Shuck Dieback Of Pecan Trees: Learn About Pecan Shuck Decline Disease

Shuck Dieback Of Pecan Trees: Learn About Pecan Shuck Decline Disease

By: Mary Ellen Ellis

Pecans are prized in the South, and if you have one of these trees in your yard, you likely enjoy the shade of this regal giant. You may also enjoy harvesting and eating the nuts, but if your trees are hit with pecan shuck decline and dieback, a mysterious disease, you could lose your harvest.

Signs of Pecan Shuck Decline Disease

If your pecan tree has shuck decline or dieback you’ll see the impact on the shucks of the nuts. They start to turn black at the end and, eventually, the entire shucks may blacken. The shucks will open as normal, but early and there will either be no nuts inside or the nuts will be of lower quality. Sometimes, the entire fruit falls off the tree, but in some cases they remain on the branch.

You may see white fungus on the outside of the effected shucks, but this is not the cause of the decline. It is only a secondary infection, a fungus taking advantage of the weakened tree and its fruits. The ‘Success’ cultivar of pecan trees, and its hybrids, are the most susceptible to this disease.

What Causes Shuck Decline?

Shuck dieback of pecan trees is a mysterious disease because the cause has not actually been found. Unfortunately, there is also no effective treatment or cultural practices that can manage or prevent the disease.

There is some evidence that pecan shuck decline disease is caused by hormones or some other physiological factors. It does seem to be that trees that are stressed are more likely to show signs of the shuck decline.

While there are no treatments or accepted cultural practices for managing this disease, anything you can do to keep your pecan trees happy and healthy may help prevent shuck decline. Make sure your trees get enough water but are not in standing water, that the soil is rich enough or that you fertilize them, if necessary, and that you prune the tree to maintain good airflow and to avoid an overload of nuts.

This article was last updated on


Pecan Shuck Decline And Dieback – What Causes Shuck Decline Of Pecan Trees - garden

I have two Western Schley pecan trees planted seven years ago. Last year both produced very well. This year one suddenly dropped all of its pecans, but the other didn't. They are only 20-feet apart and are managed the same. What happened and how do I prevent it next year?

Last year our 20 year old pecan tree had pecans with black shucks that stuck tight. When we removed the shuck,the pecan meats were also dark and shriveled. This year the tree doesn't have a dozen nuts on it. Before last year the nuts were large and very good. We put pecan food on it every year and water often. Please advise us.

Answer:

I spoke with Dr. Esteban Herrera, NMSU Extension Horticulturist working with grapes, tree fruit crops, and pecans. He feels that the answer to the first question involves the hot dry weather this year and that irrigation is the ultimate cause of the problems described.

He stated that the maximum water needs for pecans occur during August in Las Cruces, at a physiological point of pecan nut development called "water stage." Pecan shells harden in late August. Water deficiency in August, before the shell hardens, causes the trees to drop of most of the crop as described in the first question. This drop follows a good rain or irrigation following the dry period, especially in sandy soil. Since it follows a rain, drought often doesn't seem to be the cause.

Insufficient water late in the season will make the trees weak and the shucks will not open, producing "stick tights." An early freeze will do the same, but the pecans will be well filled. However, there are years when pecans are affected by other factors at the time the pecan shell (not the shucks) are hardening in late August. Wind or insect damage before shell hardening will make the nut drop, but if it happens at the end of shell hardening, the pecan will not drop, but it will not fill, producing the empty nuts called "pops." Pops are stick tights with no meat inside.

He also thinks that a characteristic of pecans, alternate bearing, is also involved. Some pecan varieties, like many other fruit crops tend to produce heavy crops one year followed the next year by light crops. If there is a problem with orchard management(especially water and fertilizer), he said this cycle can go wild. According to Dr. Herrera, established pecan trees need 150 to 200 gallons of water on the hottest day of the summer. In very dry weather, especially in a lawn setting in which there are not many trees around to break the wind, it is difficult to provide sufficient water to the trees. He also observed that few pecan trees in a lawn are provided that much water in the first place. Remember that in a lawn, the trees compete with other plantings and the grass for the water provided, so all that is provided does not go to the pecan tree. You did not describe the landscape in which these trees were growing, but consider the amounts of water that Dr. Herrera said the pecans need.

