Pflanzenpathologie und Pflanzenschutz

Epidemiological and phytopathological studies on wheat blast (Magnaporthe grisea) - characterisation of pathotypes and resistance in wheat

Occurrence and host range

Wheat blast “brusone” (Iragashi et al., 1988) caused by Magnaporthe grisea (Hebert) Barr (anamorph Pyricularia grisea (Saccardo) is a relatively new disease of wheat. It has been reported for the first time in Brazil from the North of the State of Paraná in 1985 (Iragashi et al., 1986). Since this report, the disease has been found to occur in other wheat-growing areas of Brazil including Mato Grosso do Sul, São Paulo and Rio Grande do Sul, federal district of Brasília and the state of Goiás, as well as in the Cerrados in central-western Brazil where irrigated wheat is cultivated (Igarashi, 1990; Prabhu et al. 1992; Goulart and Paiva 2000). The disease is now also found in Paraguay (CRIA), Uruguay, Argentina and Bolivia. M. grisea can infect all major small cereal grains such as barley, rye, triticale and oats. Besides cereals, M. grisea also infects several species of grass weeds that are common in farmers’ fields (Urashima et al. 1993).Although both blast inducing pathogens from wheat and rice can cross on culture media, there is molecular genetic evidence that the wheat pathogen M. grisea found in Brazil does not originate from rice (Prabhu et al., 1992; Urashima, 2005; Valent and Chumley, 1994) and differs to the extent that the rice pathogen is now considered as a separate species, M. oryzae (Couch and Kohn, 2002).

Symptoms and factors of disease development

Typical symptoms include partially infected spikes that turn bleached and are easily distinguishable from healthy plants. Leaves and stems are commonly not or only poorly affected. Bleached ear portions do not produce grain. The wheat pathogen is considered to be favored by warm (>25°C) and humid conditions (rain in the flowering stage) but in the absence of detailed epidemiological studies it is difficult to determine potential risk areas for this new pathogen.

Economic importance

The yield losses, lack of resistant varieties and lack of efficient fungicide application schemes as knowledge on the inoculum sources and epidemiology of the disease is insufficient have made wheat blast a major disease problem in the Southern Cone (Urashima et al. 2004). Yield losses of 10% have been reported but 90-100% of the spikes can be affected if favorable climatic conditions and critical growth stages for infection coincide (Iragashi, 1991; Urashima et al., 1993), leading to significantly higher yield loss. Presently, the disease is a major limitation to wheat growing in areas such as the Cerrados. Likewise, in the Bolivian lowlands, the disease has caused a decline in the area sown to wheat. Although wheat blast has not been reported outside South America, there is an increasing fear that it could become a threat to wheat in other parts of the world. Since the disease is seedborne and able to survive on alternative hosts, with uncertainties related to climate and the absence of satisfactory genetic resistance, more information is needed to forecast epidemics and protect other regions and cropping systems from infection.

Outline of research

We are interested to improve the knowledge on the epidemiology, infection process and host specificity of wheat blast. Furthermore, we will characterize Magnaporthe pathotypes from wheat and also make comparisons with M. oryzae isolates from rice, with regard to a genotypic differentiation and host range. We are also interested in the characterization of resistance in wheat lines to Magnaporthe ear infection. Investigators: Xia Ha, PhD student; Tingting Wei, PhD student Supervisor: Prof. Andreas von Tiedemann Collaborators: Dr. Etienne Duveiller, CIMMYT, Mexico Dr. Ulrich Schaffrath, RWTH, Aachen, Germany The Wheat Blast Consortium.

References:

  • Goulart, A.C.P., Paiva, F.A.(1992) Incidência da brusone (Pyricularia oryzae) em diferentes cultivares de trigo (Triticum aestivum) em condições de campo. Fitopatologia Brasileira 17:321–325.
  • Couch, B.C. & L.M. Kohn.(2002) A multilocus gene genealogy concordant with host preference indicates segregation of a new species, Magnaporthe oryzae, from M. grisea. Mycologia, 94(4), 683–693.
  • Prabhu, A. S., Fillipi, M.C. and Castro, N.(1992) Pathogenic variation among isolates of Pyricularia oryzae affecting rice, wheat, and grasses in Brazil. Tropical Pest Management 38: 367-371.
  • Igarashi, S.(1988) Uma análise da ocorrência de “brusone” do trigo no Paraná. In: Seminário sobre Melhoramento para a Resistência a Enfermidades, 1988, Passo Fundo. 19p.
  • Igarashi, S., Utiamada, C.M., Igarashi, L.C., Kazuma, A.H., Lopes, R.S. Pyricularia sp em trigo. (1986) Ocorrência de Pyricularia sp. no Estado do Paraná. In: Reunião nacional de pesquisa de trigo, 14., 1986, Londrina. Resumos. Londrina: IAPAR 1986. p.57.
  • Iragashi et al., (1986) Pyricularia in wheat. Occurrence of Pyricularia sp. in Paraná State Fitopatol. Bras. 11: 351-352.
  • Igarashi, S.(1991) Update on wheat blast (Pyricularia oryzae) in Brazil. In: Saunders DA (ed) Proceedings of the International Conference-Wheat for nontraditional warm areas. CIMMYT, Mexico, pp 480–483.
  • Urashima, A. S., Igarashi, S. and Kato, H.(1993) Host range, mating type and fertility of Pyricularia grisea from wheat in Brazil. Plant Disease 77:1211-1216.
  • Urashima, A.S., Lavorent, N.A., Goulart, A.C.P., Mehta, R.(2004) Resistance spectra of wheat cultivars and virulence diversity of Magnaporthe grisea isolates in Brazil. Fitopatologia Brasileira 29:511–518.
  • Urashima A.S. et al.,(2005) DNA-fingerprinting and sexual characterization revealed two distinct populations of Magnaporthe grisea in wheat blast from Brazil. Czech J. Genet. Plant Breed. 41: 238-245.
  • Valent, B. and Chumley, F.G. (1994) Avirulence genes and mechanisms of genetic instability in the rice blast fungus p.111-134. Rice Blast Disease Eds. Zeigler, R.S., Leong, S.A. and P.S. Teng. CAB international Wallingford, UK.