296 Arboriculture & Urban Forestry 2008. 34(5):296–299. Mmbaga: Downy Mildew in Lilac Downy Mildew in Lilac Margaret Mmbaga 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. Key Words. Plant health care; Plasmopara spp. leaf spot diseases; Syringa species. 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). Dis- ease resistance is the best management approach for lilac dis- eases, 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 ‘Pocahon- tas’ 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 south- ern 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. MATERIALS AND METHODS New hybridized selections, Syringa ‘Old Glory’ and ’Declara- tion’, were field-grown for observation at the USDA National Arboretum germplasm evaluation plots in McMinnville, Tennes- see State University (TSU) Otis L. Floyd Nursery Research Cen- ter. In May 2005, disease symptoms exhibiting necrotic lesions on the foliage were observed in ‘Old Glory’, but not in ‘Decla- ration’. Disease symptoms intensified in 2006 making the ‘Old Glory’ hybrid unsightly (Figure 2). The upper side of affected ©2008 International Society of Arboriculture 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 sugges- tive 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 be- tween 18°C and 28°C (64.4°F and 82.4°F) (Cotner 1930; Spen- cer 1981). The sporangiophores and sporangia were isolated from underside leaf lesions and were characterized under a com- pound microscope. 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 L/L. The spore suspension was adjusted to a concentration of1×105 spores/mL. Twelve dis- ease-free leaves of Syringa ‘Old Glory’ were detached and sur- face-sterilized using 10% Clorox bleach (Clorox Company, Oakland, CA) for 2 min and rinsed in sterilized water. Six ster- ilized leaves were aseptically inoculated with the sporangiospore suspension by using an atomizer to deliver the inoculum uni- formly; leaves were sprayed to runoff. The noninoculated con- trols were sprayed with sterile water. Inoculated and noninocu- lated leaves were placed in Petri dishes over triple-layered ster- ilized 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 under- side of the leaves was observed under a dissecting microscope
September 2008
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
