APS Abstracts of Presentations

Occurrence of Rubbery brown rot or Phytophthora root rot of carrots (Daucus carota L.) in Michigan
C. C. SAUDE (1), M. K. Hausbeck (1)
(1) Dept. Plant Pathology, Michigan State University, East Lansing, MI
Phytopathology 96:S104

In fall 2005, Phytophthora spp. were isolated from diseased carrot roots grown for the processing industry. Samples were collected from fields exhibiting foliage blight and crowns with blackened petioles located in Mason, Oceana and Newaygo counties, MI. The Phytophthora spp. caused symptoms associated with rubbery brown rot/Phytophthora root rot, previously described in other carrot producing areas. Inoculations were carried out in fulfillment of Koch’s Postulates. Three to seven days after inoculation, symptoms on wounded and nonwounded carrot roots occurred and included water-soaked lesions that progressed to a dark brown to black color over time. Infected tissue remained firm but watery on some carrots, while on others sunken and watery. On V8 juice agar, the homothallic Phytophthora spp. isolates produced papillate, obpyriform, caducous sporangia and oospores with paragynous antheridia. Sporangia mean lengths were 35.0 to 45.2 µm and mean widths were 26.2 to 33.2 µm. Oospore mean diameters were 27.2 to 32.0 µm. Optimum temperature for mycelium growth was 25°C with significant growth at 30°C and no growth at 5°C. These morphological characteristics were consistent with Phytophthora cactorum, cited as one of the causal agents of rubbery brown rot/Phytophthora root rot of carrots. Molecular studies are in progress to confirm the identity of the Phytophthora spp. isolated from carrot roots from Michigan production fields.

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