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DISEASES

Overview
- Variety of causal agents
- Resistant varieties
- Air circulation
- Pruning
- Crop sanitation
- Last line of defense

Common diseases
- Pecan scab
- Stem end blight (Water stage nut drop)
- Shuck die back ( Physiological, nut drop, variety reaction)
- Downy spot (Foliage loss, Stuart)
- Brown leaf spot (Late season, defoliation, weak trees)
- Fungal leaf scorch (Defoliation, susceptible varieties, leaf curling)
- Powdery mildew (White powdery fungus, high temperatures, dry conditions)
- Vein spot (Leaf veins, brown lesions, defoliation)
- Leaf blotch (Weak trees, old foliage)
- Articularia leaf mold (Minor leaf disease, white tufts, weak trees)
- Pink mold (Secondary diseases, behind scab lurks pink fungus)
- Bunch disease (Mycoplasm, sucker growth)
- Fungal twig die back (Black pustules, drought)
- Kernel discoloration (Several fungi, insects, drying)
- Cotton root rot (Soil-borne disease, rapid death, fungus)
- Root knot on pecans (Slow decline, zinc deficiency, nematodes)
- Crown gall (Bacteria, weakened tree)
- Regional disease pressure
- Honeydew sooty mold

Variety selection and fungicide applications


 

Overview
Variety of causal agents
Diseases affecting pecans are caused by bacteria, fungi, nematodes, mycoplasm, and physiological problems. Although viruses have not been shown to occur on pecans, it is suspected that with time and closer observation some of the problems involved with obtaining maximum production may be a result of a viral infection. Diseases are controlled with a variety of practices.


Resistant varieties
Use of resistant varieties is one of the most important methods of reducing insect and disease problems. Resistant varieties can reduce the amount of pesticides required to produce a crop. For example, resistance can delay the occurrence of scab during periods of inclement weather when spraying is not possible. The cultivar 'Pawnee' is less susceptible to aphids than most others and 'Cheyenne' is quite susceptible resulting in greater concern about aphids on this variety.

Air circulation
Plant spacing is also important in reducing diseases. As plants are grown closer together, the more likely serious disease epidemics are to occur. Poor air circulation slows down the drying of foliage and increases the potential infection periods within an orchard. Where possible, the rows should run with the prevailing wind. This will allow for better movement of wind through the grove. If the rows are planted perpendicular to the prevailing wind, the outer rows will block movement to the inner orchard. This is particularly important to the more closely spaced trees. The black pecan aphid also increases in severity as densely shaded areas become more prevalent, as occurs in orchards needing thinning.

Pruning
Pruning to remove low hanging limbs can also help to reduce the disease build-up within an orchard. By removing the lower hanging limbs, air movement is encouraged beneath the trees, and this not only encourages drying but also facilitates better movement of the released spores away from the tree. This also reduces black pecan aphid problems.

Crop sanitation
Sanitation within the orchard is extremely important because many of the diseases and insects overwinter in old shucks, leaves, leaf petioles, and twigs. Shallow disking or removal of the old plant debris will help to reduce the amount of fungal pathogens and insects present in a grove.

Last line of defense
If used properly, fungicides can prevent significant losses to disease. Total coverage is important to achieving effective control. When spraying, always check foliage to see if you are leaving a moist film on all of the foliage. If you are not achieving thorough coverage, check the tractor speed, pump pressure, nozzle size and nozzle arrangement.

