Pecan Disease

Credits: Dr. Charles Rohla, Noble Research Institute; Becky Carroll, Oklahoma State University; Dr. Eric Stafne, Mississippi State University

Name Latin Name Description Life Cycle Symptons
Pecan Scab Cladosporium caryigenum Some varieties are resistant, but many grafted varieties are susceptible. Producers should keep in mind that most commercial varieties were at one time resistant to pecan scab and have now become susceptible because of genetic changes in fungus virulence. Native pecan trees in Oklahoma exhibit a high degree of genetic variability in resistance to scab. The scab fungus overwinters as a small, tight mat of fungal material called a "stroma" on shucks, leaf petioles and stems infected the previous season. With warmer temperatures and rainfall in the spring, fungal spores are produced on the stroma. Dew and rain spread spores locally within a tree, and the wind spreads them over long distances to adjacent trees or orchards. First appears as small, circular, olive-green spots that turn to black on the newly expanding leaves, leaf petioles and nut shuck tissue. All tissues are most susceptible when young and actively growing. Lesions expand and may coalesce. Old lesions crack and fall out of the leaf blade, giving a shot-hole appearance. Nut infections cause the greatest economic damage. Early infections may cause premature nut drop but more commonly cause the shuck to adhere to the nut surface, causing sticktights. Late infections can prevent nuts from fully expanding and decrease nut size.
Powdery Mildew Microsphaera penicillata or alni? The fungus that causes powdery mildew can infect both leaves and nuts but under typical orchard conditions it is observed much more frequently on nuts. Generally, powdery mildew does not cause significant crop damage, but it can reduce nut quality in severe infections. The disease is sometimes a problem on pecan foliage in greenhouses and nurseries. Powdery mildew occurs sporadically in Oklahoma and rarely is a serious disease in the state. The life cycle of all species of powdery mildew fungi is nearly identical. These fungi overwinter as small black fruiting bodies (cleistothecia) or as fungal threads (mycelium) on leaf debris and on stems, spurs, or dormant buds of plants. In the spring, the cleistothecia produce spores that initiate primary infections on susceptible hosts. Secondary spores are produced from these initial infections and cause additional infections. Spores produced on over wintered mycelium can also start spring infections. The result of infection is the appearance of a white, powdery growth on the leaf surface. It is composed of fungal threads (mycelium) and chains of small summer spores. After the fungus has become established, small black fruiting bodies are produced. This completes the life cycle of the fungus. The fungus is first visible on nuts as a white powdery coating. As the nuts age, the white fungal growth disappears, but the nut shuck develops a brownish patina instead of the typical bright green. It occurs on foliage and nuts alike and appears as a superficial powdery white growth. Early-infected nuts may remain small and have an early shuck split and shriveled kernels. In some other states, it sometimes causes defoliation.
Vein Spot Gnomonia nerviseda Vein spot disease can cause defoliation during late summer and fall. Infection and defoliation vary with cultivar; some cultivars have been recorded with a defoliation of over 70%. Vein spot is a common disease of pecan leaves. It does not occur on shoots or nuts. In Oklahoma, this disease is usually not serious Spores of the fungal pathogen are produced in leaf debris from the previous year’s disease. Infection of the current season foliage usually develops following spring rainfalls. If there is less rainfall in the spring months than normal, infection may occur following rainfall in the summer period. Lesions begin to appear in late May. Lesions of vein spot disease always occur on vascular associated tissues including leaflet veins, midribs, petiolules, and rachises. The lesions are similar in size to scab lesions, but have a smooth surface and become gray in the center as they age. Vein spot infections (lesions) are very similar to those caused by the scab fungus. Vein spot lesions, however, tend to be linear rather than round and also tend to be restricted to veins.
Liver Spot Gnomonia carvae Liver spot can cause severe defoliation, particularly during prolonged periods of wet weather. Weak trees are more susceptible to liver spot than are healthy trees. This disease sometimes causes defoliation in the more humid southeastern states but is not considered a serious disease in Oklahoma. The first sign of the disease appears in May and June. Black pycnidia form in the lesions in late summer. Circular, dark brown spots appear along the midrib on the lower surface of the leaves. In late summer the spots turn a cinnamon brown or liver color. Liver spot is characterized by the appearance of reddish brown (liver colored) circular spots one-eighth to five-eighths inch in diameter on the lower side of leaflets. In the fall, the spots turn cinnamon brown and contain small, dark fungal-fruiting bodies.
