Scab (fungus – Cladosporium effusum): The fungus invades growing shoots, leaves and later developing nuts. Severely infected nuts fall or fail to fully develop. Infected foliage prematurely drops during periods of frequent rainfall and mild temperatures. These conditions are optimum for rapid development and spread of the scab fungus. Infection takes place when leaves or nuts are covered with free moisture for at least 2 hours. The scab fungus overwinters in infected shoots, old shucks, and leaf petioles. In the spring when temperature and moisture conditions become favorable, the fungus becomes active and within a few days spores are formed at the surface. They are spread by wind and rain to developing leaves and nutlets. If weather conditions are favorable, the spores germinate and enter the host tissue. Within 7-10 days new lesions are formed on the host. Lesions formed on the lower leaf surface (See Photo) are characteristically olive brown in color, somewhat elongated in shape. They range in size from a barely discernible dot to lesions one-fourth inch or more in diameter. Frequently, adjacent lesions coalesce, forming large, chocolate brown irregularly shaped spots. Scab lesions occur on or along the leaflet veins, but can be found between the veins. On nuts, the scab lesions are small black dots which become sunken as they mature. Adjacent lesions on the nuts may coalesce, forming large sunken black lesions (See Photo). When infection is severe, the entire nut surface is black, development is arrested and the nut drops prematurely or fails to grow in the area of infection. Pecan Varieties (See Table 3 Below) vary in their susceptibility to the scab fungus. Strains of the pecan scab fungus can develop that will infect a resistant variety. As the inoculum from these strains increase, a once resistant variety is increasingly infected by the fungus. Even though a variety possibly will not be resistant throughout its entire life, it is best to plant varieties that are productive and resistant to the scab fungus. Trees should be spaced to allow for air circulation between the trees. This encourages rapid drying of the foliage. Properly spaced trees will have a higher percentage of their leaves exposed to sunlight. Removal of the lower limbs of a tree will aid in air circulation and reduce the time leaves remain wet following rain or heavy dews. Trash trees surrounding the field should be removed. Pecan trees that can not be adequately sprayed should be removed if they are susceptible to the scab fungus. Spores of the pecan scab fungus produced on the unsprayed trees are spread by air currents to surrounding trees. Sanitation measures such as removal of old shucks and leaf petioles from around trees wiil help to reduce disease inoculum in the orchard. If left in place, spores produced from old lesions are carried by air currents up to the tree. Fungicides (See Table 1 Below) are an important part of the pecan disease management program. Although some of the newer fungicides have post infection activity, they are most effective when applied as a preventive application. Thorough coverage of leaf, nut and shoot surfaces with a fungicide must be maintained to prevent infections.
Brown Leaf Spot (fungus – Cercospora fusca): Brown leaf spot affects only mature leaves and does not appear until the latter part of May or mid-June. Primary lesions develop on the lower leaf surfaces as small dots which gradually enlarge and become reddish-brown with a gray cast. The shape of the lesions may be circular or irregular, especially where two or more lesions develop adjacent to each another. In seasons favorable for brown leaf spot development, pecan trees may be completely defoliated if the fungus is not controlled. Only the foliage is infected by this fungus. Pecan trees that are making active growth are less likely to have brown spot. Leaves on trees that are fertilized properly sedlom are infected with the brown leaf spot fungus. The problem is most often observed on pecans growing in sandy soil. Heavy rains during the spring will leach nitrogen below the root zone of the trees. Fungicides (See Table 1 Below) applied as preventative sprays will protect the foliage.
Vein Spot (fungus -Gnomonia nerviseda): Foliage symptoms of this disease are similar to the lesions formed by the scab fungus. Unlike scab, vein spot only affects the leaves. Lesions of vein spot disease develop on veins of leaflets and are characteristically dark brown to black. Severely infected leaflets shed soon after symptoms are observed. The fungus overwinters in fallen leaves beneath the tree. Fungicides and sanitation procedures will control this fungus. Fungicides applied for scab control are also effective against the vein spot fungus.
