Rescuing a popular nut from attack - pecan diseases and pests

Agricultural Research, Dec, 1992 by Doris Stanley

Long before Europeans arrived on this continent, Native Americans enjoyed the exceptional taste of nuts produced by majestic, wild pecan trees.

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."