Bunch Disease phytoplasma Movement of the disease within a tree and between trees can occur slowly over a period of many years or be fairly quick spreading throughout an orchard in three to four years. Often, the disease appears to move into a grafted orchard from seedling trees in wooded areas near the orchard. The pathogen also can be graft-transmitted. Terminals with the disease do not produce nuts, and thus a significant level of the disease can cause economic yield loss. Terminals that are not affected by the disease can produce good quality nuts. Some varieties are more resistant than others and they may be successfully topworked onto susceptible trees. It is not known how the pathogen infects trees but it is suspected that insect vectors are involved because leafhopper vectors transmit phytoplasm pathogens in other plant diseases. It is believed that the pathogen is insect transmitted, probably by leafhoppers. This disease causes shortened internodes that produce a “witches-broom” appearance on affected limbs. Terminals affected by bunch disease produce crowded shoots from numerous buds that normally would not be active. These dense, compact shoots often look like a broom head sticking out of a limb. Terminals with bunch disease usually start growing earlier in the spring than the unaffected terminals. This premature shoot growth provides an easy way to identify the disease in the spring. Terminals with bunch disease can be more difficult to recognize after trees are in full foliage. The disease may occur throughout a tree or be limited to a few scaffold limbs. The characteristic symptom of bunch disease is bushy growths of slender willowy shoots, resulting from an abnormal forcing of lateral buds. Symptoms may appear on only one branch or on many branches. Bunch disease is very conspicuous in the spring and early summer because the diseased shoots leaf out earlier than noninfected shoots.
Hypoxylon Canker Hypoxylon atropunctatum Though this fungus occurs on oaks in Oklahoma, it has not been found on pecans. Stressed trees, particularly those that are water stressed, will very likely become infected. There is no control of the fungus once it infects a tree. The problem can be prevented by maintaining a proper balance of water in the root zone, painting over wounds, preventing compaction of soil around trees, avoiding cultivation around trees, and maintaining a satisfactory rate of growth by fertilization. Research reports indicate that the organism enters branches through wounds. The fungus then grows through the wounds, then through the sapwood causing decay. The first outward symptoms that may be evident are yellowing and wilting of leaves and death of top branches. Inocula­tion experiments in Georgia have shown that the fungus is capable of spreading up to 3 feet above and below a point of inoculation within one growing season. Researchers at the University of Arkansas have been able to isolate the fungus from seedling oak trees which showed no symptoms of infec­tion. This would indicate that the fungus may be active in the trees for a number of years before disease symptoms are noticed. When trees are weakened, particularly by drought, or injured, the disease is capable of overcoming resistance of the host, and the tree dies. The disease progresses through branches, causing dieback symptoms. After the death of branches or of the tree, the outer bark sloughs off, exposing a thin stroma (a mass or mat of fungal hyphae packed together to form a hard crust in or on which spores are formed). At first the stroma produces brownish, dusty masses of conidia (asexual fungus spores) that are easily blown from tree to tree and which cause new infections. The stroma color soon changes to silver and then to black as the sexual state of the fungus develops. The stroma becomes thicker and harder as the sexual state develops. The sexual state produces masses of dark spores inside the stroma, which are “oozed” out onto the sur­face, where they can be transferred by various means (rain, insects, etc.) to other branches or trees. A large tree may be killed within one to two years, depending on the vigor of the tree; however, because early stages of the disease may not be noticed, trees may appear to die within a period of a few weeks. A stroma may be limited in its development or may extend the entire length of the tree. The disease is characterized by sloughing off of bark and a reddish powdery substance formed on the wood under the bark. This should not be confused with the normal red powdery material formed on the underside of the bark itself. A chocolate brown to black crusty substance will eventually form on infected wood. Infected limbs and trunks will eventually die.
Twig Dieback Botryosphaeria berengeriana It is most often found on limbs that are under stress. The stress may be caused by over-production, defoliation by scab and other foliar pathogens, drought, or excessive shading. Prune out dead wood and destroy it. This fungal disease infects through natural openings in the bark or through pruning wounds. The fungus is dormant in the winter, and spores are spread by wind in the spring. Twig death is usually general over a tree except under shady conditions, where it will occur only in the shaded areas. Some limbs may die back as much as 24-30 inches.
Crown Gall Agrobacterium tumefaciens A bacterial disease that girdles trees. Introduced through wounds and manifested by winter injury. Large tumor-like growths occur near base of trunk, but can also be seen on other areas. Bacteria live in the soil on infected tissues. Enters plant through pruning and grafting wounds, as well as other damage like winter injury, hail damage, insect entry points, and mechanical damage. Galls usually form annually in late spring or early summer, and reduce tree vigor by restricting the flow of water and nutrients. The bacterium can be spread from infected trees to other areas by transfer of the soil on equipment (mowing and tilling). Initially appear as fleshy, cream colored galls and later turn brown and woody. Affected trees are weak and may die. Infected trees can be more susceptible to cold damage. Young trees may be girdled in one season and die.
