American Potato Journal

, Volume 70, Issue 11, pp 779–795 | Cite as

Testing for resistance to metalaxyl inPhytophthora infestans isolates from northwestern Washington

  • K. L. Deahl
  • D. A. Inglis
  • S. P. DeMuth


Outbreaks of late blight occurred in commercial potato fields in northwestern Washington in 1989 and reached epidemic proportions in 1990. Since most blighted fields reportedly had received 2–4 late-season applications of metalaxyl, the possibility existed that pathogenic strains ofPhytophthora infestans had developed resistance to this systemic fungicide. In testing this hypothesis,P. infestans was isolated consistently from diseased leaves, stems, and tubers of potato when small tissue pieces were surface-disinfected in 0.5% NaOCl for 3 min. Virulence of isolates was assessed on detached leaflets of eight differential (R-gene) potato genotypes. Although simple races (single and double) were recovered, there was also a high frequency of complex race combinations. When tested in metalaxyl-amended media, 81% of the 73 isolates recovered from diseased tissues were highly resistant to 10 μg metalaxyl/ml and 19% yielded intermediate resistance. Sensitive isolates were not recovered. Fifty-five percent of 40 isolates sporulated on potato tuber disks in the presence of ≥10 ug metalaxyl/ml. On leaf tissues, 85% of 20 isolates sporulated in the presence of 10 μg metalaxyl/ml, and 50% sporulated in the presence of 100 μg/ml. This is the first report ofP. infestans resistance to metalaxyl in the U.S.

Additional Key Words

Late blight fungicide resistance 


En 1989 se presentaron brotes de tizòn tardío en campos comerciales de papa del noroeste de Washington los que alcanzaron niveles de epidemia en 1990. Desde que la mayoría de los campos afectados, según se informó, habían recibido 2–4 aplicaciones de metalaxyl, al final de la temporada, existía la posibilidad que las razas patogénicas dePhytophthora infestons hubieran desarrollado resistencia a este fungicida sistémico. Al probar esta hipótesis, se aisló consistentementeP. infestans de hojas, tallos y tubérculos de papa cuando pequeños trozos de tejido fueron desinfectados en NaOCl al 0.5% por 3 minutos. Se determinó la virulencia de los aislamientos sobre foliolos desprendidos de ocho genotipos diferenciales (gen-R) de papa. Si bien se recobraron sólo razas simples (solas y dobles), hubo también una alta frecuencia de combinaciones complejas de razas. Cuando se probaron en medios con adición de metalaxyl, 81%, de los 73 aislamientos obtenidos de los tejidos afectados, fueron altamente resistentes a 10 g metalaxyl/cm3 y 19% mostraron resistencia intermedia. No se obtuvieron aislamientos sensibles. Cincuenta y cinco por ciento de 40 aislamientos esporularon sobre discos de tubérculos de papa en presencia de 10 g metalaxyl/cm3. Sobre tejidos de hojas, 85% de 20 aislamientos esporularon en presencia de 10 g metalaxyl/cm3, y 50% esporularon en presencia de 100 g/cm3. Este es el primer informe de resistencia delP. infestons al metalaxyl en los EE.UU.


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Copyright information

© Springer 1993

Authors and Affiliations

  • K. L. Deahl
    • 1
  • D. A. Inglis
    • 2
  • S. P. DeMuth
    • 1
  1. 1.Agricultural Research Service, Vegetable LaboratoryU.S. Department of AgricultureBeltsville
  2. 2.Washington State University-NWREUMount Vernon

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