Biology and Control of Bacterial Leaf Blight of Cornus

HORTSCIENCE 41(3):721–724. 2006. Disease incidence was evaluated in the affected nursery for two growing seasons. Observations were made monthly from early May to July. Biology and Control of Bacterial Leaf Since the prevalence of this disease appeared during early spring, we suspected that cool Blight of Cornus mas temperature and moisture were important in 1 disease development. Plant propagation from M.T. Mmbaga and E.C. Nnodu stem cuttings taken from apparently healthy Tennessee State University, Otis L. Floyd Nursery Research Center, McMinnville, branches of infected plants was observed and TN 37110 this practice may have perpetuated the disease at the nursery. A small study was conducted to Additional index words. dogwood, Pseudomonas syringae, fungicide/bactericide evaluate whether the stem cuttings from healthy Abstract. Cornelian cherry (Cornus mas L.) has been free of disease and pest problems until looking branches transmitted the disease to new recently when a bacterial leaf blight caused by Pseudomonas syringae was reported. Since plants. In 2000, 50 stem cuttings were taken its fi rst observation in middle Tennessee in 1999, the disease has become endemic in the from seemingly healthy branches of infected nursery where it was fi rst discovered. The objective of this study was to assess the disease, plants at the nursery fi eld, 50 other cuttings were evaluate factors that favor disease development, and develop disease management strate- propagated from noninfected C. mas ‘Redstone’ gies. Cool temperatures of 20 to 24 °C (day) and 10 to 15 °C (night) were most favorable to plants grown at the University landscape evalu- the disease and young leaves were highly susceptible while mature leaves were resistant to ation plots. In 2002, additional 40 cuttings were infection. Leaf wounding increased the susceptibility of leaves and mature leaves developed taken from the propagated plants, 20 from each infection at 28 °C, temperature at which nonwounded leaves were completely resistant group. The new cuttings were rooted under mist to infection. Results from this study also showed that plant propagation from seemingly in greenhouse environment at 25 ± 3 °C and they healthy branches of infected plants may have perpetuated the disease at the nursery. Six were monitored for disease symptoms during chemicals—Phyton-27 (copper sulfate), Camelot (copper salt of fatty acids), Agri-Mycin and after rooting. 17 (streptomycin), Kocide 101 (copper hydroxide), Basicop (elemental copper 53%), and, Disease development was evaluated Bordeaux mixture (cupric sulfate + lime) were evaluated for disease control. Phyton-27, and outside the nursery location using C. mas Agri-Mycin—were most effective and reduced disease severity to 10% of foliage showing ‘Redstone’ plants inoculated with a fi eld disease symptoms. Information from this study will be useful in designing effective disease isolate of P. syringae from the nursery. The management strategies. plants were grown in 3.75-L plastic containers and maintained in a shade-house (50% shade Cornelian cherry (Cornus mas L.), a native (Pscheidt et al., 2001; Sinclair and Backman, cloth) environment under drip irrigation. The to eastern Europe and western Asia, has demon- 1989; Sinclair et al., 1993) on other plants. bacterium was grown in nutrient agar and strated longevity and adaptability where it has The bacterium isolated from infected plants inoculum consisting of 106 (colony form- been grown with no apparent disease and pest was identifi ed as Pseudomonas syringae and ing units) CFU/ml was prepared in sterile problems. It is credited as an enduring dogwood fatty acid analysis showed that it was similar cultivated since the ancient time; it has brought to P. syringae pv. maculicola (Mmbaga and beauty and fl avor to the landscapes (Dirr, 1990). Sheng, 1999, 2000). Bacterial pathogens that In North America, it is used solely as a landscape attack woody plants are known to survive on ornamental, as a specimen tree or shrub or as a or within their hosts (Agrios, 1997). Bacterial large sheared hedge. Cornelian cherry is one of infections occur through natural openings or the earliest blooming spring fl owering trees and wounds and free moisture on plant surfaces shrubs, with fl owers emerging before the leaves. is critical during bacterial infection. Bacterial In some parts of Europe, it has a place on the table cells are transferred easily from plant to plant by as a fruit (fresh or pickled), in soft drinks and in handling or cutting instruments, water splash, the production of wines and liqueurs. and budwood (Agrios, 1997, Chase and Benson, A bacterial leaf blight caused by Pseudomo- 2001). Thus, cultural practices associated with nas syringae has been observed in one nursery in plant production can affect disease development Middle Tennessee. The location of the affected and spread. nursery was highly shaded, gorge-like