Cotton Root Rot Phymatotrichum omnivorum There is no resistance to severe infestations. Avoid areas where this disease occurs. Can be controlled in non-calcareous soils below 6.5 pH. Infects roots of mature plants, rotting the tap root. Grows as mycelial strands on root surface and has unique cross-shaped hyphae. Symptoms usually noticeable in summer months. First symptom of infection is leaf wilting. Plants may die quickly with leaves still on the tree.
Armillaria Root Rot Armillaria mellea Old forested areas or new orchard sites where trees grew previously may be at risk. Most obvious in late summer when tree may completely collapse. Trees infected with Armillaria can have reduced growth, rapid mortality, wood decay (white rot), and susceptibility to windthrow. Armillaria species contribute to tree decline by the interaction with other factors such as site, tree age, drought, insects, etc. Fungus spreads from tree to tree through root contact. Fungus can survive for years on dead roots and old stumps in soil. Fungal mats occur near soil line under the bark. Fungus also produces mushroom fruiting bodies in clumps and dark, string-like fungal tissue. The fungus spreads from tree to tree through roots, and via rhizomorphs [a thick strand of organized hyphae (tubular cell array) resembling a fine root] which infect intact bark. These fungi produce a honey-colored mushroom around the base of the tree in late summer or fall during moist periods Stunted shoots, yellow or red leaves, wilting and premature defoliation. Fungal mats occur near soil line under the bark. Distinct mushroom odor when infected tissues are moist.
Bacterial Leaf Scorch Xyllela fastidiosa Not yet found in Oklahoma, but may appear in southern counties near Red River in the future. An infected branch may die in one year or may live for a few years with the infection depending on genotypic and environmental factors. The disease can cause large economic loss over the lifetime of infected trees. Caused by a bacterium that lives in the xylem tissue of the branch and is spread by leafhoppers and spittlebugs. It is not known how the pathogen infects trees in nature, but it is suspected that it is transmitted by xylem-feeding insects, such as leafhoppers, because that is the typical method of pathogen spread in other hosts. The pathogen can be transmitted through pecan scion wood and this may be a significant source of infection because pecan cultivars are clonally propagated. It is not known how the environment affects disease development. Symptoms usually begin in mid-summer; leaflets turn brown beginning at the tip and outer margins and leaflets abscise soon after they begin to turn brown. Symptoms may occur throughout a tree or be only on a few of the scaffold limbs. Defoliation of over 60% has been recorded on trees with severe symptoms. Infections are chronic and the disease is usually present every year to some extent. Leaves show scorching on margin going inward and drop off, leaving petiole attached to shoot. Flower clusters may set berries, but do not mature. In fall, infected shoots mature in patchy manner, leaving spots of green tissue surrounded by dark brown mature wood. May delay budbreak and new shoots are stunted.
Anthracnose Glomerella cingulata This disease can be severe in years with heavy rainfall. the fungus can infect a wide range of hosts. Infections typically occur on ripening and senescing tissues. Anthracnose is primarily a concern on pecan fruit; however, recently the fungus causing anthracnose has been implicated in a foliar disease complex called fungal leaf scorch. The distinction between fungal leaf scorch and foliar symptoms of pecan anthracnose is unclear. The fungus overwinters in orchards on peduncles of the previous year's crop. Spore (conidia) release in early spring coincides with budbreak. Spores are spread by splashing rain. Spore production and spread are encouraged during periods of excessive rain and cool temperatures (~68 F). Infection usually occurs during early stages of fruit development. Stresses such as drought, tree crowding, heavy crop load, insect infestations can accelerate the life cycle of the pathogen resulting in rapid and severe disease development. Shiny, dark brown, sunken lesions develop on green pecan fruit during wet periods. Lesions can expand and eventually cover the entire fruit. During periods of excessive wetness, salmon-colored masses of spores may be visible on the lesions. Yield and quality are reduced when the fungus penetrates the shell and kernel. Decreased kernel size, fruit abortion, and sticktights can result from infections by the fungus.
Brown Leaf Spot Spirosporium diffusum this disease occurs on the mature pecan leaflets. 'Kanza,' 'Stuart,' and 'Dodd' appear to be susceptible in mid-summer, as are some native pecans. Occurrence is mainly in orchards and groves that are not sprayed for pecan scab. The fungus overwinters on leaf tissue. Since this is a minor disease issue in most areas, in-depth understanding of its life cycle is not available. Brown leaf spot presents with reddish-brown, small circular lesions. As the lesions develop, they become more irregular in shape and cross over veins. Older lesions turn gray. Severe infestations can cause premature defoliation.