2º Congreso Argentino De Fitopatología Libro de Resúmenes

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2008) for 11 years prior to 2009, with plant populations comprised of Intermediate wheat grass and an old variety of alfalfa called Grimm. In all cases where voucher seed from high-incidence outbreaks could be examined, the percentage of seedborne infection was too low to account for seed as a source of inoculum. South Dakota adapted soybean germplasm is very susceptible to Northern stem canker Extensive pathogenicity studies of soybean varieties entered in annual crop performance testing program at South Dakota State University over three seasons (2004-2006) demonstrated that nearly all prospective commercial germplasm had a high degree of susceptibility. These studies were based on toothpick inoculations of field grown plants (Chase and Geppert, 2007). Although the toothpick method may be a good measure of resistance to spread of DPC within plants, it is not clear how well it represents the actual infection process as it occurs naturally. No evidence of major resistance genes to Northern stem canker was found in these studies. What does this mean for soybean growers who are at risk for Northern stem canker? It is clear that under some circumstances Northern stem canker can be a very damaging disease of soybeans. Given the widespread occurrence of the pathogen in soybean seed and as latent infections in immature plants, one might ask the question why has Northern stem canker not become more damaging in areas where soybeans are intensively cultivated? In point of fact, it seems that NSC only becomes a real factor when soybeans are planted in fields where soybean residues and ostensibly high concentrations of Dpc inoculum must have been lacking. What is the source of inoculum and why is canker formation triggered so uniformly in these fields? Several researchers have reported high isolation frequency of Dpc from apparently healthy immature soybean plants (Sinclair, 1991), and it is also known that seedborne infection occurs under conditions where visible disease is lacking (Xue et al., 2007). It is tempting to hypothesize that Dpc is primarily an endophytic fungus, capable of infecting soybeans early in the growing season, but then normally surviving as an. It may be that only under specific environmental or biological conditions that Dpc is triggered to be actively pathogenic 25
2 º Congreso Argentino de Fitopatología and to cause cankers. Many endophytyic fungi cause little apparent disease in infected plants and only become visible as they fruit on senescing or dead tissues. The closely related fungus Diaporthe phaseolorum var. sojae (Dps), the causal agent of pod & stem blight is a good example. As a corollary to this hypothesis it may be that other endophytic fungi and/or bacteria build up as resident micro-organisms in continued soybean cultivation and suppress Dpc. Hildebrand (1954) demonstrated a higher occurrence of pathogenic Diaporthe in soybeans grown on soils not previously planted to soybeans than in plots where soybeans had been grown continuously for four years. Alternatively, there may be a specialized set of physiological stresses associated with soybeans in the first rotation that trigger Dpc to become actively pathogenic. At this point, these concepts are purely speculative and much work will be required to test them. It is clear that a standard model of a residue born stem disease will not suffice to explain the occurrence of Northern stem canker in soybeans in the North Central region of the United States. References Anonymous. 2008. “Conservation Reserve Program.” Natural Resources Conservation Service. USDA. http://www.nrcs.usda.gov/programs/CRP/ Chase, T.E. and R.L Geppert. 2007. Susceptibility to Northern stem canker in South Dakota soybean varieties. Phytopathology 97:S20 Chase, T.E., and C. Tande. 2001. Occurrence of stem canker on soybeans in South Dakota. Phytopathology 91:S15. Crall, J.M. 1952. A toothpick tip method of inoculation. Phytopathology 42:5-6. Hildebrand. 1954. Observations on the occurrence of the stem canker and pod and stem blight fungi on mature stems of soybean. Plant Disease Reporter 38: 640-646. Kulik, M.M. 1983. The current scenario of the pod and stem blight-stem canker-seed decay complex of soybean. International Journal of Tropical Plant Diseases 1:1-11. Sinclair, J.B. 1991. Latent infections of soybean plants and seeds by fungi. Plant Disease 75:220-224. Xue, A.G., M.J. Morrison, E. Cober, T.R. Anderson, S. Rioux, G.R.Ablett, I. Rajcan, R. Hall and J.X. Zhang. 2007. Frequency of isolation of species of Diaporthe and Phomopsis from soybean plants in Ontario and benefits of seed treatments. Canadian Journal of Plant Pathology 29: 354-364. Zhang, A.W., L. Riccioni, W.L. Pedersen, K.P. Kollipara and G. L. Hartman. 1998. Molecular identification and phylogenetic grouping of Diaporthe phaseolorum and Phomopsis longicola isolates from soybean. Phytopathology 88:1306- 1314. 26
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2º Congreso Argentino De Fitopatología Libro de Resúmenes

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