valley and Since the fi rst report of this disease (Mmbaga the environment appeared to be highly suited and Sheng, 2000), some C. mas landscape plants to the disease proliferation. The infected plants displayed similar symptoms during early spring had blighted leaves and the young shoots had in 2000 and 2005, but the symptoms were in- dark brown necrotic lesions and some had tip signifi cant. To date, this disease has remained dieback (Fig. 1a). During the early stages of destructive at only one nursery where it has infection, leaves developed discrete lesions, become endemic. The objective of this study angular in shape and surrounded by a chlorotic was to determine when disease developed natu- halo. These lesions eventually coalesced to rally in the fi eld and the conditions infl uencing form large dark necrotic patches that covered disease development and evaluate bactericides a large part or the entire leaf (Fig. 1b). Disease and fungicides for controlling this disease. symptoms occurred only during the early spring when the weather was wet and cool. When the Materials and Methods temperature increased, all new growths were symptomless (Mmbaga and Sheng, 1999; Disease development and nursery environ- Mmbaga and Sheng, 2000). ment: Disease developed naturally starting Disease symptoms on C. mas were consistent from a group of plants imported from Europe. with those caused by Pseudomonas syringae Received for publication 28 Jan. 2006. Accepted for Fig. 1. Disease symptoms of bacterial leaf blight publication 4 Mar. 2006. caused by Pseudomonas syringae. (a) Severely 1To whom all correspondence should be directed; infected trees in the nursery fi eld. (b) Disease e-mail [email protected]. progress on infected leaves. HORTSCIENCE VOL. 41(3) JUNE 2006 721 11428-PlantPathol428-PlantPathol 772121 44/12/06/12/06 77:34:36:34:36 AAMM distilled water using 7-d-old cultures. Plants lay out. The study was initiated in early spring, symptoms and the percentage of healthy tissue per were sprayed to run-off using an atomizer to soon after the last frost date. Plant foliage were plant continued to increase over time. Although deliver inoculum uniformly. Inoculated plants spray-inoculated with a bacterial suspension the trend of disease development in 2004 and were incubated for 24 h at 100% RH and then of 106 CFU/mL prepared as described above, 2005 was similar, disease development started placed back to drip irrigation. In 2004, plants noninoculated control was sprayed with wa- one week earlier and overall disease severity was were inoculated during the fi rst week of May ter. The inoculated plants were incubated at higher in 2005 compared to 2004 (Fig. 2). The while in 2005 inoculation was done during 100% RH for 24 h and monitored for disease 1-week delay in inoculation and subsequently the last week of April. Disease development symptoms. Chemical treatments were initiated delayed disease establishment may have caused was evaluated weekly using a scale of 0 to 5 when the inoculated plants started showing the lower disease severity in 2004 compared to in which 0 = no infection, 1=1% to 10%, 2 fi rst disease symptoms. 2005 (Fig. 2). The temperature in May 2005 was = 11% to 25%, 3 = 26% to 50%, 4 = 51% to Chemicals were applied using a CO2-pres- cooler than in May 2004 and the total rainfall was 75%, and 5 =76 to 100% of foliage showing surized sprayer with 12 psi pressure; all plants higher in May 2005 (Figs. 3 and 4). Thus, the disease symptoms. Temperature and precipita- were sprayed to run off. The pesticides were environment in 2005 was more favorable to the tion were recorded daily. reapplied at 10-d intervals until the end of June disease and may have contributed to the differ- Effect of temperature, leaf age, and leaf or early July when disease spread stopped and ences in disease severity in the 2 years. wounding on disease severity: This study was new leaves did not show disease symptoms. Effect of temperature, leaf age, and leaf conducted using C. mas ‘Redstone’ grown Disease severity was evaluated using the rating wounding on disease severity. Disease symp- in 10.3-cm plastic containers and treatments scale of 0 to 5 described above. All data were toms were most severe at 24/15 °C followed consisted of four temperatures, two leaf ages analyzed using General Linear Model Proce- by plants at 20/15 °C (Table 1). Young leaves (young and old), and leaf wounding (wounded dure for analysis of variance with chemical were highly susceptible to infection and they and nonwounded). Four growth chambers treatment as the treatment effect (Schlotzauer developed disease symptoms within 7 d and the (Percival Scientifi c Inc., Boone, Iowa) were and Littell, 1987). wounded leaves developed symptoms in 5 d. set at four temperature regimes [28/15, 24/15, The nonwounded mature leaves were resistant 20/15, and 15/10 °C (day/night)] with 12 h day Results to infection (Table 1).

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