Abstract
Syringa spp. ‘Old Glory’ is a disease-resistant selection of lilac that was developed from a controlled hybridization between Syringa ‘Sweet Charity’ and Syringa × hyacinthflora ‘Pocahontas’. ‘Old Glory’ is credited for high-level disease resistance to powdery mildew, bacterial blight, and other foliage diseases common in the southern region of the United States, but in 2005 and 2006,‘Old Glory’ plants developed symptoms of downy mildew in McMinnville, Tennessee, U.S. Symptoms started in late April as chlorotic lesions and later became necrotic. Upper leaf surface symptoms appeared similar to common leaf spots with necrotic lesions, but underside leaf lesions were covered with masses of sporangiophores and sporangia. Morphologic features of the sporangiophores and sporangia matched that of Plasmopara spp. The leaf lesions were circular or irregular and developed between veins. Coalesced lesions formed large necrotic patches; severely infected leaves defoliated prematurely. Surface-sterilized healthy leaves were spray-inoculated with sporangiospores and placed in Petri dishes over triple-layered wet paper towels at 23°C to 25°C (73.4°F to 77°F). Disease symptoms were reproduced in approximately 12 days. Noninoculated control leaves did not develop disease symptoms. Oospores were not observed. Downy mildew has not previously been reported in lilac and this is the first report of the disease in Tennessee.
Lilac (Syringa spp.) is one of the most common flowering shrubs, known for its beautiful and fragrant bloom in spring. Several foliage diseases affect lilac in Tennessee, U.S., the most common of which are bacterial blight (Pseudomonas syringae pv syringae), powdery mildew Erysiphe (Sect. Microsphaera) syringae, Cercospora spp., and Alternaria altinatum (Westcott 1898; Hibben et al. 1977; Sinclair et al. 1993; Clement et al. 1994; Pscheidt and Moorman 2001; Mmbaga et al. 2005). Disease resistance is the best management approach for lilac diseases, and cultivars that are resistant to individual or multiple diseases have been identified in field evaluations (Hibben et al. 1977; Mmbaga et al. 2005). Syringa ‘Old Glory’ is a product of the lilac hybridization program at the U.S. National Arboretum, representing a selection from a controlled hybridization between Syringa ‘Sweet Charity’ and Syringa × hyacinthflora ‘Pocahontas’ made by Don Egolf in 1978 and released in March 2006 (U.S. National Arboretum 2006). ‘Old Glory’ was selected for its abundant fragrant bluish purple flowers, rounded growth habit, and foliar disease tolerance (Figure 1). Syringa ‘Old Glory’ is credited for high-level disease resistance to powdery mildew, bacterial blight, and other foliage diseases common in the southern U.S. region (US National Arboretum 2006). Common leaf spot symptomatology was observed on ‘Old Glory’ in Summer 2005 and 2006. The objective of this study was to identify the causal agent of this disease.
Syringa ‘Old Glory’ hybrid introduction (NA 62974; P1641803) US National Arboretum Plant Introductions, Floral and Nursery Plants Research Unit (www.usna.usda.gov/Research/Lilacs_Release.html).
MATERIALS AND METHODS
New hybridized selections, Syringa ‘Old Glory’ and ’Declaration’, were field-grown for observation at the USDA National Arboretum germplasm evaluation plots in McMinnville, Tennessee State University (TSU) Otis L. Floyd Nursery Research Center. In May 2005, disease symptoms exhibiting necrotic lesions on the foliage were observed in ‘Old Glory’, but not in ‘Declaration’. Disease symptoms intensified in 2006 making the ‘Old Glory’ hybrid unsightly (Figure 2). The upper side of affected leaves yielded distinct symptoms consistent with common leaf spots, but the underside of the leaves showed the lesions were covered with white mass of sporangiophores and sporangia that later turned grayish in color. The symptomatology was suggestive of downy mildew disease, but the disease has not previously been observed on more than 50 lilac accessions grown in the local area (Mmbaga et al. 2005). Temperatures were ideal for downy mildew with April temperatures ranging between 15°C and 23°C (59°F and 73.4°F) and May temperatures ranging between 18°C and 28°C (64.4°F and 82.4°F) (Cotner 1930; Spencer 1981). The sporangiophores and sporangia were isolated from underside leaf lesions and were characterized under a compound microscope.
Necrotic lesions caused by downy mildew in Syringa ‘Old Glory’.
