Watch out for new ornamental plant and lawn diseases

Linda Benedict, Ferrin, Donald M.  |  2/21/2012 3:51:08 AM

Donald M. Ferrin

The introduction into Louisiana of new plants produced in other regions provides an opportunity not only for the introduction of new diseases, but also new hosts for pathogens already in Louisiana. Furthermore, ever-changing weather patterns continue to influence the occurrence of endemic diseases of ornamentals and lawn grasses across the state. For instance, Louisiana experienced an unusually large number of cases of large patch (also referred to as brown patch), caused by the fungus Rhizoctonia solani, in home lawns during the extremely wet weather in July 2011, even though it was much warmer than usually associated with this disease.

Following are a few noteworthy developments of ornamental plant diseases:

A new bacterial leaf spot disease of Knock Out and Double Knock Out roses
has been reported from Florida, but it has not yet been observed in Louisiana. The pathogen involved is a new strain of Xanthomonas, which is also pathogenic on Indian hawthorn. It is a problem on roses primarily during propagation and nursery production where the plants are subjected to overhead watering that promotes the spread of the bacterium and subsequent disease development. Symptoms include small black lesions with well-defined margins that are often delimited by the leaf veins. These lesions are generally found along the margins of the leaves. The control of bacterial diseases still relies primarily on the use of copper-based fungicides. Identifying and testing potential new products for their control continues to be a high priority of the national IR-4 Project, which is funded through the U.S. Department of Agriculture and based at Rutgers University in New Jersey. The project’s goal is to facilitate the registration of needed pest management technology for specialty crops, including horticultural crops.

Downy mildew of impatiens, caused by the fungal-like pathogen Plasmopara obducens, has been observed sporadically in the northeastern and north central United States since 2004 and could make an appearance in Louisiana at any time. This disease develops during periods of cool, wet weather and is primarily restricted to the foliage. Initial symptoms are the yellowing of infected leaves, which eventually drop off leaving only bare stems. The grayish-white growth of the pathogen that is found on the lower surface of affected leaves is a good diagnostic feature. Should you suspect that you have downy mildew on your impatiens, please send samples to the LSU AgCenter Plant Diagnostic Center for confirmation.

Fusarium wilt of Canary Island date palms has been found in several locations in New Orleans. This disease is caused by the fungus Fusarium oxysporum f. sp. canariensis and is almost always fatal. It may have been introduced into the state when previously infected palms were brought in and installed in landscape plantings. Once established, the pathogen is then spread from infected to healthy trees during regular pruning to maintain the classic pineapple shape of the crown.

Armillaria root rot, caused by the fungus Armillaria tabescens, has recently been implicated in the decline of older plantings of landscape roses in Louisiana. Like other species of Armillaria, this pathogen is normally associated with hardwood forests and is found in urban landscapes where previously wooded areas have been cleared for development. It is also commonly associated with the roots of oak trees from which it may spread to other more susceptible hosts, such as roses. Little can be done to control this disease once it becomes evident and no fungicides are available for its control. One management practice that may help is to avoid irrigating and mulching around the base of the plants as the moisture favors pathogen growth. Instead, soil and mulch should be removed from the area of the root collar to promote drying, which helps to prevent further growth of the pathogen.

Donald M. Ferrin, Associate Professor, Department of Plant Pathology & Crop Physiology, LSU AgCenter, Baton Rouge, La.

(This article was published in the winter 2012 issue of Louisiana Agriculture magazine.)

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