There are two fungal diseases you may encounter on ornamental shrubs in your landscape now that the weather is cooling off. These are Ravenelia rust disease on cassia and downy mildew on awabuki sweet viburnum.
Cassia, Senna surratensis, is a small flowering tree commonly used in landscape plantings. Cassia trees bloom profusely throughout the year with their peak bloom period varying depending on the variety. Cassias have experienced significant injury in the fall/winter of 2012 and 2013 due to a rust fungus identified as Ravenelia sp.
Ravenelia rust fungus is not new to Florida, but rust disease on cassia was previously mild and limited to foliar damage. Ravenelia rust disease the past two years has resulted in severe defoliation and twig dieback on cassia trees on the east and west coasts of Florida. Early leaf symptoms appear as brown spots with yellow haloes. Rust-colored spores will be visible on the leaf undersides as the disease progresses. Trees may defoliate and entire branches may die. Preventive fungicide applications are recommended to limit defoliation and dieback. Surviving branches will produce new growth in the spring. Disease development is favored by the cool nights and foggy mornings we’re starting to experience throughout the state.
Visit Collier County Extension Agent Dr. Doug Caldwell’s informative fact sheet for information about Ravenelia rust disease and descriptive symptom photos: http://lee.ifas.ufl.edu/Hort/GardenPubsAZ/Senna_Cassia_Rust.pdf.
Awabuki sweet viburnum, Viburnum odoratissimum var. awabuki, may be grown as a tree or hedge and is favored for its lustrous, leathery leaves and heavily scented flowers. This variety of viburnum is susceptible to a downy mildew fungus, Plasmopara viburnum, that usually causes mild disease but occasionally results in severe defoliation. Downy mildew initially appears as light green to reddish-brown leaf spots on the upper leaf surface and white tufts of fungal mycelium may be visible on the leaf underside. Defoliation is possible as the disease progresses. Removing fallen leaves will help limit the spread of downy mildew but preventive fungicide applications provide the best control.
Visit http://hillsborough.ifas.ufl.edu/documents/pdf/ornamental_production/A-Z_pubs/Viburn.pdf for more information about downy mildew on awabuki sweet viburnum, including management suggestions.
Consult with your county extension agent to determine which fungicides will be appropriate and how to legally and safely apply them if you decide to make preventive fungicide applications this fall and winter.