Rescuing a popular nut from attack.
The nutritious pecan--the most important of the hickory (Carya) species--lends its rich, nutty flavor to luscious confectioneries such as pecan pie.
And Agricultural Research Service scientists are guardians of this American contribution to the world of fine foods.
"We're working to protect pecans from diseases and insects," says ARS horticulturist Bruce W. Wood. "And we're trying to make the crop more productive."
At the Southeastern Fruit and Tree Nut Research Laboratory in Byron, Georgia, Wood leads a team of horticulturists, plant pathologists, and entomologists trying to solve the disease and insect problems confronting southeastern pecan growers.
Pecan production in this region dropped to 83 million pounds in 1990, less than half the 173 million grown there in 1988. Much of this decline is attributed to diseases.
Total U.S. production has not followed the same trend; in 1990 it was 205 million pounds and in 1991, 299 million. The forecast for 1992 is 205 million.
Fungal diseases plague pecan crops in the humid Southeast. So much so, says ARS' Charles C. Reilly, that the pecan industry is a significant user of fungicides.
This is because the strategy for managing pecan diseases is based on controlling pecan scab, the number-one disease problem that threatens both foliage and fruit. Other fungal diseases are considered secondary and are usually taken care of by the fungicides applied to control scab.
However, this may not be the case with a recently discovered disease.
"After several days of rain in August 1988, I noticed signs of rot around the stem ends of some of the pecans on trees here at the station," says Reilly, a plant pathologist at Byron. "Within a day or two, the rot had circled the shuck and moved all the way to the tip of the fruit."
Within 4 days, the pecans were rotted. Closer inspection showed the shucks (the immature nut's hard, green covering) had become almost black and were moist and spongy.
A New and Virulent Disease
"I realized then that we weren't dealing with pecan scab, which attacks both the leaves and fruit," Reilly says.
His investigation and further research identified a new disease, which he named Phytophthora shuck and kernel rot. Already, this disease, caused by P. cactorum and spread by rain, was cutting yields up to 50 percent in some orchards in south and central Georgia.
"The surprising thing was that those orchards were well managed, irrigated, and mature," Reilly says.
A soilborne fungus, R cactorum, can destroy the whole crop if it develops early in September. Damage from later infection (late September) results in discolored nuts, which reduces kernel quality.
Early identification of the new disease is difficult, according to Reilly, because the rot begins on isolated nut clusters throughout the tree. Growers can suspect Phytophthora shuck and kernel rot if there are "sticktights" on the pecan trees in mid-August after prolonged periods of rain. A sticktight is the infected, discolored pecan shuck, dried and adhering tightly to the pecan shell.
Temperature is also important for this fungus to grow, he says. Wet, cool days with temperatures between 50[degrees]F and 88[degrees]F are ideal for the disease. Threat of the disease is decreased in months when the weather is very hot or very dry. The fungus has been found in orchard soils throughout the Southeast and can infect pecan roots. It's thought that insects that spend part of their life cycle in the soil may move the fungus into the 100-foot-tall trees. Fortunately, the disease can be managed by judicious application of existing fungicides registered for use on pecans.
A Disease So New It's Unnamed
Reilly has identified an even newer disease that was first noticed on last year's crop.
"We know that this disease is caused by a fungus in the genus Phomopsis, but we haven't identified the particular fungus," he says.
This new disease, as yet unnamed, not only causes the nut to rot, but also causes dieback of twigs on which the fruit forms. Dieback occurs when the infection moves into the stem and destroys the part of the tree to which the fruit is attached.
Stress, such as heavy fruit set or drought, seems to increase the severity of the disease.
Reilly has found this disease in pecan orchards he sampled throughout Georgia. One reason for the widespread occurrence could be that many of the leading commercial pecan varieties are very susceptible to the pathogen.
Fungi in the genus Phomopsis, he says, are very common worldwide. They are spread by rain splashing from infected parts of the tree.
The new disease can be controlled, Reilly suggests, with the same fungicides used against pecan scab.
"Although, in the past, pecan growers have considered pecan scab their primary concern, this new information proves that Phytophthora shuck and kemel rot, the Phomposis disease, and pecan anthracnose--a long-known disease that has recently become more severe ---can destroy the crop too," Reilly reports. "This demands a rethinking of control measures."
For example, since growers have always concentrated on controlling scab, they apply their last cover spray of fungicides in early August.
"Unfortunately, this leaves pecans unprotected from September through harvest, which is from late October through November," says Reilly.
Bad Bacteria and Bugs Team Up
In addition to identifying two new diseases and rediscovering an old one, Reilly has clarified another problem for growers.
He has shown that a bacterium--Erwinia herbicola--is associated with late-season nut drop and kernel disorder. E. herbicola is normally present on pecans.
Nut loss during the water stage and early shell hardening (July through August) has been attributed to various conditions. Demands on the tree during this stage create extreme stress and may cause what is called physiological fruit abortion.
It so happens that this same period is when the pecan is most appealing to two insects--stinkbugs and the hickory shuckworm.
When Reilly examined internal damage in the dropped nuts, he found it remarkably similar. "The kemels were dark brown to black, shrunken, and slimy, regardless of whether the damage was inflicted by insects or simply resulted from the tree' s physiology."
Since the bacteria are also present on healthy nuts, Reilly concluded that an injury such as insect feeding or some other kind of wounding activates the bacteria, allowing entry into the nuts.
"We' re working on a control strategy that would reduce the incidence of disease by suppressing insect populations,"he says. Insects cost pecan growers in Georgia alone about $25.8 million in 1990.
