Evaluation of Foliar Fungicide Sprays for the Control of Boxwood Blight, Caused by the Fungus Cylindrocladium buxicola

Cylindrocladium buxicola causes a damaging blight disease on boxwood which has spread rapidly throughout Europe since introduction of the pathogen in England in the mid 1990s. The pathogen has also been recently identified in the USA and British Columbia. The disease is difficult to control using cultural methods and information about chemical control is lacking. To address this, preventative and curative foliar fungicide sprays previously shown in laboratory tests to have efficacy were evaluated over two autumn/winter seasons in the UK. Results from the first autumn/winter season showed that the premix product Opponent (epoxiconazole + kresoxim-methyl + pyraclostrobin) was the best treatment when applied preventatively 3 days before inoculation. In the second autumn/winter season, curative treatment of diseased plants was best achieved with a weekly program of fungicides starting 3 days before inoculation and alternating with two or three products including Bravo (chlorothalonil), Signum (boscalid + pyraclostrobin), and Octave (prochloraz). The only fungicide tested and available to amateur growers in the UK, Fungus Clear (penconazole), was found to give moderate control of boxwood blight. Introduction Boxwood blight, caused by Cylindrocladium buxicola nom. cons. prop. (= C. pseudonaviculatum) (7,15,16), is the most destructive foliar disease of boxwood (Buxus spp.). The fungus causes leaf spots and black stem lesions followed by defoliation and dieback. Severe dieback can lead to death of plants, especially young seedlings. Cases of the disease have been reported widely throughout Europe since it was first recorded in 1994 in the UK (3,5,6,12,15,18,24,25). Outside Europe the disease occurs in New Zealand (15) and has also recently been found in the USA and British Columbia (9,20). C. buxicola is a threat to boxwood growing in commercial nurseries, landscapes, public gardens, and parks as well as native populations of boxwood around the world. The fungus appears to be confined to the family of Buxaceae (17) and infects many species in the genus Buxus including B. balearica, B. bodinieri, B. glomerata, B. harlandii, B. macowanii, and B. riparia. These species grow naturally over the four different continents Africa, Central America, Asia, and Europe (1). In the UK, the disease is widespread with an increasing incidence in gardens (14) and has been confirmed on native B. sempervirens growing wild, posing a threat to this unique habitat. The fungus may be spread in wind-blown rain but needs a vector to travel long distances (15), and can remain viable for at least 5 years on fallen boxwood material (17). It can also survive as microsclerotia in the soil (8). C. buxicola has also been spread widely on infected plants in plant trade. Control measures include pruning of infected twigs, destruction of fallen leaves, and increased sanitation to help reduce inoculum. 24 October 2013 Plant Health Progress Data is lacking on fungicide efficacy for this disease. Preliminary in vitro experiments showed that the fungicides Stroby (kresoxim-methyl), Opponent (epoxiconazole + kresoxim-methyl + pyraclostrobin), Opera (epoxiconazole + pyraclostrobin), and Signum (boscalid + pyraclostrobin) were the most effective at inhibiting growth of mycelium and germination of conidia (17). Other active ingredients such as carbendazim, chlorothalonil, prochloraz, and penconazole showed mixed activities in vitro. Penconazole was the only fungicide tested that was available to amateur gardeners in the UK. The current study was conducted to evaluate the efficacy of these fungicides against the disease under field conditions. The fungicides were tested on English Boxwood, B. sempervirens ‘Suffruticosa,’ a variety associated with most cases of the disease and also the most susceptible in pathogenicity assays (17). In the first year, fungicides were initially tested as single sprays, evaluating their potential as protectant and curative applications. During the second year of the project, various combinations of the most successful treatments from year one were evaluated as spray programs. Infection Trials on Container B. sempervirens (September 2006-March 2007) Three-year-old B. sempervirens ‘Suffruticosa’ grown in 15-cm-diameter (1liter) pots were imported from the Netherlands at least one month before the application of fungicides. The experiment took place at the agricultural and environmental consultancy ADAS Arthur Rickwood (Cambridge, UK). The plants were placed outdoor on a woven polypropylene groundcover MyPex. The area was covered and had a central line of overhead irrigation. Plants were given a dressing of Basacote Plus 16-18-12 slow-release fertilizer (six months release) at the start of the experiment at a rate of 3 g per pot. Overhead sprinkle irrigation was provided for three daily periods during the first two weeks and then once daily until the end of October. A single spore culture of C. buxicola (PT25, genotype G1, K. Heungens, personal communication) was obtained from a naturally infected B. sempervirens. It was selected for its sporulation abilities and aggressiveness on B. sempervirens ‘Suffruticosa.’ Suspensions of conidia used for infection were prepared as follows. The cultures were grown on potatodextrose agar (PDA; Oxoid, Basingstoke, UK) at 25°C in the dark. Several mycelia plugs from 7-day-old PDA cultures were placed onto carnation leaf agar (CLA) (11) and synthetic nutrient agar (SNA;1 g KH PO , 1 g KNO , 0.5 g MgSO 7H O, 0.5 g KCl, 0.2 g glucose, 0.2 g sucrose, and 20 g agar/liter distilled water, pH 6-6.5) media and incubated at 25°C under near ultraviolet light for 10-15 days. The spore suspension was made by adding sterile distilled water to which Tween 20 (0.1 % v/v) was added. The number of conidia was counted with a haemocytometer and a suspension of 1.2 × 10 spores/ml was used to inoculate plants. Plants were inoculated on 11 September 2006 using a Hoselock hand pump sprayer. The upper surfaces of all leaves of the experimental plants were inoculated with 25 ml of spore suspension per plot; guard plants were not inoculated. Distilled water with added Tween 20 (0.1 % v/v) was sprayed on each of the uninoculated control plants. The disease was encouraged to develop by providing humid conditions under polyethylene. The cover was removed after 24 h. Seven fungicides (Table 1) were applied as protectant and curative foliar sprays. These were kresoxim-methyl (Stroby WG; BASF, Cheadle Hulme, UK), carbendazim (Delsene 50 Flo; Nufarm UK, Belvedere, UK), chlorothalonil (Bravo 500; Syngenta Crop Protection UK Ltd, Cambridge, UK), prochloraz (Scotts Octave; Scotts, Ipswich, UK), azoxystrobin (Amistar; Syngenta Crop Protection UK Ltd, Cambridge, UK), penconazole (Fungus Clear; The Scotts Company Ltd, Godalming, UK), epoxiconazole + kresoxim-methyl + pyraclostrobin (Opponent; BASF, Cheadle Hulme, UK ). All products were applied at label rates either for ornamentals, or if not available for other crops under Long Term Arrangements for Use. When applied preventatively, products were applied 3 days before inoculation and not followed by further applications. When applied curatively, products were applied when symptoms first developed, 6 days after inoculation. These were applied three times, at 7-day intervals, except for Delsene Flo and Opponent which were applied twice at a 14-day 2 4 3