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Common diseases
Pecan scab
Scab fungus (Cladosporium caryigenum) invades young, rapidly growing shoots and leaves [clad2] and later the developing nuts [clad1]. Severely infected nuts [scab2] on highly scab-susceptible varieties fall or fail to develop, resulting in a total nut crop loss. Early season defoliation often occurs in seasons of frequent rains and high humidity which facilitates the rapid development and spread of the scab fungus. The scab fungus overwinters in infected shoots and in old shucks and leaves in the trees. In the spring when temperature and moisture conditions become favorable, the fungus resumes its growth in the old lesions, and within a few days, produces great numbers of spores. Based on work in Georgia, 70 degree F appears to be the lower temperature limit at which spores are formed. These spores are spread by wind and rain to newly developed leaves where they germinate and invade the tender tissues, initiating primary infection. It is to control this primary infection that the bud break and pre-pollination sprays are applied. The fungus produces a great abundance of spores on the surface of these primary infection sites and become visible to the naked eye within 7 - 9 days depending and then spreads throughout the trees infecting young shoots, leaves, and nuts [scab3]. On the leaves, primary infection lesions occur on the lower leaf surfaces and are characteristically olive brown, somewhat elongated in shape, and variously sized from a barely disconcernible dot to lesions 1/4 inch or more in diameter. Frequently, adjacent lesions coalesce forming large, chocolate brown lesions. Primary scab lesions commonly occur on or along the leaflet veins but may be found between the veins. On the nuts, scab lesions [scab1] appear as small black dots which become sunken with age. Adjacent lesions on the nuts may coalesce forming large, sunken, black lesions. The inner portion of the lesion will be crusty in appearance. When infection is severe, the entire nut surface is black, kernel development is stopped, and the nut drops prematurely. Pecan varieties vary in their susceptibility to scab disease. Scab disease development is favored by rainy periods and cloudy days when the leaf surfaces are wet. Under these conditions, spores of the fungus in contact with the wet leaf surface of a pecan leaflet germinate rapidly, invade the tender tissues, and initiate infection within 6 hours. Lesions resulting from these infection sites become visible to the naked eye within 7 - 9 days depending on environmental conditions. Control of pecan scab disease depends primarily on protection of tender leaf, nut, and shoot surfaces with application of an effective fungicide. A thin film of the fungicide prevents the scab fungus from developing by killing spores before they can invade susceptible tissues. Unfortunately, once the fungus has invaded the tissues, it becomes protected from most fungicides and can continue to produce spores. Therefore, thorough coverage of leaf, nut, and shoot surfaces with a fungicide must be maintained during the season to prevent secondary infections following rains providing wetting periods sufficient to allow germination and penetration. Sanitation measures, such as plowing or disk harrowing under fallen leaves and shucks, help reduce primary infections. Pruning to open up the tree for better air circulation will help reduce scab occurrence by reducing the number of infection periods occurring during the year.

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Water stage nut drop - Stem end blight (fungus - Botryosphaeria ribis) is associated with insect feeding; a fungus which attacks nuts in latter part of July and August. Shucks turn black rapidly and drop soon after infection. Lesions are black, sunken, and shiny. When nuts are cut open, the liquid in the kernel has turned brown. This can be controlled with foliar sprays of a fungicide applied at initiation of water stage and a second application 10 days later. Losses to this fungus can be reduced, but complete eradication has not been achieved. Benomyl type fungicides have been the most effective. Losses to this fungus should not be confused with other drops that occur in the fall due to other pathogens and physiological problems.

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Physiological, nut drop, variety reaction - Shuck die back (Physiological - possible hormone imbalance) is commonly associated with Success and Success hybrids. Nuts infected with this disorder drop from 1 to 2 weeks early. They do not fill properly due to the peduncle being girdled earlier. This results in what is known as "pops." The shucks turn black at the tip and open in a normal manner, but no kernel is formed. No effective control has been found.

