Characterization of Phytophthora spp. on Floriculture Crops in North Carolina

Isolates of Phytophthora in floriculture crops were collected from North Carolina commercial greenhouse facilities in 2007 and 2008, identified, and characterized for mating type and mefenoxam sensitivity. In all, 163 isolates from 13 host species at 11 locations were identified primarily as Phytophthora nicotianae (59%), P. drechsleri (23%), P. cryptogea (9%), and P. tropicalis (4%). Multiple Phytophthora spp. were found at five locations. DNA sequencing was more reliable for differentiating P. cryptogea and P. drechsleri. Only the A1 mating type of P. drechsleri was collected; however, both mating types of P. nicotianae, P. cryptogea, and P. tropicalis were found. Overall, 66% of Phytophthora isolates were resistant or intermediate in resistance to mefenoxam at 1 μg a.i./ml. Three groups of P. drechsleri isolates had effective concentration of mefenoxam providing 50% growth inhibition (EC₅₀) estimates over 700 μg a.i./ml. EC₅₀ estimates for P. nicotianae ranged from 246 to 435 μg a.i./ml. Isolates of P. nicotianae exhibited phenotypic heterogeneity at several locations, suggesting multiple separate introductions to the facilities. In contrast, P. nicotianae and P. drechsleri isolates from two locations were phenotypically identical to a previous survey, suggesting that the facilities have Phytophthora spp. established onsite.