Stem Scab

Pecan scab
     Pecan scab occurs on leaves, leaf petioles, and nut shuck tissue. It forms small, circular, olive-green to
black spots. In more severe cases, it can also attack new twigs and even catkins. The lesions often coalesce,
causing the terminals to die and the catkins to drop. Pecan tissues are most susceptible when they are young
and actively growing. They become less susceptible as they mature. When scab attacks expanding leaves and
nuts, it stunts and deforms them. The photosynthetic area is decreased, and photosynthetic capacity is retarded
so that the leaf or nut becomes much less efficient. The lesions grow until they penetrate the leaf.
     When the leaf is mature, the fungus can no longer colonize its tissues. The old lesions tend to dry, crack,
and fallout of the leaf blade, giving the leaf a tattered or shothole appearance.
     The greatest scab damage occurs from nut infections. Early-season infections can reduce yield and crop
quality tremendously. Nuts attacked shortly after nut set usually abort and fall.
     Nut shucks which become infected early in the season often crack slightly where the scab lesions are
coalescing, allowing other fungi such as pink mold (Trichothecium roseum) to penetrate and cause further
decay of the shuck and nut. Severe scab infections can also cause the shuck to adhere to the nut surface,
causing "stick-tights" or nuts that will not fall free of the shuck at harvest. As the season progresses, scab
infections become less damaging to both nut yield and quality.                      More information.

Alabama Spray Guide         Georgia Spray Guide       Louisiana Spray Guide

Cultivar susceptibility

2003 - Worst Pecan Scab Ever (Article)

Weather-based spraying:   Instructions,    weather info from AWIS

Downy Spot

Downy spot
     The symptoms of downy spot first appear on the lower surface of young foliage in late spring or early
summer as small yellow spots, ranging from 0.06 to 0.12 inch in diameter. These spots may turn white as
spores are produced. With age, the spots develop a dark yellow to light brown color, and lesions begin to
appear on the upper surface of leaves.
     Downy spot disease significantly reduces photosynthetic activity. Infection levels of 20 percent of the leaf
surface reduce photosynthetic activity by as much as 40 to 45 percent. The disease does not defoliate the trees
 in early summer, but heavily infected leaves drop earlier than healthy ones in the fall. This early drop can
drastically reduce tree vigor and productivity the following season.    More information.

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Zonate Leafspot
     On pecan, this disease occurs only as a leafspot, and its symptoms are similar to those described on maple
and other plants. Leafspots on the upper surface of pecan leaves are grayish brown with the characteristic
concentric ring formations less distinct than they are on the lower side of the leaf. Leafspots on the lower
surface appear light brown to tan in the center, becoming darker brown toward the edge. The small lesions are
circular; the large lesions are more irregularly shaped but have pronounced con- centric rings within the leafspot.
A film of crystalline-like material forms over the leafspot surfaces.
     Zonate leafspot is identified by the pyramidal or cone-shaped, cream to tan-colored fruiting bodies
(conidiophores) that can be seen on the leafspot surfaces with a hand lens. These conidiophores appear only
on large lesions (10 to 20 mm) and are erect and scattered some- what randomly over dry leafspots.
     A dense growth of conidiophores can be found on large lesions that develop in high humidity or shortly after
 rainy periods. Leaves with extensive lesions become desiccated, curl up from the margins, and eventually fall
from the tree.
     The conidiophores themselves are between 0.5 and 1.0 mm long and are made up of a stalk and a conidium
or head. The upper portion of the conidium may be branched, forming a structure 160 to 560 microns long and
80 to 210 microns wide.          More information

