Pathotype Distribution and Fungicide Sensitivity in Michigan.

Identifying the pathotype structure of a population is crucial for the effective management of Phytophthora stem and root rot of soybean (PRR). . has been successfully managed with major resistance genes, partial resistance, and fungicide seed treatments. However, prolonged use of resistance genes or fungicides can cause pathogen populations to adapt over time, rendering resistance genes or fungicides ineffective. A statewide survey was conducted to characterize this pathotype structure and fungicide sensitivity of . within Michigan. Soil samples were collected from 69 fields with a history of PRR and fields having consistent plant stand establishment issues. Eighty-three isolates of . were obtained, and hypocotyl inoculations were performed on 14 differential soybean cultivars, all of which carry a single gene or no resistance gene. The survey identified a loss of effectiveness of genes 1b, 1k, 3b, and 6, compared with a previous survey conducted in Michigan from 1993 to 1997. Three effective resistance genes were identified for . management in Michigan; 3a, 3c, and 4. Additionally, the effective concentration of common seed treatment fungicides to inhibit mycelial growth by 50% () was determined. No . isolates were insensitive to the tested chemistries with mean values of 2.60 × 10 μg/ml for ethaboxam, 3.03 × 10 μg/ml for mefenoxam, 2.88 × 10 μg/ml for oxathiapiprolin, and 5.08 × 10 μg/ml for pyraclostrobin. Results suggest that while there has been a significant shift in gene effectiveness, seed treatments are still effective for early season management of this disease.