Pecan Scab Disease - Pecan Scab Disease

Pecan scab disease is the most economically damaging disease of pecan production in the southeastern United States. The disease affects stem, leaf and nut growth, causing reduced yield. This publication includes information on the development, symptoms and control of the disease.

Importance and Damage
Pecan scab disease is the most economically damaging disease of pecans
production in the southeastern United States. It is often the cause of lower
production and profit in commercial orchards. The disease affects stem, leaf
and nut growth, causing reduced yield. The cost of disease control reduces the profitability of growing
pecans. Pecan scab also is one of many factors that prevent yard trees and small, unmanaged plantings
from producing high quality nuts.
Disease Development and Symptoms
Pecan scab disease is caused by the fungal pathogen Fusicladosporium effusum. The pathogen can
infect actively growing tissue including stems, leaves and nut shucks (Figures 1 & 2). Rapidly
growing stems can be infected in early spring. Leaves can be infected from bud break until they are
fully expanded, usually in June. Nuts are susceptible from the time they are formed until they have
ceased growth in late summer. Nuts are especially susceptible to infection during the period of rapid
enlargement, usually from late June through July.
Infections are started by spores of the fungus. Lesions left from the previous yearâ€™s infections begin
to produce spores as the weather warms in early spring. These old lesions as well as new infections
can continue to produce spores throughout the summer, compounding the level of infection that
results in dramatic increases in the amount of disease.
Following infection, visible lesions develop in seven to 14 days. Lesions are generally circular, range
in size from pinpoint to about one-quarter inch in diameter. They are light brown to black (Figures
1 & 2). On leaves, lesions are often most numerous along the veins, but they can occur on any part
of the leaf. Numerous lesions can combine to form larger dead areas on any infected tissue.
Severe infection can kill current season stem growth. Leaf lesions reduce the photosynthetic
potential of the foliage and cause early leaf loss. Lesions produced by early spring infections will
produce spores that can infect the nuts during the summer.
Lesions on nuts become sunken and often hardened and crack as they age. Severely infected nuts
can be distorted and may stop growing. These may drop prematurely or remain attached to the tree
throughout the season. Less severe infections and infections that occur later in the season reduce the
growth of the nuts and lower yields. Depending on the stage of nut development and severity of
infection (Figure 2), damage can range from total nut loss to little yield loss.
Factors that Influence Disease
The severity of pecan scab disease depends, to a large extent, on weather. Moisture, generally from
rainfall, is needed for spore germination and infection. Several hours of wetness are needed for
infections to become established. The frequency of rainfall and length of time tissue remains wet are
more important in creating infection periods than is the total amount of rainfall. Late afternoon
rains that cause trees to remain wet overnight provide prime conditions for development of scab
disease. The amount of time it takes for an infection to occur is influenced by temperature, but
temperatures are normally adequate for infection during the spring and summer months. Factors
that tend to maintain high humidity conditions in an orchard also contribute to infection periods.
These include close tree spacing, low limbs and tall, dense ground vegetation.
Pecan cultivar (cultivated variety) susceptibility to the pathogen also plays a major role in disease
development. Levels of susceptibility can extend from very susceptible to seldom being infected.
The pathogen frequently adapts to the genetics of a cultivar, however, and resistance can be lost
over time. Many cultivars that were originally regarded as resistant to scab disease (ca. Schley and
Desirable) are now considered quite susceptible because the pathogen has adapted to the cultivar.
Control
The primary method of scab disease control preventive applications of fungicides. For prevention of
disease, apply fungicide before an infection period. Once an infection is established, fungicides do
little to stop disease development. Because there is susceptible tissue in the orchard throughout the
growing season, it is generally necessary to apply fungicide several times during each season to obtain
efficient control. The number of fungicide applications needed for disease prevention depends on
weather, amount and virulence of the pathogen spores, and susceptibility of the cultivars.Generally
fungicides need to be applied every two to four weeks from spring through summer.
Apply fungicides at a concentration sufficient to prevent infection and completely cover the trees. To
apply the proper concentration, use the amounts advised by the manufacturer on the product label.
The best coverage is obtained with ground equipment designed to spray large trees. Almost all
cultivars will need to have some fungicide protection, but those with more resistance will need to be
sprayed less often.
In noncommercial pecan plantings such as small orchards and yard trees, it is usually not feasible to
apply fungicides because of the cost of equipment and chemicals. In these instances it may be possible
to reduce the level of scab disease by the use of sanitation. Sanitation involves removing old leaf and
nut debris from the ground or plowing the debris under. Shucks remaining in the tree at the end of a
year should be removed and infected stems pruned away. When choosing trees for yard planting, it is
best to use cultivars that are rated as resistant to scab disease. There is no guarantee, however, that the
resistance will be maintained because the pathogen historically has overcome pecan tree resistance.
The names of pecan cultivars recommended for commercial and home planting can be obtained from
the LSU AgCenter Extension office in each parish. Recommendations for control of disease and
insect problems of pecan also are available from these offices.
R.S. Sanderlin, Associate Professor (Plant Pathology)
J.R. Pyzner, Associate Specialist (Horticulture)
Louisiana State University Agricultural Center, William B. Richardson, Chancellor
Louisiana Cooperative Extension Service, Jack L. Bagent, Vice Chancellor and Director
Pub. 2025 (5M) 10/00 Rev.
Issued in furtherance of Cooperative Extension work, Acts of Congress of May 8 and June 30, 1914, in cooperation with
the United States Department of Agriculture. The Louisiana Cooperative Extension Service offers equal opportunities in
programs and employment.

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