Pecan diseases are a harvesters worse nightmare. With pecan trees grown across the southeast, varieties respond to diseases and infections differently. Pecan diseases and infections impact trees in two ways. First, disease can reduce the tree’s vigor, which in-turn causes the foliage and branches to shed, resulting in a loss of shade value. Secondly, diseases can also infect the nuts and reduce both nut quality and quantity.
At times the disease pressure can be so great that no viable nuts are available. The majority of diseases are difficult to control with either natural or chemical fungicides because their application requires special, expensive equipment that most home owners do not have. In addition, the entire tree canopy must be treated by the fungicide application for effective control.
Most Common Pecan Diseases
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.
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.
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.
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 portionsof 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, andits 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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You can access the Georgia Spray Guide: 2020-Pecan-Spray-Guide
You can also access the pecan weather for the Southeast:: http://www.awis.com/Misc/Pecan/Pecan_weather.htm