GardenDrumLoropetalum bacterial gall spreads in southern USA

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Bacterial gall on Loropetalum (Photo by Yan Chen, LSU AgCenter)

A bacterial gall that kills Loropetalum shrubs is spreading through southern USA, carried by rain, irrigation, poor pruning hygiene and infected plant cuttings.

Bacterial gall on Loropetalum. (Photo by Yan Chen, LSU AgCenter)

Bacterial gall on Loropetalum. (Photo by Yan Chen, LSU AgCenter)

First reported in Alabama in 2012, the bacterial gall, Pseudomonas savastanoi, has spread throughout the humid southern USA states of Alabama, Georgia, North Carolina, and now Louisiana. Host plants are commonly in the Oleaceae family, such as ash, privet (Ligustrum), olives, and Forsythia, but also the unrelated oleander (Nerium).

The gall enters plant tissue through a natural break in the bark such as a leaf scar, or a small wound perhaps caused by hail of frost, or pruning. The local tissue is infected and after only a few months, bacterial galls appear which soon girdle the whole stem, cutting off the food supply and causing it to die back. Often smaller shoots are infected first. Bacteria oozes from the galls during overcast, wet and humid weather, and then rain or irrigation splash spreads them over the plant, with the bacteria able to live for extended periods on the surface of the bark until an entry point arises. Eventually infection spreads to the main trunk, killing the whole shrub.

The Pseudomonas gall looks like an irregular, rough, dark-coloured callus on the stem.

The bacterium is spread by rain splash or overhead irrigation, as well as poor tool hygiene during pruning. Propagating plants from diseased stock plants also spreads the disease through nurseries and to new sites. All newly-arrived Loropetalum in a nursery should be quarantined for 3-4 months to make sure they are disease-free.

Although the galls can’t be cured, when they are on smaller stems careful pruning of all the infected plant material, starting at least 2 inches (50mm) below the gall, can help reduce the spread of the disease. Those growing Loropetalums in the warmer areas of the USA should avoid overhead watering, especially during the summer, and take care that any pruning tools are properly disinfected both before and after.

See more at Auburn University Alabama and Louisiana State University AgCenter

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