by Enrico Villamaino

Geranium devotees should be vigilant against a pair of plant-based plights this year.

Margery Daughtrey is an expert on all arboreal ailments and horticultural havocs. For 42 years she has been senior Extension associate at the School of Integrative Plant Science Plant Pathology and Plant-Microbe Biology Section at Cornell University. According to Daughtrey, two of the biggest threats to geraniums this season are Ralstonia and Xanthomonas.

Ralstonia solanacearum, also known as bacterial wilt, first affects geranium plants at the lower leaves and the stalks connecting them to the plant stem. Wilting leaves develop a yellowish coloring due to a lack of chlorophyll, and the edges of leaves can turn necrotic. Eventually, the entire plant can collapse.

Ralstonia is considered a particularly troublesome threat, and the USDA has added it to its Select Agents and Toxins List, which enumerates biological agents and toxins “determined to have the potential to pose a severe threat to both human and animal health, to plant health or to animal and plant products.” “A big problem with the Ralstonia bacterium,” said Daughtrey, “is that it can spread from geranium to potato plants. In potatoes, this manifests in the form of brown rot disease and can result in substantial yield losses.”

Margery Daughtrey, senior Extension associate at Cornell University, warns of two bacterial strains threatening geranium plants this season. Photo courtesy of Margery Daughtrey

Daughtrey explained that its identification by the USDA as a select agent means that, under federal authority, infected crops are subject to a federal quarantine and can be ordered destroyed. “There is no effective treatment for Ralstonia. The best thing we can do is engage in a disposal protocol, containing the infection and throwing the contaminated plants away,” she said.

Bacterial blight in geraniums is caused by a bacterium named Xanthomonas campestris pv. pelargonii. Affected plants can develop small water-soaked spots on their leaves between an eighth of an inch and a quarter of an inch in diameter, which become sunken, well defined and eventually die. As the disease progresses the stalks begin to weaken and wilt, giving the plant a more umbrella-like appearance.

Daughtrey said the threat from Xanthomonas bacteria has been abated over the years by measures taken to prevent Ralstonia outbreaks. “It’s become rarer than it used to be because of Ralstonia safety procedures. Unfortunately, it’s made a bit of a comeback and is big this year. Fortunately, this only affects geranium plants and can’t jump from one species to another, so it doesn’t present the same threat to our food supply. Also, unlike Ralstonia, which has no treatment, Xanthomonas outbreaks can be mitigated somewhat with measured introduction of copper based treatments,” she said.

Daughtrey noted that while Xanthomonas is not on the federal Select Agents and Toxins List, there are a number of individual states that do enact quarantines.

“Whether you’re dealing with Ralstonia or Xanthomonas,” Daughtrey cautioned, “the best course of action is to be proactive and to catch any signs and symptoms early on. It’s better to nip it in the bud.”

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