So, why did two trees only 20-feet apart have different responses? There are several factors to consider. Soil conditions can change over that distance, so one had a soil which held water better than the other, contributing to the problem, or one was a variety which tolerated the conditions better. Pecans are grafted onto seedling rootstocks, and each is different from the others. This can influence a better root system for one tree, allowing a better start when the tree was planted.

Many other factors also affect the development of pecan crops. Insect problems, disease, mineral deficiencies, especially micro-nutrient deficiencies, can affect the crop in pecans. There are many good NMSU Extension publications written by Dr. Herrera available to you at your County Extension Office and on the World Wide Web at http://www.cahe.nmsu.edu/cahe. You might wish to get some of the publications to read for more information.

We seek to improve the lives of New Mexicans, the nation, and the world through research, teaching, and extension.


UGA Pecan Extension

For the last few years, I have had many calls related to the leaves of young pecan trees scorching and often dropping from the tree in mid-summer. We’ve investigated this from a number of angles and while a host of factors can contribute to the problem (nutritional imbalance, cold damage, etc.), it appears to me that what we are seeing is primarily an environmentally related problem affecting tree establishment. Many times when the leaves of a tree scorch in mid-summer, its an indication that there is a problem with the root system or vascular system of the plant.

When trees are transplanted in to the orchard from the nursery they will inevitably go through a period of “transplant shock”. If everything is done correctly and growing conditions are suitable, the trees recover from this within the first month or two following budbreak as the roots grow and the tree gets its root foundation established. At this point, the tree begins developing vigorous shoot growth if all is well.

I’ve talked and written a lot about the need for irrigation on young pecan trees. But, there is a love/hate relationship with pecan trees and water. Soil moisture is the key to making young pecan trees grow but this must be in combination with well-drained soil. Pecan trees like water but they don’t like to be water-logged. This is why they do not survive well when planted in low-lying, poorly drained soils. The tree’s roots can’t receive enough oxygen and they essentially drown, causing the roots to die-back. Even in a well drained soil, after periods of heavy rainfall when the soil may stay wet for an extended period, you can get a little root die-back. As the soil dries out, new roots grow and replace those that were lost. This is a normal process and is usually not a big deal for a tree that is already established. The root system is large enough to compensate for the losses.

But, for a tree still trying to get its roots established, this situation can cause problems because the root system is already very limited. In my research, I do a lot of measurement of tree water stress. First year trees are usually much more water-stressed than more established trees, even under the best management as a result of their limited root system.

The nutritional status of the trees coming out of the nurseries is usually excellent and this is what most trees’ growth feeds off of for their first year following transplant (even though many growers apply a lot of fertilizer in the first year). As a result, these young trees often attempt to push out with vigorous shoots. The harder a young tree is pushed for growth with nitrogen, the greater potential for problems in this situation. When the limited root system cannot keep up with the growth that tree is trying to make, issues can arise.

For the last three years, we have had some pretty wet weather in the spring, as the graph below indicates. 2012 was an exceptionally dry spring with about a 2″ deficit in rainfall for April. However, from 2013-2015, we have had April rainfall amounts from 1″ to over 5″ above the average. In addition, 2013 remained wet season-long.

Following April, the weather conditions got hot and dry in 2014 and 2015. When the limited root system of young trees gets water-logged and many of the remaining roots suffer or die from lack of oxygen, the tree must re-grow these roots to support the growth of foliage. As the weather turns hot and dry immediately following such a period of heavy rainfall, the tree does not have enough time to develop a new root system adequate to meet the increased water demand under these conditions. When this occurs, the tree responds by shutting down and sacrificing some of its foliage in order for the tree to have the resources it needs to survive. This leads to the scorching and in some cases, dropping of leaves.

While nutritional problems like N:P or N:K imbalance can also lead to scorching, most of the time when we have compared leaf tissue analysis of these scorched trees with healthy trees from the same orchard, there is no difference. I have compared water stress on scorched vs. healthy trees as well, and in most cases the scorched trees are extremely water stressed compared to healthy trees, even though they are under adequate irrigation. In such a case, the root system is simply not capable of transporting that water to the tree as it needs to because the root system is still too limited.

Cold injury that damages the tree’s vascular system causes a similar response (and we have certainly had some of that as well over the last few years). But, if cold damage is not apparent, it is likely, the problem is simply that the tree has not yet established enough of a root system to support its growth. The problem may be more likely to show up on sandy or heavy clay soils or those with poor fertility, such as that found on sites planted in pine trees immediately prior to pecan planting.

