Foliar symptoms of downy mildew on impatiens. (Photo by Don Ferrin. Click on photo for downloadable image.)

Downy mildew was recently found on impatiens in the south Baton Rouge area.

This is the first known occurrence of this disease in Louisiana, according to LSU AgCenter plant pathologist Don Ferrin.

“The infected plants had been purchased at a local ‘big box’ store, and subsequent inspection of their remaining stock confirmed the presence of downy mildew in the garden center,” Ferrin said.

The plants had been supplied by a wholesaler from a neighboring state, he said.

Although this is most likely an isolated introduction, it does point out the need to be on alert for this disease, which has become a major problem across the northern United States, southern and coastal California and south Florida in the last year or so, Ferrin said.

“Depending on the cultivar, initial symptoms of downy mildew may appear as a gray discoloration of the new growth or a yellowing and downward curling of the leaves,” Ferrin said.

The pathogen produces spores on the underside of infected leaves and appears as a white, powdery growth reminiscent of powdery mildew.

“As the disease progresses, the infected leaves fall, and the plants become severely defoliated, leaving only bare stems that eventually rot,” Ferrin said. “The pathogen that causes downy mildew on impatiens belongs to a group of organisms referred to as water molds.”

Impatiens is one of the primary hosts of this pathogen, and all varieties are reported to be susceptible, whereas New Guinea impatiens is tolerant to this disease, Ferrin said.

“As the name suggests, an adequate supply of water is necessary for disease to develop,” he said. “However, dew and leaf wetness resulting from overhead irrigation are sufficient to allow disease development.”

Because optimum temperatures for growth of the pathogen are in the range from 60 to 73 degrees, conditions will become less conducive for disease development as temperatures become warmer during summer.

Because impatiens is typically grown in the shade where the microclimate may allow the disease to persist even during the heat of the summer, it may survive the summer and become a problem again in the fall.

“The first step in managing this disease is to inspect plant materials for any signs or symptoms of disease at the time of purchase to prevent introducing it into the landscape,” Ferrin said. “If it is not discovered until it has been installed in the landscape, remove and destroy any infected plants and those in the immediate vicinity as symptoms may not become visible for up to 14 days or more after the plant has become infected.”

Ferrin said to also be sure to remove and destroy any potentially infected leaves that may have already fallen from the plant because these could potentially be sources of inoculum for subsequent plantings.

“Above all, avoid overhead irrigation if at all possible as this serves to spread the pathogen and promote disease,” Ferrin said. “If this is not an option, irrigate in the morning so that the leaves dry off as quickly as possible.”

For more information on impatiens downy mildew, contact your local LSU AgCenter office.

Johnny Morgan