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Foliage loss, Stuart - Downy spot (Fungus - Mycosphaerella caryigena) - Only the foliage is susceptible to the fungus [downspot]. Primary infection of new leaves in the spring occurs from spores produced in old, overwintered leaves. The downy spots usually appear during the late summer months on the under surface of leaflets. Infection occurs in spring near budbreak. The downy character of lesions is due to production by the fungus of thousands of minute spores on the surface of each spot. The spores are spread by wind and rain to adjacent leaves and to neighboring trees. After spore dissemination is complete, lesions visible on both surfaces of the leaf are 1/8 to 1/4 inch in diameter and greenish-yellow. Later in the season, lesions turn brown due to the death of leaf cells in the disease area. Eventually, the lesions become frosty in appearance. Moneymaker and Stuart varieties are most susceptible to downy spot disease although all pecan varieties are moderately to slightly susceptible. Disk under old fallen leaves in early spring before leafbuds begin to swell. This practice covers leaves with soil and prevents the discharge of spores into the air, thereby controlling primary infections of new leaves. Fungicides at budbreak will reduce the primary infections.

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Late season, defoliation, weak trees - Brown leaf spot (Fungus - Cercospora fusca) affects only mature leaves [brwnlfsp] and usually does not appear until mid-June or July. Primary lesions develop on the lower leaf surfaces as small dots which gradually enlarge and become reddish-brown with a grayish cast. The shape of lesions may be circular or irregular, especially where two or more lesions develop adjacent to one another. In seasons favorable for brown leaf spot development, pecan trees may be completely defoliated within 3 to 4 months if the disease is not controlled. Most pecan varieties which are maintained in a vigorous state of growth are not as susceptible to this disease. The fungus has been observed causing the most damage in the West Cross Timbers and on the 'Burkett' variety.

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Defoliation, susceptible varieties, leaf curling - Fungal leaf scorch (Fungus - several) - Although all varieties are susceptible to this group of fungi [funlfsc1], Shoshoni, Chickasaw, Cheyenne, Shawnee, and Stuart appear to be some of the most susceptible. The fungus [funlfsc2] causes premature defoliation in the fall. Infected leaves turn a reddish-brown, and infection occurs along the leaf margin or at the tip. The infected area rolls upward. With age, the reddish-brown area becomes a dull brown with small black spots scattered over the lesions. The disease often occurs within one area of the tree rather than randomly scattered over the canopy. The use of fungicides will help reduce losses from these fungi.

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White powdery fungus, high temperatures, dry conditions, immature foliage - Powdery mildew (Fungus - Microsphaera alvi) - When pecans become infected with this fungus [powmil], they appear to be covered with a white, powdery material. The fungus develops on the outside of the shuck and only feeds on the outer layer of plant cells. Thus, although it appears to be causing considerable damage, no significant yield losses can be attributed to the presence of the fungus. Powdery mildew can develop at very low humidity, and is a problem during the mid-summer months. Once the fall rains begin, the fungus is washed off the shuck leaving only the dead epidermal cells of the shuck. When the foliage is infected, it becomes slightly distorted and covered with a faint white, powdery substance. Foliage infection is a problem only on the lower, immature leaves of a tree and on nursery trees. Control is based on following a scab fungicide program with no special sprays. In nurseries where the foliage is all immature, Benlate sprays may be required should mildew begin to develop.

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Leaf veins, brown lesions, defoliation - Vein spot (Fungus - Gnomonia nerviseda) - Similar to leaf lesion symptoms of scab fungus, but fungus which causes vein spot, unlike the scab fungus, affects only the leaves. Lesions of vein spot disease develop on the veins or petioles of leaves and are usually less than 1/4 inch in diameter and are characteristically dark brown to black. Leaves which are severely affected drop resulting in premature defoliation. The fungus lives in fall leaves over the winter. Fungicides applied for scab plus orchard sanitation will help reduce losses due to this disease. The disease has not been observed to cause significant economic losses in Texas.

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Weak trees, old foliage - Leaf blotch (Fungus - Mycosphaerella dendroides) occurs mainly in trees of poor vigor. The fungus overwinters in fallen leaves. The disease symptoms first appear on the under surface of mature leaves in early summer as small, olive-green, velvety spots. By mid-summer, black pimple-like dots become especially noticeable in the leaf spots after the surface spore masses have been removed by wind and rain giving the diseased areas of the leaves a black, shiny appearance. When the disease is severe, infected leaflets are killed causing defoliation of the trees in late summer or early fall and resulting in reduced tree vigor and increased susceptibility to invasion by other pathogens. Leaf blotch can be controlled in the early spring by disking under old fallen leaves that harbor the fungus. In areas where a spray program for the control of scab is carried out, leaf blotch usually is not a damaging absence of other pecan diseases, two applications of fungicides will control the disease effectively. The first spray should be applied after pollination when the tips of the nutlets have turned brown, and the second spray application should be made 3 to 4 weeks later.

