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Fungicides were evaluated for preand post-infection control of ramorum blight, caused by Phytophthora ramorum, on Rhododendron cvs. Cunninghams White and Irish Lace, Camellia japonica, Pieris japonica, and Viburnum tinus. Cyazofamid, dimethomorph, mefenoxam, pyraclostrobin, and fenamidone applied as foliar sprays consistently provided preventative control as indicated by reduced lesion size compared to water controls. These fungicides provided preventative activity for at least 28 days in the tested species except in Rhododendron where fungicides were active for at least 14 days following application. With preventative fungicide applications, the pathogen was recovered from most fungicide-treated leaves by isolation onto selective media. Dimethomorph consistently reduced the percent recovery from diseased Rhododendron leaves. With post-infection treatments, the fungicides did not significantly reduce lesion growth and percent recovery of the pathogen. The pathogen was recovered from lesions consistently at least 6 weeks after fungicide application in Rhododendron regardless of treatment on intact and fallen diseased leaves. However, the cultures resulting from isolations of diseased tissue treated with cyazofamid and dimethomorph were significantly slower growing than those cultures from other fungicide treatments. P. ramorum management issues relating to fungicide use in commercial nurseries are discussed. Introduction Phytophthora ramorum is the causal agent of the disease known as sudden oak death (SOD). The pathogen causes trunk cankers and widespread mortality on tanoak (Lithocarpus densiflorus), and oak (Quercus spp.) (18), and leafspots and blights on numerous other native hosts in California and Oregon woodlands (7). The pathogen was described as a new Phythophthora species in 2001, but it was observed as early as 1993 to cause leaf blights and mortality on Rhododendron and Viburnum in nurseries and public gardens in Germany and The Netherlands (21). With the recognition that this newly identified pathogen caused SOD, intensive nursery stock and public garden inspections ensued, and P. ramorum was found in several European countries. In December 2000, P. ramorum was first discovered infecting Rhododendron nursery stock in California (9). By 2003, agricultural inspectors found the pathogen infecting Rhododendron, Camellia, Viburnum, and Pieris nursery stock in California, Oregon, Washington, and British Columbia, Canada. In 2004, the disease became a national concern when a large wholesale nursery in California shipped camellia plants infected with P. ramorum to nurseries and other customers in 40 states. Presently the Animal and Plant Health Inspection Service (APHIS) Plant Protection and Quarantine (PPQ) Agency lists 110 plant taxa as proven or associated hosts (1). Of plants on that list, Rhododendron, Camellia, Viburnum, and Pieris are some of the most commonly cultivated and important host species worldwide. 8 February 2008 Plant Health Progress There is much concern that infected or infested nursery stock may move through the nursery trade to locations favorable for disease development and provide inoculum to infect native forest and woodland hosts. In England and The Netherlands there is evidence of inoculum moving from infected Rhododendron and causing mortality on Fagus sylvatica, several Quercus species (rubra, sylvatica, and ilex) and Aesculus hippocastrum. Since 2001, there have been various state, federal, and international quarantine restrictions placed on the movement of plants or plant parts of known hosts of P. ramorum. Official inspections are an important part of those regulations and concerns exist that fungicide use may mask symptoms in nursery stock or reduce the effectiveness of laboratory detection. In contrast, as part of a comprehensive disease control program that includes exclusion, scouting, and sanitation, fungicides may play a role in the control of P. ramorum in nursery stock. We evaluated fungicides for efficacy against P. ramorum on four commonly grown and important ornamental nursery hosts. Our first objective was to screen a wide range of commercially-available and experimental fungicides. The second objective was to select the best performing fungicides from objective one and further test their efficacy and residual action for preventative control. Treatments were also judged by their effects on pathogen recovery when isolated from treated leaves. The third objective was to determine the eradicative potential of these fungicides by applying them to active lesions and evaluating subsequent lesion growth and pathogen recovery after isolation from those lesions. Experimental Plants and Environmental Conditions Fungicides were evaluated on four genera of plants: In the first year, rhododendron (Cunninghams White) and azalea (Rhododendron Irish Lace); in the second year, camellia (Camellia japonica Elena Nobile), Japanese pieris (Pieris japonica Whitewater), and laurustinus (Viburnum tinus