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Fungal Leaf Scorch  
     Like most fungal diseases, fungal leaf scorch develops most rapidly in wet conditions. It usually appears in
July and August and becomes severe by September. The characteristic symptom of fun- gal leaf scorch is a
blackened area on the leaf between healthy and dead tissue. This symptom does not occur with other scorch
problems.  The disease usually begins at the base of the leaflets and advances toward the mid-vein. The dead
areas are dark brown or ash, and there is usually a distinctive black zone between the green and dead portions
of the leaflet. The disease gradually affects more and more healthy tissue, and the leaflet soon drops from the
leaf. As more leaflets drop, eventually the entire leaf is lost.
     A second type of fungal leaf scorch has appeared in a high density planting under sprinkler irrigation that
was also severely infested with aphids. The symptoms in this case were circular areas of dead tissue on the
leaves and defoliation occurred earlier in the season than with more typical fungal leaf scorch.
     Extreme nutrient imbalances of nitrogen or potassium cause leaf scorch that is similar in some respects to
fungal leaf scorch. However, neither shows the distinctive black zone between healthy and dead tissue of fungal
 leaf scorch. In addition, nitrogen scorch usually appears in June and July- earlier than the common fungal leaf
scorch. Potassium toxicity also differs from fungal leaf scorch in that it affects the apex of the leaflet only, and
its first symptoms appear as minute necrotic spots that coalesce as conditions become more acute.
     Scorch symptoms resulting from nutrient imbalances do not continue to progress but subside when
conditions are corrected. With fungal leaf scorch, the condition of the trees continues to deteriorate.
     Spider mites will also cause the leaflet to deteriorate from the mid- vein to the leaflet margin.
More information.

Bunch Disease
     Bunch disease 0£ pecans is an infection caused by a mycoplasma- like organism (MLO). The organism has
been consistently associated with diseased tissue and has not been identified in healthy trees.  Affected trees
displays proliferation of stem shoots on large scaffold limbs. It is not systemic within the tree but localized on
individual limbs. The shoots on the affected limbs are brittle and short-lived.
     The bunches of shoots that develop may be isolated on one or two limbs or distributed throughout the
canopy. As the shoots grow, they develop lateral buds which further increase the size and density of the
bunches. The leaves on affected shoots are larger than normal and are flexible. Diseased trees have lower
yields and inferior nut quality.
More information.

Crown Gall
     Crown gall transforms normal plant cells into tumor cells which become wart-like growths of disorganized
tissues. The galls range from a few inches to a foot or more in diameter and are normally confined to large roots
 and bases of trunks. However, they may also appear on the lower limbs of young trees and on smaller roots.
Initially, the tumorous growths can be confused with callus tissue, but later they become round, rough, and dark.
     The bacterium that causes crown gall can survive in the soil for several years. It enters pecan roots or stems
near the soil line through wounds often caused by insects, grafting, and cultivation. Once it has entered the plant,
it transmits pieces of genetic material into the plant cells. The affected cells grow faster and larger than normal.
As they are transformed from healthy to tumorous tissues, the affected cells be- come independent of the
bacteria and can continue to grow and divide abnormally without it being present. The galls reduce tree vigor
by reducing water and nutrient flow in the vascular tissue.
     The external portions of the galls deteriorate from lack of water and slough off. These tissues often contain
 the bacteria and provide an avenue of entrance back into the soil.
More information.

Powdery Mildew

Powdery Mildew
     Powdery mildew is a fungal disease of both pecan foliage and nuts caused by Microsphaera alni.  It appears
sporadically throughout the pecan belt in July during hot, humid weather.
     This disease produces characteristic powdery-like growth on leaflets and nuts. Infected leaflets are not
usually damaged by the fungus, but severely infected leaflets may lose up to 40 percent of their photosynthetic
capacity.  The amount of damage powdery mildew causes nuts depends on their stage of development at the
time of infection. Nuts infected early may abort or be undersized with poorly developed kernels. Nuts infected
when they are mature sustain little or no injury from the disease.          More information.
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Vein Spot
     Vein spot, a foliar disease, is caused by the fungus, Gnomonia nerviseda Cole. The fungus over winters in
infected leaf debris on the ground. From spring through August, spores are released into the air immediately
after rain showers and infect susceptible pecan foliage. The greatest spore release usually occurs from late
April into early June.
     Vein spot lesions on pecan foliage closely resemble scab and must be examined very closely to be
distinguished from it. When observed in direct sunlight, vein spot usually looks shiny or greasy while scab
appears dull. The lesions are first visible as dark brown or black, pin-point-sized spots. On the leaflets, the
lesions are always centered on veins on midribs. Vein spot infections seldom increase in diameter to more than
0.25 inch. The lesions are usually circular, but they can increase in length along a vein, midrib, or leaf stem
(rachis).
     The lesions first become visible in mid to late May. They can appear on leaf stems, leaflet stalks, midribs,
or leaflet veins. Rarely, a few lesions infect new shoot growth. They frequently occur at the junction of the
leaflet stalk and leaf stem and on the lower half of the leaf stem. A few infections at these locations can result
in leaflet or leaf drop, even if there acre few infections elsewhere on a leaf.          More information.
      

