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%.

Phytophthora citricola constituted about 70% of all Phytophthora isolates recovered from rhododendron leaves used as baits for detection of that group of organisms in water. The species was found in 4 rivers, 2 hardy nursery water reservoirs and nursery drainage canal from May to October, 2006. Analysis of spots’ number on rhododendron leaf baits as a measure of P. citricola density showed that place of holding baits had a significant influence on the species occurrence. Significantly more spots, especially in July survey, were observed on baits held in Skierniewka and Zwierzynka rivers swimming through agricultural and forest area than in Ner, the river of horticultural area. Significantly more spots/rhododendron leaves were observed when they were held in rivers downstream of nursery and in the middle of hardy nursery borders. In nursery water containers and drainage canal higher Phytophthora density was recorded in August than in other periods of surveying. Using water from reservoir for sprinkling of Picea omorika nursery trees caused the development of tip blight and from diseased twigs P. citricola was isolated.

We examined whether peroxidase activity in cutting bases and leaves and starch content in cutting bases can be used as rooting phase markers in the elepidote rhododendron cv. ‘Babites Baltais’ (Rhododendron L.). Changes in peroxidase activity in cutting leaves and bases, as well as starch content in cutting bases, were determined in relation to anatomical stages of rhizogenesis in leaf bud cuttings treated with 1% indole-3-butyric acid (IBA+) or without IBA (IBA-). The pattern of change of peroxidase activity was similar in cutting bases and leaves of IBA- leaf bud cuttings. Three phases of adventitious root formation were identified: induction, initiation and expression. During the induction phase peroxidase activity decreased, but no anatomical changes were observed in the cuttings. Peroxidase activity increased in the initiation phase when adventitious root initials were formed. Peroxidase activity decreased during the expression phase when adventitious root primordia developed. The starch content of IBA- leaf bud cuttings decreased during the first few days and then gradually rose to maximum, followed by a sharp reduction and another increase at the end of the experiment. The changes of starch content did not coincide with rooting phases as peroxidase activity did, and cannot be used as a rooting phase marker in rhododendrons. Adventitious root formation did not occur in IBA+ leaf bud cuttings, so distinct rooting phases could not be observed. There was a significant correlation between peroxidase activity in cutting bases and leaves of IBA- leaf bud cuttings. Peroxidase activity in leaves of rhododendron leaf bud cuttings are potentially useful as a marker for rooting phases, but that requires further anatomical and physiological study of rooting in leaf bud cuttings.