agricultural research campus in Wooster. The US Department of Agriculture-Agricultural Research Service (USDA-ARS) soon followed suit by adding three scientists, a plant pathologist (1967) and an entomologist (1967) to focus on disease epidemiology and a viral biochemist to add expertise in the emerging field of molecular biology. Later (1980), OSU added a faculty member specializing in plant disease modeling. These scientists joined forces with a veteran maize pathologist and two established corn breeders, one each from OSU and the USDA, on the Wooster campus. Excluding the cost of major equipment purchases (e.g. electron microscopes, ultra-centrifuges, etc.) the annual budget to support the maize virus program in 1968 was ca. $100,000 each for OSU and the USDA.
This cadre of Wooster-based OSU/USDA scientists (maize virus team) collaborated with plant pathologists, entomologists, and plant breeders from other Corn Belt states as well as maize specialists from the southeastern United States who reported similar disease problems. These university and federal scientists met yearly with representatives from the major seed corn companies and other agribusinesses under the authority of the S-70 regional project to share data and plan collaborative research.
The aphid-transmitted maize dwarf mosaic virus (MDMV), a strain of sugar cane mosaic virus, was isolated in early investigations (1964) and thought to be the main cause of stunting disease in the Corn Belt. However, the maize virus team could not consistently associate MDMV with maize stunting disease in the region. Something was missing; a search for additional pathogens continued! Data from greenhouse and field studies, electron microscopy, and insect vector studies ultimately led to the isolation and characterization of maize chlorotic dwarf virus (MCDV) in 1971. It took four years to uncover this unusual, semi-persistently-transmitted, leafhopper-borne virus.
Maize chlorotic dwarf virus along with MDMV proved to be the primary causes of maize
stunting disease, not only in the Corn Belt, but in the southeastern US as well. The distribution of MCDV overlaps that of its native vector, Graminella nigrifrons, and its over-wintering host, the introduced, perennial weed, johnsongrass. Johnsongrass also is the primary alternate host for MDMV. Fortunately, the northerly distribution of johnsongrass is limited to the southern fringe of the Corn Belt; the leafhopper and aphid vectors migrate to as far north as Canada. Neither virus has ever spread to the heart of the Corn Belt. Control of MCDV and MDMV has been principally by the development of virus tolerant and or resistant varieties and management of johnsongrass. These control measures proved timely and highly successful in returning maize in infested regions to pre-disease production levels.
Spin-off from the formation of the maize virus team in Wooster and its collaboration with workers from more than 20 states has been multifold. Among the discoveries by the team in its first decade was uncovering another complex of corn stunting pathogens endemic to Texas, Florida, and other Gulf Coast states. Causal agents are the corn stunt spiroplasma, the maize bushy stunt phytoplasma, and the maize rayado fino virus. All three are transmitted by the corn leafhopper, Dalbulus maidis. Also discovered were the maize mosaic and the maize stripe viruses in these same Gulf Coast States; both are obligately transmitted by the corn delphacid, Peregrinus maidis. The beetle vector of the maize chlorotic mottle virus, a virus introduced into Nebraska and Kansas from the Andes was also discovered. So too were the eriophyid mite-borne wheat streak mosaic and wheat spot mosaic viruses in maize as well as the mite-induced phytotoxin, the cause of kernel red streak disease in the Great Lakes region. Information derived from the etiology of these maize pathogens provided researchers elsewhere (in the United States and internationally) with new and better tools to detect and manage maize diseases.
The OSU/USDA maize virus team on the Wooster campus remains as the foremost center, both nationally and internationally, for the study of maize virus diseases. There is a new generation of scientists who have replaced retired members of the original team. These scientists have brought new skills, especially in the molecular arena, to bear upon the discovery, characterization, and management of vector-borne maize pathogens. Dozens of graduate students, post doctoral scientists, visiting faculty and other researchers have been educated, trained or have advanced their careers working with the maize virus team. It should be noted that several “alumni” are now investigating vector-borne citrus pathogens, including HLB.
What lessons learned from the experiences of the maize virus team is relevant to the strategies for the study of HLB? Many factors contributed to the successes of the maize virus team but the following were most critical.
The team was formed by recruiting young scientists who were trained in the varied disciplines required for solving the problems associated with vector-borne plant pathogens. Just as important was that they joined forces with experienced faculty familiar with maize diseases and corn breeding in the Corn Belt states.
Members of the team were committed to a common goal. All were located on the same campus, many in the same suite of offices and research laboratories. The OSU and USDA scientists, their staff, and the students interacted daily, freely exchanging information and utilizing each other’s equipment and resources. Team members formed and, as circumstances dictated, reformed flexible units of two or more specialists to tackle specific problems. Team members met frequently with regularly scheduled meetings and seminars.
Success in solving HLB and other vector-borne pathogens in citrus is most likely to be achieved if a team of scientists, with the relevant skills, tools, and experiences, engage one another and willingly work toward agreed upon goals. Scientists with requisite expertise likely now are in place and conducting HLB research, but they are scattered among a number of institutions and locations. It may not be possible to house key scientists of an HLB team in one laboratory or campus setting. The question then is how to form a virtual laboratory where those tackling HLB can best take on the advantages of working together as a team.