Contents
Routing Pecan Scab -- Protecting a
Popular Nut
Horticulturist Bruce Wood (left) and plant pathologist Charles Reilly inspect
greenhouse plants for pecan scab damage.
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Sweet, tasty pecans remain one of the most popular foods native to North
America. Last year, U.S. consumers ate 134 million pounds of them.
But this staple of pies and ice cream and other confections faces a tiny,
yet powerful enemy: pecan scab disease. If unstopped, the fungus can destroy a
crop, forcing food companies to import pecans to meet demand.
Agricultural Research Service
scientists Bruce W. Wood and Charles C. Reilly are doing their part to ensure
that consumers have an adequate supply of pecans. Wood, a horticulturist, and
Reilly, a plant pathologist, work at the ARS Southeastern Fruit and Tree Nut
Research Laboratory in Byron, Georgia. There, they are developing new
strategies to combat pecan scab and protect this native nut crop.
Wood, who heads the Byron lab, says two of the main advances leading to the
U.S. pecan industry's success have been the introduction of fungicides and
airblast spray technology for quickly and effectively dispersing pesticides
throughout an orchard.
"We're studying ways to build and improve on these advances,"he
says. "The approaches have quadrupled yields and boosted farmers' profits.
But they've also increased operating costs, at a time when wholesale prices for
pecans have declined. So we're developing ways to effectively control scab
while reducing spraying costs and minimizing environmental impact."
Pecan scab, caused by the fungus Cladosporium caryigenum, is
generally the most damaging pecan disease. Discovered on pecans in 1888, it
started to become a problem in the early 1900s, when farmers first domesticated
the crop.
Scab attacks the foliage, twigs, and developing fruit. The disease causes
lesions to appear on leaves as small, charcoal gray to black, concentric
circles. Severe infection causes defoliation, fruit abortions, and poorly
filled fruit, thus reducing yields. It is active throughout the growing season
and, if left unchecked, can cause near total crop losses.
If unchecked, lesions like these caused by pecan scab can lead to tree
defoliation and reduced yields.
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The disease continues to be a problem, particularly for the primary
commercial pecan tree varieties --Desirable, Schley, and Stuart --grown in the
Carolinas, Georgia, Alabama, Florida, Mississippi, Louisiana, and east Texas,
where the climate is generally moist. Pecan scab is not a problem in arid
western regions, like New Mexico, Arizona, and California. Overall, it costs
pecan growers at least $22 million each year in losses and control costs.
Holding the Threat at Bay
Current scab control strategies often require 9 to 11 fungicide sprays per
season. Growers typically spray first when leaves begin to emerge from swelling
buds, around the first or second week in April.
But scientists have found that young leaves emerging from breaking buds in
the spring are not vulnerable to scab. It is after the leaves have grown for
about 10 to 14 days that foliage becomes susceptible, Reilly says.
"This means that growers can abbreviate their spray schedule by
eliminating or delaying the typical first spray. This could save about $15 to
$18 an acre in control costs and also cut pesticide use."
Reilly says weather conditions are the driving force in scab infection and
that the disease worsens during periods of high rainfall. "We're
developing new predictive spray methods, so growers can potentially reduce
costs and pesticide use by cutting the spray schedule during dry years,"he
says.
Grower complaints of substantial crop losses to scab --even though their
orchards received several fungicide sprays per year --have raised suspicions
about emerging scab resistance to fungicides.
"We haven't been able to prove increased resistance,"says Reilly.
"But we have discovered major deficiencies in the timing and techniques of
fungicide application that may give the false impression of resistance."
To the scientists' surprise, Reilly says, they discovered the disease
protection from fungicides only lasts about half as long as the 2 to 3 weeks
previously assumed. "Standard control strategies traditionally used in
commercial orchards leave developing fruit unprotected from scab about half of
the time,"says Reilly.
A second discovery was that the approximate 4-day kickback activity --the
fungicide's ability to kill scab after infection --was closer to being
nonexistent for most fungicides, under commercial field conditions.
"This new information will allow growers to adjust spray strategies to
spray orchards quickly once infective conditions develop, or to spray before
conditions are right for the disease to take hold,"Reilly says
Better Spray Coverage
A major scab control problem for pecan growers is the time required to spray
an entire orchard. Airblast sprayers typically used in orchards carry 500 or
1,000 gallons of fungicide, travel about 2 miles an hour, and deliver about 100
gallons of spray per acre. This restricts spray coverage to about 75 acres a
day. Many commercial pecan orchards are over 400 acres and require more than
one airblast sprayer to protect the crop in a timely manner.
This new nozzleless sprayer uses one-half to two-thirds less spray to achieve
superior coverage and penetration of the tree canopy.
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To measure fungicide application, the researchers studied how much and where
spray materials were being deposited in the trees, some of which are up to 70
feet tall.
"We discovered that a primary reason for poor scab control was uneven
coverage,"says Wood. "The top third of the tree receives very little
spray, and coverage of the lower two-thirds is often excessive."The
scientists concluded that contemporary spray equipment contributes to
scab-related losses because foliage and fruit are often poorly covered by
pesticides.
Wood and Reilly are evaluating a new nozzleless sprayer developed by
Michigan State University researchers that will allow for improved protection
against disease and insect pests. The sprayer penetrates the tree canopy
better, while reducing spray volumes by one-half to two-thirds.
"Preliminary evaluation of this new spray technology for pecans appears
highly promising,"says Reilly. "The strategy includes use of an
atomizer that creates uniform droplet size, giving superior penetration of
fungicides carried by a high-energy airstream into the tree canopy. The nozzles
on traditional sprayers either clog up or wear out, causing a need for frequent
inspection and replacement.
"On average, growers produce about 286,000 million pounds of pecans per
year,"Reilly says. "They're very excited about having new scab
control strategies that lessen crop losses and save money, while reducing
pesticide usage." --By Tara
Weaver, Agricultural Research Service Information Staff.
Bruce W. Wood and
Charles C. Reilly are at the USDA-ARS
Southeaste rn
Fruit and Tree Nut Research Laboratory, P.O. Box 87, Byron, GA 31008; phone
(912) 956-6420, fax (912) 956-2929.
"Routing Pecan Scab -- Protecting a Popular Nut" was
published in the August 1998 issue of Agricultural Research magazine.
Click here
to see this issue's table of contents.
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