LSU AgCenter plant pathologist Don Ferrin shows the difference between bird damage and sweet orange scab at a recent citrus training session for county agents at the Hammond Research Station. (Photo by Johnny Morgan. Click on photo for downloadable image.)

A citrus disease fairly new to Louisiana may cause problems for commercial growers, but not so much for backyard growers, according to LSU AgCenter experts.

Sweet orange scab is among the latest problems faced by citrus growers in the state, but experts say the problems the disease causes are more cosmetic than quality.

When considering the scab disease, two different organisms affect the industry.

Louisiana growers have noticed the citrus scab for a number of years, but the sweet orange scab was first detected in Louisiana last year, according to LSU AgCenter plant pathologist Don Ferrin.

The sweet orange scab is primarily a disease of sweet oranges, satsuma, grapefruit, lemons and limes. Ferrin said the disease affects the fruit and leaves, causing irregularly raised corky or scabby-looking areas.

“Citrus scab and sweet orange scab are similar, but the main difference in the two is the host range,” Ferrin said. “Citrus scab rarely occurs on sweet oranges, while sweet orange scab does.”

There is not much for the backyard grower to fear from the sweet orange scab because it is mainly a cosmetic disease that doesn’t affect the quality of the fruit, he said.

“But for commercial growers, the look of the fruit will probably make it hard to sell,” Ferrin said.

He said the symptoms of the two scab diseases will vary with citrus variety. Both occur in humid areas and are primarily dispersed by splashing rain and wind.

“We don’t know how long it has been in the state because for a number of years, it was believed to be bird damage,” Ferrin said.

For growers to control this disease, applying the proper chemicals at the right time is the key to success.

“Copper fungicides, tribasic copper sulfate or Kocid

LSU AgCenter entomologist Natalie Hummel explains how to identify the Asian citrus psyllid at a recent training session at the Hammond Research Station. (Photo by Johnny Morgan. Click on photo for downloadable image.)

e, sprayed after bloom beginning when the fruit are pea size and repeated twice over the next six weeks, will control scab,” Ferrin said.

There are some restrictions, however, Ferrin said. These fungicides shouldn’t be sprayed during bloom or when the pH of the water is low.

The two other issues facing growers are the Asian citrus psyllid and the citrus greening disease that it spreads.

LSU AgCenter experts say tree owners have ways to control the Asian citrus psyllid, but citrus greening disease is more serious because it is fatal to the trees.

Citrus greening disease was first found in Orleans Parish by a backyard grower in 2008, said LSU AgCenter entomologist Natalie Hummel.

That tree and trees in Washington Parish were destroyed in order to keep the disease from spreading.

“So far, the disease has only been found in Orleans and Washington parishes,” Hummel said.

Symptoms of citrus greening include yellowing of the leaves, blotchy mottling of the leaves, poor flowering and stunted growth.

Hummel said the symptoms may not show up for several years after the tree has been infected.

Asian citrus psyllid, the insect that spreads the greening disease, has so far been detected in nine Louisiana parishes.

“My job is to advise management strategies to control the insect,” Hummel said. “That’s what we’ve done with our publications, websites and training videos.”

The goal is to inform people how to identify the insect, then recommend management strategies to keep the population down and to avoid disease transmission, she said.

The Asian citrus psyllid has been found in Florida, Texas, Arizona and California. So far, sweet orange scab has been detected in Texas, Mississippi, Florida and Arizona.

This year, there have been heavy psyllid populations on the south shore of Lake Ponchartrain, Hummel said. “Hammond is the only area with infestation on the north shore; none have been found west of the Atchafalaya River.”

Johnny Morgan