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Phytophthora ramorum, causal agent of ramorum blight of woody shrub species, has caused serious damage to cultivars and species of camellia in commercial nurseries. Reports of relative susceptibility of camellia to P. ramorum have indicated a range from high to low susceptibility, both in nurseries and under experimental conditions. We inoculated a series of cultivars of camellia to determine their relative susceptibility to infection, and then compared lesion size to the capacity of the pathogen to produce sporangia on the lesions. We found, as did others, a wide range of susceptibility among cultivars, but lack of correlation between susceptibility (lesion size) and potential to produce sporangia that might spread the pathogen within the nursery. These results indicate that on some cultivars the pathogen might produce small or inconspicuous lesions, yet still produce copious numbers of sporangia that could spread the disease, both within the nursery and from nursery to nursery. Introduction Ramorum blight, caused by the pathogen Phytophthora ramorum (21), has been reported on a wide range of trees, shrubs, and ornamental plants in nurseries and landscapes (20). As a result the disease and the pathogen are regulated by quarantine to prevent geographic dissemination (10). Numerous plant species have been shown to be susceptible (2,20), including camellia (1,13,16,19). Currently, nurseries are inspected to detect infected plants and thereby prevent dissemination of the disease geographically (9). However, the pathogen has been disseminated geographically from nursery to nursery on camellia and other host plants. In the United States, national nursery surveys were performed in 48 states in 2005, and seven states were found to have nurseries with plants that tested positive for P. ramorum (10). In 2004 infected camellia plants were found in nurseries in 22 states, shipped from supply nurseries on the west coast. How that dissemination occurred, in spite of inspections, remains unknown. One explanation, however, is that the pathogen spread within the disseminating nurseries and caused insignificant lesions on camellia leaves that were not detected. Shishkoff (16) described the symptoms and susceptibility of species and cultivars of camellias, and indicated that sometimes lesions were small, and on some cultivars the symptoms were difficult to see on the upper leaf surfaces. Furthermore, infected leaves on camellia often abscised and were no longer on the plants during inspections. If sporangia were produced on leaves that had fallen onto the medium surface, they could have washed down onto the roots, causing root infections that eventually progressed into the shoots, such as was shown for rhododendron (5). The purpose of this study was to test the hypothesis that some camellia cultivars were more susceptible than others, and that variation also occurred in the potential for the pathogen to produce sporangia on lesions on different genotypes of camellia. 22 August 2007 Plant Health Progress Inoculum Production, Inoculation, Disease Evaluation, and Sporulation Potential The P. ramorum used in this study, 03-74-N10-A (N10A), was isolated from rhododendron (Rhododendron sp.) in an Oregon nursery, and is a North American genotype of the A-2 mating type. Cultures were maintained and stored under refrigeration on agar slants until used, and then were transferred frequently on dilute V8 juice agar plates (30 ml/liter of clarified V8 juice instead of the normal 150 to 200 ml/liter) (6). Sporangia to be used as inoculum were produced on dilute V8 juice agar plates (150-mm diameter), starting from a sporangial suspension spread on the plates that were then incubated in a dark incubator at 20°C for 8 days. Sporangia were removed from the plates by flooding with 5 ml of sterile distilled water and scraping the surface of the agar with the edge of a spatula. The aqueous suspension of cauducous sporangia was then poured into a beaker and gently swirled using a magnetic stirrer. Sporangia were used to inoculate leaves from cultivars of Camellia japonica and C. susanqua, and six camellia hybrids (Fig. 1). Three leaves were randomly taken from each of 3 plants per cultivar grown in one-gallon containers. Just prior to inoculation, each leaf was wounded once on the abaxial side, away from the midvein, using a needle probe. A 10-μl drop of sporangial suspension containing approximately 2600 sporangia per ml was pipetted into the wound. The concentration of sporangia (or released zoospores) was determined by plating on PARP medium (4,14) and counting colonies. A single culture plate of N10A was used to inoculate 6 cultivars at a time. Inoculated leaves were placed in plastic box chambers with moist vermiculite in the bottom to maintain high humidity and free water on the leaf surface. Inoculated leaves in boxes were allowed to sit for 5 h in a 20°C incubator, after which they were manually sprayed with distilled water, enough to wet leaves to provide some free-standing droplets. The sealed containers were then placed in a 20°C incubator with a 14-h light regime (average 33 μmol/sec/m) to incubate for 7 days. Leaves were misted daily to maintain free water on the surface for 6 days. Boxes were rotated 180 degrees daily to balance light distribution among replications during the entire incubation time. Quantification of sporulation and lesion size was done on day 8. 22 August 2007 Plant Health Progress

Comparative Host Susceptibility and Sporulation Potential of Phytophthora ramorum