Leaf Blotch (fungus – Mycosphaerella dendroides): This disease occurs mainly on low vigor trees. It overwinters in infected leaves on the orchard floor. Symptoms first appear on the under surface of mature leaves in early summer as small olive-green velvety spots. By midsummer, black pimple-like dots are visible in the spots. When severe, infected leaflets are killed, causing leaf shed in late summer or early fall. Premature shedding adds to the reduced plant vigor. Fungicides applied following the pecan scab spray program will also control this fungus.
Crown Gall: (See section on Crown Gall) Also see Table 1 Below for Fungicide Information.
Crown Gall: (See section on Crown Gall) Also see Table 1 Below for Fungicide Information.
Downy Spot (fungus – Mycosphaerella caryigena): Foliage on trees infected with this fungus shed soon after symptoms are observed. Primary infection occurs on developing leaves in the spring from spores produced on last years foliage. Symptoms usually appear during the summer months on the lower surfaces of leaflets. Lesions are one-eighth to one-fourth inch in diameter and greenish-yellow. Later in the season, the lesions turn brown due to the death of the leaf cells in the disease area. Spores are spread by wind and rain to nearby leaves and neighboring trees. Although all pecan varieties are moderately to slightly susceptible, Pawnee, Moneymaker and Stuart varieties are the most susceptible to the downy spot fungus. Disk or rake under fallen leaves in the fall or early spring before the leafbuds begin to swell. By removing the leaves from the soil surface, spore discharge is stopped. Fungicides (See Table 1 Below) applications between bud break and casebearer are the most critical in the control of downy spot.
Powdery Mildew (fungus – Microsphaera alni): Infected nuts are covered with a white powdery substance. The fungus most often develops between mid-July and mid-August. Due to rainfall washing the spores from the nuts or leaves, the fungus seldom is a problem during months of frequent rain. Pecans which were previously infected by the powdery mildew fungus are brown in color when the fungus is washed by rainfall. The fungus is sometimes associated with nursery trees or sucker growth around the base of mature trees. Infected leaves are somewhat distorted in shape and are covered with a faint gray colored dusty material (See Photo). Powdery mildew infected nuts or leaves are not significantly damaged by the fungus. Its feeding is limited to the outer cells of the shuck. This does not restrict the movement of nutrients into the developing kernel. Fungicides (See Table 1 Below) applied in the course of the regular scab spray program will control powdery mildew.
Fungal Leaf Scorch (fungus – several): Infected leaves turn brown along the leaf margin or tip. The necrotic areas may extend from only a small area at the leaf margin to completely encircle the margin of the leaf. As the lesion matures it becomes gray. Infection can result in leaf shedding from mid July to mid August. Symptoms develop on a few leaves and then spreads to nearby leaves. Seldom is the entire tree affected. Because the infected leaves shed, orchard insepections made late in the season are not effective in diagnosing the disease. Several fungi are reported to be the cause of this disease. All of the fungi are controlled with approved fungicides. Fungicide applications applied for scab are effective when applied in early to mid summer. The varieties Western, Cheyenne, Chickasaw, Shoshoni, Stuart, and Cape Fear are some of the more susceptible varieties to this group of fungi.
Bunch Disease (mycoplasma): Although the cause of bunch disease is not completely understood, evidence indicates it is a mycoplasma (MLO). Trees affected with bunch disease show a typical bunching symptom, caused by excessive growth of lateral buds. Normally only the most distant bud will break dormancy and develop into a twig and finally a limb. A large percentage of the buds on bunch infected trees will break dormancy at one time. This results in a dense growth of small twigs. In some cases only one or two limbs will be infected by the organism. Nut production is very limited on bunch infected trees. Nuts from infected trees are restricted in size. There is no effective control for bunch disease, except tree removal.
Lichens: (See section on Lichens.)
Articularia Leaf Mold (fungus – Articularia quercina): The disease occurs most commonly following rainy periods, in areas of high relative humidity and on leaves of poor vigor trees. The fungus grows on the lower surface of the leaves, a growth of white tufts that contain spore masses. Articularia leaf mold does not occur on trees in orchards that have been sprayed regularly for disease control. A single fungicide application at the first sign of disease is usually sufficient to control articularia leaf mold disease. The materials applied for scab control will also provide some protection against this fungus.