The good news is that these trees will usually grow out of this scorching problem as more suitable growing conditions arise. It may take them 4 or 5 years to reach that point but they do seem to grow out of it with no long term damage.


Pest & Disease Control for Pecan Trees

Every tree has the future potential for disease and insect damage. Factors such as location and weather will play a part in which issues your tree encounters. If available, disease-resistant trees are the best option for easy care and for all trees, proper maintenance (such as watering, fertilizing, pruning, spraying, weeding, and fall cleanup) can help keep most insects and diseases at bay.

NOTE: This is part 7 in a series of 11 articles. For a complete background on how to grow pecan trees , we recommend starting from the beginning.

Crown Gall

Trees appear stunted and slow growing leaves may be reduced in size, little or no fruit. If tree is dead, inspect roots for hard, woody ‘tumors’. Note: many things can cause stunted trees.

  • Consult County Extension Agent

Appear as small circular, olive-green spots that turn black on new leaves, leaf petioles and nut shuck tissue. Lesions expand and may coalesce, then fall out giving a shot hole appearance. Early infections may cause premature nut drop, but more commonly cause shuck to stick to nut surface (stick tights). Late infections can prevent nuts from fully expanding and decrease nut size.

  • Bonide® Citrus, Fruit & Nut Orchard Spray

Aphids

They are the size of a pinhead and vary in color depending on the species. Cluster on stems and under leaves, sucking plant juices. Leaves then curl, thicken, yellow and die. Produce large amounts of a liquid waste called “honeydew”. Aphid sticky residue becomes a growth media for sooty mold.

  • Bonide® Insecticidal Soap
  • Bonide® Citrus, Fruit & Nut Orchard Spray

Pecan Nut Casebearer

Eggs are minute and change from white to pink. Larvae change from olive-gray to gray-brown and measure 1/2”, reddish brown head and sparsely covered with fine, white hairs. Adult moths are slate-gray with ridge of long, dark scales on laser end of forewings. They are 1/3” long with wingspan of almost an inch.
Larvae leave cocoon (located at junction of bud and stem) in early spring about time buds open, feed for about 2 weeks on exterior of opening buds. Then bore into tender shoots where they mature. Late May to early June, about time for pollination to occur, adults emerge and lay eggs on young nuts. 8-9 days later eggs hatch and larvae bore into nuts at stem end. Infested nuts are held together by frass and webbing and larvae feed inside nut for 3-4 weeks, pupates and 2nd generation of adults emerge in mid-July (in Missouri) and the cycle is repeated. A third generation of adults emerges in late August and September and larvae feed in nut shuck and on the leaves.
First generation is most damaging. Treat when all catkins have fallen and tips of nuts turn brown (after pollination), early June in Missouri. Timing is important and varies from year to year and from area to area.

  • Consult County Extension Agent

Brown Leaf Spot

This is common in southern areas with high rainfall and neglected orchards. Reddish-brown spots often with gray rings. Can cause early leaf drop in fall, weakening tree.

  • Consult County Extension Agent

Powdery Mildew

Appears as whitish-gray powdery mold or felt-like patches on leaves and nuts. Leaves may fall off early and on nuts, shucks split and kernels shrivel.

  • Bonide® Citrus, Fruit & Nut Orchard Spray

Borers

Appears as a thick, gummy substance (SAP) leaking from round holes on the trunk or in crotch of the tree. Worms with brown heads and cream-colored bodies tunnel through trunk that will kill the tree. Once infested, use a fine wire to try to mash them or dig them out.

  • Dig out with thin wire or mash

  • Bonide® Borer-Miner Killer

Sticky Shuck

During nut development when water begins to fill the nut. Part of the shuck turns black nuts will not be completely filled.

  • Consult County Extension Agent

Fall Webworm

Damage the leaves by both feeding and web building. Webworms over-winter within cocoons located in protected places, such as crevices in bark or under debris and fences. Adult moths emerge in summer. They have a wingspan of about 1 1/4" and vary from pure satiny white to white thickly spotted with small dark brown dots. Females lay white masses of 400-500 eggs on the undersides of the leaves. The caterpillars hatch in 10 days and all from the same egg mass live together as a colony. They spin webs that enclose the leaves, usually at the end of a branch, to feed upon them. After they have defoliated a branch, they extend their nest to include additional foliage.
When caterpillars are mature, they leave the nest to seek a place to spin gray cocoons. The mature caterpillars are about 1 1/4" long with a broad dark brown stripe along the back, and yellowish sides thickly peppered with small blackish dots. Each segment is crossed by a row of tubercles with long light brown hairs.

