Susceptibility of Camellia to Phytophthora ramorum

Camellia is a known host of Phytophthora ramorum, the "sudden oak death" pathogen. During 2003-2004, the organism was shipped from California throughout the U.S. on infected nursery stock, leading to a nationwide effort to recover the infected plants. This paper describes the symptoms on Camellia and the relative susceptibility of nine species and four hybrids. Camellias varied widely in susceptibility, with Camellia x ‘Roni gaki’ showing the worst overall symptoms, while some other cultivars showed little or none. Obvious symptoms include leaf lesions and stem blight; defoliation, while more difficult to observe, was also characteristic in camellias. The pathogen persisted in diseased plants, notably in stem tissue, for at least a month as demonstrated by isolation. One month after inoculation, roots remained asymptomatic, but the pathogen could be recovered from washed or surface-sterilized root pieces. These results will aid the ongoing national survey for P. ramorum at nurseries and forests. Introduction Phytophthora ramorum Werres, De Cock & Man in't Veld, causes leaf spots, twig blight, and cankers on a variety of plant hosts. It was first observed on nursery stock in Europe, then in California forests around the mid-1990s. In 2003, camellias infected with P. ramorum were found in California nurseries. Initially, only one cultivar, C. sasanqua ‘Bonanza,’ was found to be infected (4), but soon it became apparent that many cultivars were susceptible. In 20032004, shipments of camellia plants from California nurseries, some of which were infected with P. ramorum, were transported to other states and Canada, triggering a massive effort to recover these plants before the pathogen could spread. Since January 2005, the Emergency Federal Order Restricting Movement of Nursery Stock from California, Oregon, and Washington Nurseries requires inspection of nurseries that ship hosts of P. ramorum outside the regulated areas for the disease. In addition, many state agencies now conduct yearly nursery surveys to determine if the pathogen is present. In these surveys, symptomatic host plants are sampled; therefore it is important to recognize the various symptoms of the disease and to establish the relative susceptibility of Camellia species. Inoculation of Plants and Rating of Disease Nine species and four hybrids of Camellia were tested for susceptibility to Phytophthora ramorum (Table 1). In general, the plants were rooted cuttings with 8 to 25 leaves; some were older plants. The pathogen isolate (5-C) used in these experiments was originally recovered from C. sasanqua ‘Bonanza’ in California in 2003. Sporangia in solution were prepared as described previously (8), by placing mycelial plugs in a sterile soil extract solution for 48 h. Plants were inoculated using a large paintbrush to apply sporangial solution (approximately 5000 sporangia per ml) to leaves, buds, and stems. Plants were inoculated in batches of 15 to 20, with individuals of each tested species or cultivar separated into at least 2 batches. Each batch included a few rooted cuttings of Rhododendron ‘Cunningham’s White’ used as positive controls; at 5000 sporangia per ml, this cultivar develops distinct, easy-to-rate symptoms that, for the genus, are considered severe. Inoculated plants were carefully 15 March 2006 Plant Health Progress spaced in a dew chamber at 20°C for 3 to 4 days until symptom development, and then removed to insure that fallen leaves could be paired with the plant that had dropped them. Upon removal from the dew chamber, plants were rated for defoliation and the percent symptomatic area of each leaf. If a particular leaf symptom was not clearly caused by the pathogen, or if defoliation of nonsymptomatic leaves occurred, leaves were plated on PARP media (2). Statistical analysis was done to compare disease severity among cultivars. Cultivars showing very little or no disease had to be