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Got pecan questions? Contact any member of the Tri-County Pecan Demonstration team:
Lonoke County - Kyle Sanders
Prairie County - Jacob Holloway
White County - Sherri Sanders
White County - Jan Yingling
or reach out to your local county extension agent.
Contact Sherri Sanders at firstname.lastname@example.org to sign up for email updates!
by Jan Yingling, CEA - Agriculture
Pecan scab is an important fungal disease caused by the pathogen Cladosporium caryigenum and is generally considered to be the most destructive disease of pecan in most growing
regions in the United States. Without management, pecan scab can cause 50-100% losses
in susceptible varieties.
Scab is favored by wet and humid conditions, especially prolonged periods of continuous
leaf wetness, and can occur on leaves, twigs, and nut shucks. Young and actively growing
tissues are the most susceptible to scab and become less susceptible as they mature.
The disease first appears as small, dark brown to black spots, which may continue
to increase in size, sometimes coalescing into large, irregularly shaped, lesions.
Infection is more often seen on the lower leaf surface, but both sides of the leaf
may be infected. Leaf infection can cause a reduction in photosynthetic output, but
nut shuck infection causes the majority of losses linked to scab.
When early infection of the nuts occurs, trees may experience significant premature
nut drop as well as a reduction in nut size and quality. As the season progresses,
initial nut scab infections become less damaging to yield and quality, with damage
mostly being cosmetic, if the disease develops after shell hardening. The likelihood
of nut drop increases greatly if lesions cover more than 50% of the shuck and may
also result in nuts that remain on the tree or will not separate from the shuck.
Pecan scab can be managed through cultural methods such as removing fallen branches
from the orchard floor and pruning trees to increase air circulation, planting scab-resistant
varieties, and following a fungicide spray program. Scab severity can vary greatly from year to year, so frequent scouting of orchards
and being observant of weather conditions are essential tasks to stay ahead of scab
See how we use SensorPush to collect weather data to determine pecan scab application timing.
When relative humidity is above 90% and the temperature is above 70° F, conditions
are very favorable for pecan scab development. Each pecan variety has its own level
of resistance, so a method for determining spray intervals has been developed using
accumulated pecan scab hours.
Accumulated scab hours are calculated by totaling the number of hours where both 90%
relative humidity and 70°F temperatures occur at the same time, resetting the calculation
after each fungicide application. The model assumes that the crop will be protected
for 14 days following a properly applied fungicide application. Thresholds have been
set at 10 scab hours for highly susceptible varieties, 20 hours for moderately susceptible
varieties, and 30 hours for less susceptible varieties.
If the threshold is met within the two-week period following a spray, it will be necessary
to reapply. If the threshold has not been met, it may be possible to delay the next
application until the hours have been met. It has been shown that using the scab hour
model rather than simply spraying on a fixed schedule can reduce the number of fungicide
applications required in many crop years.
Go to https://pecanscab.uada.edu/ to use the Pecan Scab Hour Calculator.