"Aphids," says entomologist W. Louis Tedders at Byron, "are the major insect problem pecan growers face in the Southeast. And, interestingly, fire ants worsen the aphid problem."
Under almost every pecan tree in the Southeast there is a fire ant mound, Tedders says. The ants rarely eat aphids, but prey on several beneficial insects that do. They forage in the trees and eat the eggs, larvae, and pupae of lacewings, thereby saving many pecan aphids from certain death.
The aphids seem to retum the favor by excreting honeydew for the ants to eat. "We've seen ants forage as high as 40 feet into a pecan tree to get honeydew," Tedders says.
Three types--the yellow pecan aphid, the blackmargined aphid, and the black pecan aphid--are present in pecan trees in enormous numbers. All feed on foliage.
Sap that they suck from the leaves is rich in sugars. The aphids take amino acids and other nutrients from the sap for growth and reproduction and excrete the sugars. They deposit the sugars on the foliage in the form of honeydew.
Because aphids give birth to live young, the offspring immediately begin feeding and continue for the duration of their 3-week lifespan. This feeding draws much-needed vigor from the tree, since the sugars are essential for fruit production and good nut quality.
Aphids are rapidly becoming resistant to chemicals, so biological controls with lady beetles and other beneficial insects are being used against them more often.
"We developed this biocontrol method several yea= ago and many growers are using it already ," Tedders says.
But the method takes some planning. Growers first plant crimson clover and hairy vetch on the orchard floor. These legumes harbor pea aphids that serve as prey for lady beetles and other beneficial insects.
After the legumes die in May and June, the lady beetles, deprived of their food source, move into the trees and feed on pecan aphids.
"This method works well because there are about 25 species of beneficial insects that appear with these cover crops," Tedders explains. They include assassin bugs, syrphid flies, pirate bugs, beneficial stinkbugs, and brown and green lacewings.
Since legumes and lady beetles last until June and the hot weather helps control aphids during mid-summer, some means of protection is needed for the fall. It's at this time that Tedders uses another biocontrol technique--lacewings.
Tedders started experimenting with lacewings for aphid control a few years ago. Several companies now rear the same species commercially.
Hickory shuckworms, pecan nut casebearers, and stinkbugs also attack pecans. These pests lower the quality of mature nuts and cause loss of immature nuts.
Of the three, the stinkbugs are the only ones that have hosts other than pecan. They fly into the pecan orchard after cotton, soybeans, and corn crops are gone.
Insecticides are still used to control the nut casebearer and shuckworm, but the most effective ones are no longer available because of environmental concerns. Since the casebearer has many parasites, it is usually controlled biologically in the Southeast. But control of stinkbugs is difficult because nuts are usually harvested before their damage is discovered.
The insect pest most likely responsible for transporting fungi into the trees is the pecan weevil, Tedders says. This weevil spends part of its fife cycle in the soil.
In midsummer, the adult weevil lays eggs in pecan nuts that are nearing maturity. The resulting larvae destroy the nut kernels. By this time, the grower has already made a substantial investment in the crop.
Mature larvae chew an exit hole in the nut, fall to the ground, and burrow into the soil. There they remain for 1 or 2 years, during which they pupate and become adults. Adults remain in the soil for another year before emerging.
Adult weevils, probably replete with the soilborne fungus, immediately crawl or fly into the pecan trees and mate, and the cycle begins again.
"There's simply no way to manage this pest while it's in the soil or in the nut," Tedders says. "It's protected in these stages."
Last year, Tedders designed a new type of trap that growers are already using to monitor weevil emergence.
Better monitoring means more effective control when spraying must be done. Although chemical control is available, Tedders says spraying also kills most of the beneficial insects. But knowing when the weevils peak in pecan orchards gives growers a better idea of whether and when to spray.
"Insects do more than just reduce quality of the current season's nut crop," says Bruce Wood, who is director of the Byron lab. "Aphid feeding on a tree can also affect that tree's production the following year."
Sooty Mold Cuts Photosynthesis
A feeding aphid excretes honeydew, which contains carbohydrates and nitrogen, a perfect growth medium for microorganisms. As it builds up on leaves, a heavy black fungus called sooty mold soon proliferates.
"This is like laying a lead blanket over the leaves, blocking out sunlight that is essential for photosynthesis," Wood explains. "We've observed in some cases that this mold cuts light penetration to the leaf surface up to 98 percent."
Pecan trees have a relatively high late-season energy demand. So photosynthetic activity during the fall is critical to future nut production.
Rains can wash most of the mold from the trees, but in dry periods it becomes a major problem that fungicides don't completely control. Most growers don't have a control program for this mold, Wood says.
"What we're looking for is better aphid control. After all, it's the aphids that produce the honeydew that the fungus thrives on. And the long-term aphid populations haven't responded well to our attempted control with insecticides."
It's a vicious cycle.--By Doris Stanley, ARS.
Scientists in this article can be reached at the USDA-ARS Southeastern Fruit and Tree Nut Research Laboratory, P.O. Box 87, Byron, GA 31008. Phone (912) 956-5656 fax number (912) 956-2929.
Pecans: In addition to protein and carbohydrate, pecans provide dietary potassium, and calcium, as well as small amounts of the trace elements Zinc, manganese, and copper. While the kernels are about 70 percent fat, 95 percent of the fat is unsaturated.
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|Title Annotation:||pecan diseases and pests|
|Date:||Dec 1, 1992|
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