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Title: IDENTIFICATION OF PYTHIUM SPECIES ON WEST TEXAS PEANUTS AND SENSITIVITY OF ISOLATES TO MEFENOXAM AND AZOXYSTROBIN IN PETRI DISH ASSAYS.

Author
item WHEELER, T - TEXAS AGRIC EXP STATION
item Howell, Charles - Charlie
item COTTON, J - TEXAS AGRIC EXP STATION
item PORTER, D - TEXAS AGRIC EXP STATION

Submitted to: Peanut Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2005
Publication Date: 6/28/2005
Citation: Wheeler, T.A., Howell, C.R., Cotton, J., Porter, D. 2005. Identification of Pythium species on West Texas peanuts and sensitivity of isolates to mefenoxam and azoxystrobin in petri dish assays. Peanut Science. 32(1):9-13.

Interpretive Summary: Increased occurrence of peanut pod rot incited by Oomycetes and decreased effectiveness of pod rot fungicides has indicated a need for establishing what specific pathogens are causing pod rot disease in peanut fields, and to determine whether resistance to commonly used fungicides is present. A survey of 107 peanut fields was conducted to determine the incidence of Rhizoctonia and Pythium spp. pod rots. The Pythium spp. isolated from diseased pods were collected and identified to species. The Pythium spp. were then assayed for resistance to the fungicides mefonoxam and azoxystrobin. None of the species isolated were resistant to mefonoxam, but all of the Pythium irregulare isolates were more resistant to azoxystrobin than the other Pythium spp. Poor sensitivity to azoxystrobin by P. irregulare may explain why the fungicide sometimes fails to control pod rot in peanut fields.

Technical Abstract: A survey was conducted in 107 peanut fields to determine the incidence of Rhizoctonia or Pythium pod rot. Rhizoctonia solani and Pythium spp. were isolated from rotted pods in 35 and 39% of the fields, respectively. Isolates of Pythium were collected and some were identified to species. The three most common pathogenic species identified were P. irregulare, P. myriotylum, and P. ultimum. The three most common pathogenic species identified were P. irregulare, P. myriotylum, and P. ultimum. The sensitivity of pathogenic Pythium isolates to fungicide concentrations that reduced growth by 50% (EC50’s) ranged from 1 – 270 ng ai mefenoxam/ml. None of the Pythium isolates tested were resistant to mefonoxam. EC50’s of different Pythium isolates exposed to azoxystrobin ranged from 1 – 103 micro gram ai of azoxystrobin/ml. All the isolates of P. irregulare were less sensitive to azoxystrobin than any of the other pathogenic Pythium isolates that were identified to species. Poor sensitivity of P. irregulare to azoxystrobin may explain some failures of azoxystrobin to control pod rot.