Shuck Dieback and Stem End Blight
     The causes of these two similar problems are unknown, but both kill shuck tissue and reduce nut quality.
They sometimes appear in August but more often in September and October. Both problems can appear on
the same nut cluster and even on the same nut, or some nuts on a cluster can be affected while others remain
healthy.
     Stem end blight begins as a brown or black spot on the shuck near the base of the nut.  This black area
usually enlarges to cover the entire nut or at least a portion of it.  Shortly after the black area appears, the nut
becomes easily dislodged from its stem.
     The earlier the symptoms of these two diseases appear in the season, the poorer the kernel will be. If the
shuck begins to deteriorate in September, the damage will be significant. If the diseases do not appear before
the nut is almost mature, they cause very little damage.          More information.
       

Brown Spot
     Brown spot is a foliage disease caused by a fungus, Cercospora fusca. It infects mature leaflets in June and
July. Brown spot is found primarily in neglected orchards in areas that have abundant rainfall or high humidity.
It is rarely a problem in well-managed orchards.
     Shortly after brown spot infection, circular, reddish brown spots appear on the leaflets. As the disease
progresses, the spots develop grayish concentric zones and become irregular in shape. Brown spot may
defoliate the tree by October if steps are not taken to control it.
     Brown spot symptoms can be confused with those of Gnomonia leafspot. The two diseases can be
distinguished in that brown spot lesions can develop beyond the lateral veins, but Gnomonia leafspot lesions
remain confined within the veins.    More information.
    

Gnomonia Leafspot
     Gnomonia leafspot is a minor disease that occurs in South Alabama, South Georgia, and North Florida.
It is caused by a fungus, Gnomonia dispora, that is weakly parasitic and will only infect poorly nourished trees
that are deficient in zinc.
     The first symptoms appear in June a few days after infection. They are small, circular brown spots on the
upper surface of the leaflets. As the disease progresses, the spots turn dark and enlarge up to 0.25 to 0.5 inch
in diameter. As the spots expand, they are restricted by the lateral veins. They develop large, elongated dead
areas within the lateral veins.          More information.

Liver Spot
     Liver spot is a leafspot disease caused by a fungus, Gnomonia carvae var pecanae.  Liver spot can cause
severe defoliation during late summer and early fall, particularly in orchards where prolonged periods of wet
weather exist. Weak trees are more susceptible to liver spot than are healthy trees.
     The first sign of the disease appears in May and June. Circular, dark brown spots ranging from 0.12 to 0.37
inch in diameter appear along the midrib on the lower surface of the leaflets. In late summer, the spots turn a
cinnamon brown or liver color.        More information.

Leaf Blotch
     Leaf blotch is a minor foliage disease found throughout the pecan belt. It is caused by the fungus,
Mycosphaerella dendroides, and infects only weak trees. It causes little or no damage to healthy trees.
     Leaf blotch symptoms appear in June and July. Spots with olive-green, velvety tufts form on the lower
surface of mature leaf- lets. At the same time, pale yellow blotches appear on the upper surface. Later, the
spots run together, forming black, shiny blotches.
     As the disease progresses, the trees lose their lower leaves first and continue to defoliate until only a few
leaves are left in the tops of the trees.  More information.