Compacta). Rhododendron and Camellia were established in 1-gal containers and Pieris and Viburnum were grown in 3.3-inch diameter containers. Cunninghams White represented a highly susceptible Rhododendron and Irish Lace represented a moderately susceptible azalea (19). Both Rhododendron cultivars were used in the first experiment to screen fungicides and Cunninghams White was used in all subsequent Rhododendron experiments. All plants were initially maintained outside and moved into a research greenhouse (Salinas, CA) one to several months before inoculation. The greenhouse was kept at a maximum day temperature of 73.5°F. and minimum night temperature of 51.0°F. The soil medium of the Elena Nobile contained only peat moss, while the other species were grown in approximately equal portions of Douglas fir bark, peat moss, and perlite. Inoculum, Wounding, and Inoculation The Phytophthora ramorum isolate (Pr-52) used in all experiments originated from an infected Rhododendron from a commercial California nursery in Santa Cruz County and has been previously characterized (13,18,20,21). Inoculum consisted of mycelial agar plugs. Wounding of leaf tissue was accomplished by gently poking the abaxial side of the selected leaf with a circular, 6-mm diameter, fine wire brush. In the first set of experiments there was no statistical difference when comparing the treatments effectiveness on wounded and unwounded inoculations. Since wounding increased the success of infection and increased the experiments statistical precision, it was used after the first set of experiments. The inoculum cap was placed over the wounded or selected area on the distal (1/3) end of the leaf. The lid was held in place with a modified hair curl-clip and kept there until the resulting lesion was evaluated. Screening Potentially Effective Fungicides Treatments in the fungicide screen trials included fungicides that had been either commercially registered or experimentally tested to have activity on Phytophthora species (5,6,12,16,17). Treatments included fungicides of different 8 February 2008 Plant Health Progress chemical classes with diverse modes of action and different movement characteristics within the plant. Upwardly mobile fungicides (mefenaxom and fosetyl-Al), in addition to being applied as foliar applications, were also applied as soil drenches 7 to 14 days before inoculation to allow for absorption and translocation to the foliage. All other fungicide treatments consisted of foliar sprays and were applied one day (both Rhododendron cultivars) or 7 days (Camellia, Viburnum, and Pieris) before inoculation. Treatment rates were selected based on the highest labeled rate, or in the case of an experimental compound, the treatment was applied at the highest recommended rate given by the chemical companys experimental protocol. To facilitate foliar wetting on these hard to wet species, all foliar spray treatments as well as the water check included a surfactant Nofoam B (CMR Inc., Fresno CA) at 0.1% (Rhododendron) or Silwet L-77 (Loveland Industries LTD, Cambridge, UK) at 0.05% (Camellia, Viburnum, and Pieris) (Table 1). Table 1. Experimental treatments: active ingredient, application type, trade name, percent active ingredient, rate applied, and treated plants. Rate given is for the formulated product. Foliar treatments applied until spray just began to drip from foliage. All foliar spray treatments as well as the water check included a surfactant Nofoam B @ 0.1% (Rhododendron) or Silwet L-77 @ 0.05% (Camellia, Viburnum, and Pieris). Soil treatment volume was 5 fl oz (just allowing some leachate from the bottom of 1-gal container). Treated Plants: 2 Rhododendron cultivars = R, Camellia = C, Viburnum = V, Pieris = P. Determining Residual Fungicidal Action Treatments in the residual action trials included fungicides chosen from the most efficacious fungicides in the screening experiments. Fungicide treatments were applied 28, 14, 7, and one day before the inoculation date for each experiment. Evaluation dates were dependent on the relative lesion growth rate and propensity for infected leaves to drop. This percentage was measured 6, 10, 14, and 16 days after inoculation, respectively, for Pieris, Camellia, Rhododendron and Viburnum. Treatment (active ingredient) Application Trade fungicide (a.i. concentration) Treatment rate (100 gal) Treated plants azoxystrobin Foliar Heritage 50% 2.0 oz C,V,P copper sulphate pentahydrate Foliar Phyton 27 21.36% 65 fl oz R cuprous oxide Foliar Nordox 83.9% 21.3 oz R cyazofamid Foliar Segway 34.5% 6.0 fl oz C,V,P dimethomorph Foliar Stature DM 50% 12.8 oz R,C,V,P fenamidone Foliar Fenstar 44.4% 14 fl oz R,C,V,P fosetyl-Al Soil Aliette WDG 80% 12.8 oz R fosetyl-Al Foliar Aliette WDG 80% 5.0 lb R,C,V,P mancozeb Foliar Dithane 75% 24 oz C,V,P mefenoxam Soil Subdue Maxx 22% 2.0 fl oz R mefenoxam Foliar Subdue Maxx 22% 2.0 fl oz R,C,V, P mono-di potassium salts of

Evaluation of Fungicides for the Control of Phytophthora ramorum Infecting Rhododendron, Camellia, Pieris, and Viburnum