To confirm that the observed fungal organism was a pathogen associated with the observed symptoms, a pathogenicity test was done using a detached leaf technique (Dhingra and Sinclair 1995). Sporangiospores were harvested from the leaf underside and suspended in sterilized double-distilled water containing a surfactant (Tween 20, Cayman Chemical Company, Ann Arbor, MI, U.S.) at a rate of 0.04 μL/L. The spore suspension was adjusted to a concentration of 1 × 105 spores/mL. Twelve disease-free leaves of Syringa ‘Old Glory’ were detached and surface-sterilized using 10% Clorox® bleach (Clorox Company, Oakland, CA) for 2 min and rinsed in sterilized water. Six sterilized leaves were aseptically inoculated with the sporangiospore suspension by using an atomizer to deliver the inoculum uniformly; leaves were sprayed to runoff. The noninoculated controls were sprayed with sterile water. Inoculated and noninoculated leaves were placed in Petri dishes over triple-layered sterilized paper towels soaked in sterilized water and incubated at 23°C to 25°C (73.4°F to 77°F) with 14/10 hr (light/dark) periods. A randomized complete block design with a replication of six individual leaves per treatment was used. The inoculated and noninoculated leaves were monitored for symptoms and signs of disease development. Twelve days after inoculation, the underside of the leaves was observed under a dissecting microscope and sporangiosphores and sporangiospores were harvested and characterized under a compound microscope.
RESULTS
The field-grown Syringa ‘Old Glory’ hybrids did not develop symptoms of powdery mildew nor Pseudomonas syringae, both of which are problematic in the local area. However, inoculated ‘Old Glory’ leaves developed symptoms consistent with common leaf spots, originally observed (Figure 2). The inoculated leaf lesions developed between the veins appearing angular in shape. Lesions often coalesced to form large patches of necrotic tissue covering a large part of the leaf. The symptoms were at first evident on the upper side of the leaves, appearing as chlorotic lesions that later turned brown and necrotic with indefinite edges (Figure 2). The lesions on the underside of the leaves were covered with white mass of sporangiophores and sporangia that later turned grayish in color (Figure 3A). Although the disease was first observed in spring, the necrotic lesions persisted throughout summer with the mycelia becoming grayish in color. Severely infected leaves defoliated prematurely. Symptoms were not observed on Syringa ‘Declaration’ that was growing in close proximity.
Mycelial tuft characteristic of the downy mildew fungus on foliage underside. (A) Mycelial tufted mycelia visible with the naked eye. (B) Microscopic view of sporangiophores and spororangiospores produced from the mycelial tuft (100×).
Observation of the fungal mycelia under a dissecting and compound microscope revealed an abundance of sporangiospores borne on branched sporangiophores (Figures 3B and 4). Sporangiophore branching was distinctly monopodial with smaller branches arranged at right angles to the supporting branches; tips of sporangiophore branches measured 8 to 14 μm long (Figure 4). The sporangia were hyaline and ovoid in shape measuring approximately 19.5 to 22 μm × 14 to 17 μm. Overwintering structures, oospores, were not observed.
Morphologic characteristics of lilac downy mildew pathogen at 400× magnification showing (a) primary monopodial branching of sporangiophores with (b) smaller sporangiophore branching arranged at right angles to the supporting branches and (c) obovoid sporangia.
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 characteristic of Plasmopara species.
DISCUSSION
Downy mildew fungi can survive for many years as overwintering 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 disease 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 1995, but downy mildew has not previously been detected (Mmbaga et al. 2005).
Ideal conditions for pathogenic development of downy mildew 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 midmorning for at least 4 days in a row is required for sporangiospore germ tube development and subsequent epidermis penetration (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 infection 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 develop 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 disease development, higher temperatures characteristic of the Tennessee 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 susceptible 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 hydroxide), 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 (Heritage®; Zeneca Professional Products, Wilmington, DE), strobilurin (Compass®; Norvatis Crop Protection, Greensboro, NC), mefenoxam (Ridomil Gold®; Syngenta Crop Protection, Wilmington, DE), cymoxanil (Curzate® or Tanos®; DuPont Company, Wilmington, DE), propamocarb (AgrEvo USA Co., Wilmington, 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®; Luxembourg-Pamol, Inc., Memphis, TN), and cyazofamid (Fosphite®; JH Biotech Inc., Raleigh, NC). Of these, propamocarb, cyazofamid, and dimethomorph have systemic and protective actions (www.bayercropscienceus.com/products; Westcott 1898). Literature search in plant disease, phytopathology, and mycological journals and secondary sources for lilac downy mildew (Westcott 1898; Sinclair et al. 1993; Pscheidt and Moorman 2001) showed that downy mildew has not previously been reported in lilac and this is the first report of the disease in Tennessee. Evaluation of lilac accessions for susceptibility or resistance to downy mildew is needed to provide more information on the economic potential of this disease.
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