298 Mmbaga: Downy Mildew in Lilac 1995, but downy mildew has not previously been detected (Mmbaga et al. 2005). Ideal conditions for pathogenic development of downy mil- Figure 4. Morphologic characteristics of lilac downy mildew pathogen at 400× magnification showing (a) primary mono- podial branching of sporangiophores with (b) smaller spo- rangiophore branching arranged at right angles to the sup- porting branches and (c) obovoid sporangia. approximately 19.5 to 22 m × 14 to 17 m. Overwintering structures, oospores, were not observed. Pathogenicity tests on detached leaves confirmed that the originally observed organism was pathogenically associated with the observed symptoms. All leaves inoculated with the downy mildew sporangiospores developed fungal induced symptoms in 12 days; noninoculated leaves did not develop symptoms. The lesions started as chlorotic lesions and in 10 to 12 days; the symptoms turned brown to ashy brown and necrotic similar to symptom development in the field. Symptoms and signs were characteristic of downy mildew; morphologic features of the fungus observed under a compound microscope were character- istic of Plasmopara species. DISCUSSION Downy mildew fungi can survive for many years as overwinter- ing oospores in the soil or in colonized roots and host debris (Scribner 1886; Cotner 1930; Barrett 1939; Spencer 1981). New infection may occur soon after transplanting a susceptible host in an infested area or transplanting a previously infested plant to a location where environmental conditions are favorable for dis- ease development. The source of inoculum for this disease is not clear. The location where Syringa ‘Old Glory’ and ‘Declaration’ were grown did not have lilac plants in the previous 12 years. It is probable that the infection started from infested plant material or overwintering oospores, which may have remained dormant in soils for decades (Scribner 1886; Cotner 1930; Barrett 1939). More than 50 accessions of lilac have been growing at TSU Otis Floyd Research farm in McMinnville, Tennessee, U.S., since ©2008 International Society of Arboriculture dew are cool night temperatures of 6°C to 15°C (42.8°F to 59°F) and day temperatures no greater than 25°C (77°F). Free surface moisture such as rain, condensation, or fog persisting until mid- morning for at least 4 days in a row is required for sporangio- spore germ tube development and subsequent epidermis penetra- tion (Cotner 1930; Spencer 1981). In McMinnville, Tennessee, environmental conditions favorable to downy mildew may occur during April to May when monthly mean temperature range from 15°C to 26°C (59°F to 78.8°F). Once downy mildew in- fection has occurred from previously infested plants, or from infested soil, a new crop of conidia can be produced in 4 to 5 days. The sporangiospores are disseminated by rain and wind and they can germinate within 4 hrs (Cotner 1930; Spencer 1981). As seasonal temperatures rise, plants tend to outgrow the disease. New infections may also occur in the fall when seasonal temperatures once again become favorable. Infections that de- velop in fall may pass unnoticed because of natural change in leaf colors associated with defoliation. Thus, it is not clear when the first symptoms of downy mildew infection developed on ‘Old Glory’. Downy mildew is a destructive disease on many field and vegetable crops, but it has little economic impact on woody plants except in roses and grapes (Sinclair et al. 1993). Although some defoliation may be associated with this disease, the impact of downy mildew disease on lilac is mostly aesthetic. Because cool temperatures are critical for continued downy mildew dis- ease development, higher temperatures characteristic of the Ten- nessee summers will probably not allow the perpetuation of this disease as a production problem. If other lilac accessions are susceptible, and the disease has opportunity to spread to other hosts, downy mildew infection would likely be limited to early spring. Studies on the management of this disease in lilac were not undertaken. However, recommendations for the management of downy mildew in other crops include avoiding planting suscep- tible plants in infested areas, use of resistant plants, chemical fungicides, and cultural methods that improve air circulation and avoid wetting plant foliage (www.ces.ncsu.edu/depts/pp/ cucurbit/). Broad-spectrum contact protectant fungicides such as dithiocarbamate (Mancozeb), copper (copper sulfate, copper hy- droxide), and Benzonitrile (chlorothalonil) provide some downy mildew control (Paulus et al. 1983; Horst 1990). Fungicides that specifically target oomycete fungi allow better control of downy mildew fungi. Most effective fungicides are systemic or partially systemic and have combined systemic and protectant efficacy (Horst 1990). Several systemic fungicides are now available for downy mildew, including fosetyl–aluminium (Aliette; Bayer Crop Science, Research Triangle Park, NC), azostrobin (Heri- tage; Zeneca Professional Products, Wilmington, DE), strobi- lurin (Compass; Norvatis Crop Protection, Greensboro, NC), mefenoxam (Ridomil Gold; Syngenta Crop Protection, Wil- mington, DE), cymoxanil (Curzate or Tanos; DuPont Com- pany, Wilmington, DE), propamocarb (AgrEvo USA Co., Wil- mington, DE) (Previcur Flex; Bayer Crop Science, Research Triangle Park, NC), cyazofamid (Ranman; FMC Corporation, Philadelphia, PA), dimethomorph (Forum; BASF Chemical Corporation, Florham Park, NJ), phosphorus acid fungicides (Phostrol; Nufarm Americas Inc., Burr Ridge, IL; ProPhyt;
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