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Minor leaf disease, white tufts, weak trees - Articularia leaf mold (Fungus - Articularia quercina) occurs after rainy periods and on the leaves of weak trees. On the lower surface of the leaves, the fungus produces a conspicuous growth of white tufts which contain masses of spores. Articularia leaf mold does not occur in trees or in orchards which have been sprayed for disease control. A single application of fungicide when the disease is first detected is usually sufficient to control Articularia leaf mold disease.

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Secondary diseases, behind scab lurks pink fungus - Pink mold (Fungus - Trichotecium roseum) usually occurs on nuts infected with scab fungus. The pink mold fungus apparently enters nuts through scab lesions on shucks and continues to produce masses of pink spores on shuck surfaces until late fall. The fungus sometimes invades the kernel of thin shelled pecan varieties causing "pink rot" which is characterized by an oily appearance of the nut shell and a rancid odor. Pink mold rarely occurs on the shucks of nuts in the absence of scab disease. If scab is controlled, pink mold will not be a problem.

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Mycoplasm, sucker growth - Bunch disease (Mycoplasm) - Evidence indicates it is a mycoplasm [bunchdis]. Trees affected with bunch disease show the bunching symptom caused by excessive growth of slender succulent twigs from lateral buds that normally remain dormant on the main limbs. In moderately affected trees, one or several branches will show the "bunch" growth symptom. Bunching in severely affected trees may involve all main limbs which produce thick masses of sucker-like growth and few, if any, nuts. There is no known effective control for bunch disease. Early detection of the first symptom of bunch and pruning out of the affected branch may prevent spread of the disease throughout the tree. When pruning, make sure cuts are 2 to 3 feet below the infected area. When the tree is severely affected, it should be destroyed to protect nearby healthy trees from infection.

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Black pustules, drought - Fungal twig die back (Fungus - Botrydiplodia sp.) - Infected twigs are covered with small, raised pustules with black centers. This can result in 1 to 4 feet of die back. No control is suggested at this time except sanitation and to carry out normal disease control programs and maintain adequate moisture around trees. Based on preliminary studies, it appears that this problem will be most severe during years of heavy production and low moisture. It has also been associated with young trees at the graft union and on older trees where limbs are pruned out.

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Several fungi, insects, drying - Kernel discoloration (Fungi - several) - There are several fungi which have been associated with discoloration of pecan kernels. Certain insects (stink bugs) can cause kernel discoloration. Delayed harvest can cause this problem. Do not allow pecans to lay on the ground for any length of time. Pecans should be dried before sacking. Forced ventilation in storage is recommended.

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Soil-borne disease, rapid death, fungus - Cotton root rot (Fungus - Phymatotrichum omnivorum) - Soil inhabiting pathogen that attacks a wide range of host plants including the pecan. The roots of the pecan tree are invaded and killed disrupting the transportation of water to the leaves. The fungus girdles the trunk near the soil line. Trees invaded by the cotton root rot fungus produce yellow foliage and become defoliated. Diseased trees die quickly after becoming infected. Losses have been observed 13 years after planting. An effective control for cotton root rot disease has not been developed. New orchards should not be planted in soil having a history of cotton root rot disease. Replanting is not recommended in those sites where trees have been lost to this fungus.

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Slow decline, zinc deficiency, nematodes - Root knot on pecans (Nematodes - Meloidogyne incognita) - Small swellings found on rootlets. Above-ground symptoms are stunted, rosette trees which do not respond to fertilizer and zinc applications. Growers should examine all nursery trees before planting. Chemical control is not recommended at this time. Trees found to be infected with root knot should be removed. Make sure to remove as many roots as possible.

