Phytophthora citricoia dominated among 11 genera and fungal species isolated from Rhododendron brachycarpum, R. catawbiense, R. impeditum and R. sepedonicum. In greenhouse trial isolates from Abies concolor, Chamaecyparis lawsoniana, R. catawbiense, R. impeditum and Thuya occidentalis caused dieback of rhododendron. Inoculation of leaf blades with isolates of the pathogen from 4 cultivars resulted in the spread of necrosis about 0.63 mm/hr. P. citricola was pathogenic to all tested rhododendron cultivars.

Phytophthora citricola Sawada was detected from 3 water pools situated in 2 container- grown nurseries. The highest number of spots on rhododendron leaves were observed in June whereas the lowest in October. The use of water for plant sprinkling caused browning, yellowing of shoots and root rot of Buxus sempervirens, and blight of shoot tips of Thuja occidentalis and Rhododendron sp. The disease symptoms were observed already in June and the disease developed till the first decade of October. Losses caused by the species varied from 9 to 56%.

Genotypic differentiation among 10 isolates of Phytophthora cinnamomi Rands and 24 isolates of Phytophthora citricola Sawada from 12 different plant species grown in Polish ornamental nurseries was determined. DNA was extracted from pure pathogen cultures and amplified by the PCR technique using ISSR and RAPD primers. 9 primers were used to amplify P. cinnamomi and 8 to amplify P. citricola DNA. The analyzed amplification products were between 300 and 2300 bp. The genotypical differentiation was from 17 to 35% in P. cinnamomi and from 10 to 60% in P. citricola. Isolates from host plants of the same family showed, with some exceptions, similar levels of differentiation.