Pink Mold (fungus – Trichothecium roseum): Pink mold infection occurs on scab infected nuts. Infected nuts develop a white mold growth in old scab lesions. In areas not exposed to air and sunlight, the white area becomes pink in color. The fungus may invade the kernel. The kernel becomes oily in appearance and ‘leaks’ onto nearby nuts. The kernel is rancid and not eatable. In areas where scab disease control is regularly practiced, pink mold is not a problem. If pink mold is a severe problem, it maybe necessary to remove the oily nuts or to segarate the problem load from disease free nuts.
Spanish and Ball Moss: (See section on Spanish and Ball Moss.)
Cotton Root Rot (fungus – Phymatotrichum omnivorum): Cotton root rot is caused by a soil-inhabiting fungus pathogen that attacks a wide range of host plants including pecan. Roots of pecan tree can be invaded at any time except during the coldest months of the year. The fungus is most active during the mid to late summer months. Infected roots are killed, disrupting the transportation of water to the leaves. Diseased trees die soon after becoming infected. Losses continue several years after planting. The losses increase in numbers roughly in porportion to the increasing size of the disease center. The fungus is present in many alkaline soils in central and south Texas. An effective control for cotton root rot has not been developed. Do not plant in soils that have had a history of cotton root rot.
Shuck Die Back (physiological – Possible hormone imbalance): This condition is commonly associated with Success and Success hybrids. Nuts affected by this disorder drop from one to two weeks early. They do not fill properly due to the peduncle being girdled and restricts movement of nutrients into the pecan. This results in what is known as “pops.” The shuck turns black and opens at the tip in a normal manner. No control is recommended.
Kernel Rot (fungus- Phytophthora sp.): This has been described from the Southeast but has not been confirmed in Texas. It is most often associated with very wet conditions in the later stages of nut development. The shucks turn black. The kernel becomes discolored and is not eatable. In the southeast copper based fungicides (See Table 1 Below) are applied from the water stage to shuck split on a 10 day schedule.
Stem End Blight (fungus – Botryosphaeria ribis): This disorder is associated with a fungus which attacks the nuts in the latter part of July and August. Shucks turn black rapidly and drop soon after. Lesions are black, sunken, and shiny. When infected nuts are cut open, the liquid in the kernel has turned brown. This can be controlled with foliar sprays of a fungicide applied when the pecans begin to fill.
Root Knot on Pecans (nematodes – Meloidogyne sp.) Small swellings are found on the rootlets. Infected trees develop severe zinc deficiency symptoms. Growers should examine all nursery trees before planting. Chemical control is not recommended. (Refer to Root Knot Nematodes section for further information.)
Kernel Discoloration (fungus – Nematospora spp.): Most kernel discoloration is characterized by distinct dark spot on the kernel. This condition most frequently develops after the kernel is damaged by stink bug attack which is followed by fungal invasion. Improper drying of the pecan can also result in kernel decay.
Fungal Twig Die Back (fungus – Botryodiplodia): Infected twigs are covered with small raised pustules with black centers. This results in twig dieback. Maintain trees in a healthy condition. No control suggested at this time.
|Table 1: Fungicides for Pecan Diseases|
|Scab||Benomyl, Propiconazole, Triphenyltin Hydroxide, Thiophanate Methyl, Ziram, Copper Sulfate, Dodine|
|Brown Leaf Spot||Benomyl, Propiconazole, Triphenyltin hydroxide, Thiophanate methyl|
|Downy Spot||Benomyl, Propiconazole, Triphenyltin hydroxide, Thiophanate methyl|
|Powdery Mildew||Benomyl, Propiconazole, Triphenyltin hydroxide, Thiophanate methyl|
|Shuck and Kernel Rot||Copper Hydroxide|
|Table 2: Fungicide Application Instructions|
|Table 3: Resistance of Pecan Varieties|
|Cape Fear||Moderately Susceptible|
|San Saba Imp.||Susceptible|