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Bacteria, weakened tree - Crown gall (Bacteria - Agrobacterium tumefaciens) - Infected roots have large, rough galls which may be several inches in diameter. Galls can be formed on any below-ground tissue. Although it causes weakening of the tree when it affects the lateral root system, it does the most damage when the main branch roots and the trunk are affected. Trees which are invaded by the bacteria are more subject to stress factors due to their reduced root system. Invasion occurs through breaks or tears on the roots. Once inside the root, the bacteria can move systemically within the root system. Currently, there is no control for this disease.

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Regional disease pressure - The environment within an area will determine how effective the scab resistance will be within that area. Thus, the weather within an area must be considered when selecting varieties and establishing a spray schedule. Fungicides and fungicide application costs are becoming a major part of the pecan production program. Table 1 shows the critical periods for scab development during the growing season. This represents a ten year average and should be reviewed with the realization that any one year can vary greatly from this average.

Pecan scab monitoring - Regional disease pressure

Month
Location S A J J M A M Scab Severity
Texarkana 0 0 0.5 2.5 5.2 4.6 2.4 Severe
Palestine 0 0 1 3.8 6.8 6.8 3.2 Severe
Dallas 0 0.3 0.7 3.2 5.4 5.2 2 Moderately severe
Victoria 0 0.2 0.9 2.9 4.7 3.5 1.6 Severe
Austin 0.1 0.1 0.8 3.5 7.3 5.8 2.1 Moderately severe
Seguin 0.3 1.3 3.2 8.7 15 10 4 Moderate
Weatherford 0 0.1 1.2 5 7 9 3.5 Moderately light
Brownwood 2 1.3 2.2 6 9.7 9.7 4.3 Light
Uvalde 0.5 2.3 3.3 8.3 14.5 9.7 3.8 Light

Table 1. Comparison of different areas of Texas using days above 90 deg. F: Days receiving measurable rainfall ratio on pecan scab severity


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Honeydew sooty mold - The development of damaging populations of yellow aphids varies with the part of the country we are talking about. Texas east of the Pecos River generally experiences one population peak in late July. The El Paso area will have one in May or June and another in late August. Areas where unnecessary insecticide treatments are made tend to have more aphid problems. Also when carbamates or pyrethroids are used in the early season, one can almost be assured of aphid and mite problems later.

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Variety selection

Resistant varieties should be planted which have a scab resistance level to ensure that over a ten year period they can be grown with little chance of significant crop loss. Contact your county agent or the USDA Pecan Breeding Program for the most recent information.

Limiting fungicide applications
Although the ten year average gives a clue to scab occurrence at a critical time during the spraying season, it is not sufficient to use only this to determine when applications should be applied. To determine when sprays should be applied within a specific year, the number of hours of 90% relative humidity occurring must be recorded and accumulated. Spray applications are then made based on this accumulation. Based on work done in Georgia, Alabama, and Texas, the figure of 100 accumulated hours is used as a threshold level. In humid east Texas, the accumulation of hours should begin at the budbreak application. In central Texas, it may begin at budbreak or may be delayed until the prepollination spray depending upon the weather. Those orchards in western Texas will probably not begin their sprays within that area. The accumulation of hours begins at that time. Due to the infrequent occurrence of 90% relative humidity, far west Texas does not fit the monitoring program, and sprays should be applied based on the occurrence of scab within the area. The rate of disease development is slow enough to take steps to stop the disease after it begins to show up in an area but before significant losses occur.

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For more information, contact:
Bill Ree
Extension Program Specialist
Texas A&M University-Riverside Campus
Bryan, TX 77806-2150
Phone: 979-845-6800
Email: w-ree@tamu.edu

Department of Entomology at Texas A&M University
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Texas A&M University

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