WESLACO — As the Lower Rio Grande Valley’s 1999-2000 citrus season slowly gets under way with the harvesting of early oranges, reports are trickling in to local scientists about mysterious markings on some fruit.
Drs. Mani Skaria and Vic French at the Texas A&M-Kingsville Citrus Center at Weslaco say the strange blemishes started showing up about a month ago, and a few more have been reported since.
The spots are about half a centimeter large and are yellow to orange in color, covering up to 30 oil glands on the peel. They are permanent and show up on only one side of affected fruit. To date, they have been found only on oranges,but Skaria says grapefruit will probably also be affected.
“The first fruit with these spots that we saw were on N-33 Navel oranges from the Mission area,” said Skaria, “but they’ve come in from other areas of the Valley since then. We’ve even found some in our own citrus research blocks, and now we’ve also seen them on Marrs and Everhard oranges.”
The taste of the spotted fruit will probably not be a problem, Skaria said, but fresh fruit consumers may shy away nonetheless because the markings are so obvious.
“These are unusual symptoms for any type of disease or insect damage we know about, so it is probably something new,” said Skaria, a plant pathologist. “These symptoms are not described in textbooks we usually refer to.”
Suspecting a fungus or bacteria might be involved, Skaria began taking cross sections of the affected areas. He found no evidence of a disease pathogen or signs of probing by insects that release toxins that can cause discoloration. But he did find gumming, an indication that cell walls were breaking down under stress.
“I have been in contact with a USDA scientist in California who suspects that drastic weather changes, from extremely dry to very wet, for example, could be causing these symptoms,” said Skaria. “So this could be a physiological problem, or an insect problem.”
Skaria’s colleague, entomologist French, has been looking into the possibility of the culprit being the leaf-footed bug, an insect that can show up in Valley orchards in any given year.
“A few years ago,” French said, “leaf-footed bugs had heavily infested an orchard in the Hargill area that even caused fruit to drop from the tree. This insect probes into the fruit and causes gumming, or raised mounds of gum just underneath the surface, like gum pockets. The affected fruit we’ve seen has some of these raised gum mounds, but not all of them do. So at this point, we’re just not sure yet what’s causing this.”
French said among his many efforts to determine the cause, he is exposing clean, fresh fruit to leaf-footed bugs to try and duplicate the symptoms.
“There are so many different things that can cause spotting on fruit that it is sometimes very difficult to determine exactly what is causing it,” he said. “This is the time of year for leaf-footed bugs, so that could be the problem. And there are also a lot of insects commonly known as green stink bugs out there that could be causing this.”
Since the populations of leaf-footed bugs are so spotty and transient, French said, growers don’t normally treat for them unless they become abundant in a particular orchard. Organic phosphates or carbomates, he said, are effective insecticides when necessary.
Skaria and French said they will continue studying the spotting and encourage growers who see unusual symptoms to report them to the Citrus Center.
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