excluded from data sets because the large number of zero data points made it impossible to get a normal distribution of data. Remaining data was log-transformed and analyzed by General Linear Models with Tukey’s Studentized Range test using SAS (SAS Institute Inc., Cary, NC). Infected camellia leaves developed dark, water soaked lesions with defined margins (Fig. 1). When lesions dried out, they turned brown and brittle, distorting thinner leaves. Generally lesions did not expand to encompass a whole leaf unless young leaves were involved, and then infections could travel down the petiole and into the stem. In some cultivars, the lesion was difficult to see on the upper surface of leaves, particularly in dark-pigmented leaves. Lesions on older leaves of the cultivar ‘Korean Fire’ appeared as faint ring spots on the upper surface and as dark spots on the lower surface (Figs. 2a and b). Lesions on young reddish leaves of ‘Night Rider’ were difficult to see on the upper surface, but obvious on the lower surface (Figs. 3a and b). Diseased camellia leaves frequently fell off 4 to 5 days after inoculation; some defoliation occurred even with leaves showing no visible lesions, although the pathogen could often be recovered from the surface-sterilized leaf. Significant defoliation of the cultivar ‘Elina Cascade’ occurred, although few symptomatic leaves were observed. In addition to leaf symptoms, branch die-back was sometimes observed, notably in C. japonica ‘Crimson candles’ (Fig. 4). Results of camellia inoculations are summarized in Table 1. No generalizations could be made about susceptibility at the species level; some cultivars of C. japonica and C. sasanqua were very susceptible, while others were not. The hybrid ‘Roni Gaki' showed the most severe symptoms among the taxa tested. C. sasanqua ‘Midnight Lover’ was quite susceptible, with defoliation up to 18.2% and observable symptoms on 18.8% of total leaves. C. sasanqua ‘Angel’s Kiss,’ however, showed no symptoms. Similarly, while C. japonica ‘Crimson Candles’ was relatively susceptible (with 14% defoliation and 11.9% of leaves showing symptoms), C. japonica ‘Black Magic' was asymptomatic. Disease on C. oleifera varied widely in severity from plant to plant; since this species can be propagated from seed, this may have been due to genetic variability. C. sinensis, the "tea" of commerce, was moderately susceptible. 15 March 2006 Plant Health Progress Table 1. Responses of different species and cultivars of Camellia to infection by Phytophthora ramorum. u Defoliation = the number of leaves that had fallen off the plant divided by total leaves on the plant at inoculation. Leaves that fell were not necessarily symptomatic. v % symptomatic lvs = the number of leaves showing visible symptoms after inoculation divided by total leaves on the plant at time of inoculation. This included symptomatic leaves that had dropped. w % dis/lvs tot = the percent diseased tissue averaged over total leaves. x Lesion size is meant to give an idea of the average size of lesions on symptomatic leaves, derived by adding the % lesion size for each diseased leaf and then dividing by the number of diseased leaves total. (footnotes continued) Camellia species and cultivarsz No. plants tested % Defoliationuy % symptomatic leavesvy % dis /lvs totwy Lesion sizex C. japonica 'Black Magic' 4 0.0 0.0 0.0 0.0 'Korean Fire' 10 3.8 efg 21.4 bcde 0.9 bcd 4.4 'Koto no kaori' 8 33.3 abcd 42.7 abc 0.7 bcd 18.8 'Royal Velvet' 11 2.3 2.8 0.1 1.2 'Shibori kingyo' 8 29.4 abcde 74.9 ab 1.2 bcd 12.0 'Tamo-no-ura' 8 2.5 4.33 0.2 1.8 'Tsagawa shibori' 8 30.6 abcde 74.4 ab 1.0 bcd 16.9 C. oleifera 17 7.3 12.7 3.0 8.5 seedlings 16 0.3 8.6 3.0 12.8 'Lushan Snow' 6 1.6 efg 2.7 efgh 0.1 de 4.6 C. sasanqua 'Angel's Kiss' 6 0.0 0.0 0.0 0.0 'Asa Kura' 8 36.7 abcde 55.7 ab 1.4 bcd 20.2 'Bonanza' 6 9.5 16.4 defg 1.8 cde 5.4 ‘Hatsu Hikari’ 7 13.8 abcdefg 9.8 defg 2.5 bcd 19.3 'Midnight Lover' 15 18.2 abcdef 18.8 cdef 2.6 bcd 9.4