  • Bonide® Captain Jack’s™ Deadbug Brew
  • Bonide® Thuricide Bacillus Thuringiensis (BT)
  • Remove web with rake and burn or prune out

Pecan Weevil

Larvae are creamy-white grubs, C-shaped with reddish-brown heads and 1/2” long. Adult moths are light brown to gray and are about 1/2” long. The adult emerges as early as July 15 (Missouri), feed on nuts before they are completely formed, causing them to shrivel, the nut blacken and drop. Nuts may show a tiny, dark puncture wound extending through the shuck and unhardened shell. If larvae is found inside the nut before the shell hardens, indicates damage from other insect, usually nut curculio or hickory shuck worm.
The adult lays 2-4 eggs in separate pockets within each kernel. Grubs hatch in late August and feed for about a month then exit thru a hole about 1/8” beginning in late September. Pecan weevils remain in larval stage for 1-2 years 4-12” underground. They pupate in early autumn and become adults in about 3 weeks. The adults remain in the soil until the following summer. Complete life cycle is 2-3 years. Do not move very far from the tree under which they emerge, so certain trees may be infested while trees nearby are not bothered. Can be controlled with insecticide, but ours are not recommended.

  • Consult County Extension Agent

Pecan Phylloxera

Appears as a small aphid-like insect that is seldom seen, but produces galls that are easily visible. Severe infestations cause malformed, weakened shoots that finally die and can even kill entire limbs. The insect over winters as eggs in the dead body of female adult in protected places on the branches of pecan trees. After bud break the eggs hatch and the insects feed on opening buds or leaf tissue. These are known as ‘stem mothers’. Their feeding stimulates the development of galls, which enclose the insect in a few days. The stem mother matures inside the gall and lays eggs, which emerge in mid-summer as adults and continue the cycle. Only need to treat when galls are in large numbers on shoots or nuts.

  • Consult County Extension Agent

Nut Curculio

Adults are dark-gray to reddish-brown, 3/16” long, larvae are legless, creamy-white, 3/16” long and found within immature pecans. The adults attack immature pecans from mid-July to mid-August. Make punctures in the shucks where they deposit an egg. Eggs hatch in 4-5 days and the larvae feed for 10-14 days. This causes a bleeding of brown sap on the shuck and also premature nut drop. Larvae exit from a small hole and enter the soil. Adult emerges 4 weeks later, in September and October and over winters in ground trash.

  • Consult County Extension Agent

Hickory Shuckworm

Adult moths are dark-gray nocturnal flyers, 3/8” long. Larvae are creamy-white with brownish heads, 3/8” long. Pupae, found within the shuck, are dark brown and up to 1/3” long. When larvae feed in the interior of the nut, mid-July until shell hardening in mid-August, premature nut drop can occur. Larvae pupate in the nuts and third generation moths emerge in early August. Damage from Hickory Shuck worms can be eliminated if insecticide sprays can control these moths.

  • Bonide® Captain Jack’s™ Deadbug Brew

Navel Orangeworm

Caterpillar is ¾ inch long, reddish orange to yellow. Adult moths have irregular, silver gray and black forewings and legs, snout like at front of the head. Eggs are white at first and later orange before hatching. Larvae are reddish orange then vary from milky white to pink. Pupae are light to dark brown. Larvae bore into nutmeat and later consume most of the nut. Producing large amounts of webbing and a fine powdery residue. They will over winter in mummy nuts in tree or on the ground.

  • Bonide® Captain Jack’s™ Deadbug Brew

  • Bonide® Borer-Miner Killer

Peach Twig Borers

Adults of this insect are clearwing moths, metallic blue to black in color with bright bands of orange or yellow. They are about 13 mm long with wings folded and their forewings have a black apical band. Larvae are about 18 mm long, white with brown heads.
Symptoms:

  • Build up of reddish brown frass and gummy exudates known as gummosis. Check branch crotches on larger branches or upper trunk.
  • Heavy infestation may cause branch dieback.
  • Young tree maybe girdled and killed older trees may be weakened.

  • Bonide® Captain Jack’s™ Deadbug Brew

  • Bonide® Borer-Miner Killer


Watch the video: How to grow a Pecan Tree