Back to May 2018

If You Want to Have Nuts, Protect Your Leaves

In order to prevent crop loss, growers should be sure to protect against these causes of mid- to late season defoliation.


Leaves on a pecan tree show evidence of scab. They're dotted with black, circular lesions.

These leaves show typical symptoms of pecan scab—black, circular lesions up to a quarter-inch in diameter. (Photo by Charlie Graham)

Well, as I sit down to write tonight, it is early April. Spring is here…right?

During this first week of April, the forecast was in the low 40s in Louisiana, and the weatherman said there would be over a foot of snow in the northern U.S. by the end of the week. During the last week of March, peach growers had lost part of the crop as widespread thunderstorms produced multiple hailstorms across northern Louisiana. It definitely didn’t, and to an extent still doesn’t, feel like spring.

But the pecan trees are leafing out, so it must be spring. I spent most of the first week of April in southern Louisiana visiting with many pecan growers that have been plagued by mid- to late season defoliation of their trees the past few years. This was generally followed by re-foliation of the trees, costing them next season’s pecan crop.

So, what are some of the diseases or pests that can cause such severe leaf loss? Although there are a number of causes for severe leaf loss, we’ll focus on five specific ones that appear year after year.

Well, when you talk about pecan diseases, the first one that comes to mind is scab. Pecan scab is generally considered to be the most prevalent and economically important disease of pecans. It attacks currently growing leaves, stems, and nuts. The typical symptom is black, circular lesions up to a quarter-inch in diameter. Severely infected stems can lead to terminal dieback, and leaves infected at the petiole base will usually defoliate. However, it generally does not cause major defoliation but rather reduces the photosynthetic ability of the infected leaves. So it probably isn’t the culprit causing defoliation in South Louisiana.

Infected by Neofusicoccum spp., these leaves show extensive lesions and have dried out, curling up from the margins. They will eventually fall from the tree.

Infected by Neofusicoccum spp., these leaves show extensive lesions and have dried out, curling up from the margins. They will eventually fall from the tree. (Photo by Charlie Graham)

The next possible cause is Neofusicoccum spp., the new kid on the block when discussing foliar diseases of pecans. We had not really seen it until around 2012, but it has continued to be reported more often since the initial report. I first observed it on ‘Elliott’ pecan trees being grown in an orchard in Pointe Coupee Parish, Louisiana. The symptoms appear as a necrosis of the leaflet, which moves to the rachis, often causing the entire compound leaf to die. The leaflets do not abscise independently but remain attached to the rachis even after falling from the tree. This is generally a problem in orchards receiving minimal fungicide applications during the season and can result in extreme defoliation of the tree.

Another pecan disease, Zonate leaf spot causes severe defoliation of pecan trees during July and August in rainy summers. The leaf spots’ distinguishing characteristic on pecan leaves is the concentric ring formations that are more distinct on the lower side of the leaf. Leaf spots on the upper surface are grayish brown, while the lower surface lesions are light brown in the center, becoming darker brown toward the edge. Leaves with extensive lesions will dry out, curl up from the margins and eventually fall from the tree. Zonate leaf spot is a problem in areas with high soil moisture, high relative humidity and poor air movement. It can be easily controlled with a good fungicide program.

As for pests, the infamous black pecan aphid can cause severe defoliation in late summer. Immatures and adults feed on all surfaces of the leaflet, resulting in bright yellow blotches from a quarter to a half inch in diameter. These blotches form between the veins of the leaflet. Growers should scout the lower, shaded limbs beginning in mid-summer for evidence of black pecan aphid. The population threshold for treatment is only one per compound leaf. Ignoring this pest can result in major defoliation of the pecan tree. Luckily, there is a wide range of chemicals that can effectively control black aphid populations.

Black pecan aphids have infested this pecan tree. Formed between the veins of the leaflet, these blotches serve as evidence of this pest.

Black pecan aphids have infested this pecan tree. Formed between the veins of the leaflet, these blotches serve as evidence of this pest. (Photo by Charlie Graham)

Pecan leaf scorch mite is another damaging pest which must be monitored. Mite populations can develop by June but generally occur later in the season. It takes less than two weeks for scorch mites to complete a generation during optimal conditions, so populations can explode quickly. Growers should be scouting for mites weekly, beginning in late June through early fall.

Scorch mites congregate on the underside of the leaflet, usually along the mid-vein. Early symptoms include bronzing of the leaf tissue along the midrib. When populations reach eight mites per compound leaf, a suitable miticide should be sprayed on the foliage.

The small whitish dots on the underside of this leaflet and along the mid-vein are scorch mites.

The small whitish dots on the underside of this leaflet and along the mid-vein are scorch mites. (Photo by Mike Hall, Ph.D.)

I will repeat, a miticide, not an insecticide, should be applied to control the mites. In fact, some insecticides, such as carbaryl and pyrethroids, can result in a surge in the current population and should be avoided during the late season if possible.

Lastly, the walnut caterpillar overwinters as pupae in the soil with adults emerging in the spring. If you remain unaware of its appearance, it can mean sudden and devastating defoliation. The first four instars will only consume 10 to 15 percent of the foliage on a pecan tree with the fifth instar consuming the remaining foliage. This instar can consume 85 percent of the foliage in less than three and a half days.

A group of walnut caterpillars in the fifth instar congregate on the foliar terminals of a pecan tree. In this phase, these caterpillars will quickly defoliate this tree. (Photo by Charlie Graham)

A group of walnut caterpillars in the fifth instar congregate on the foliar terminals of a pecan tree. In this phase, these caterpillars will quickly defoliate this tree. (Photo by Charlie Graham)

Why is this important?

Because by scouting and treating the early instars, you can prevent major defoliation from occurring in the trees. Walnut caterpillars do not form webs but congregate on the trunk, scaffold limbs, and foliar terminals of pecan trees. While scouting trees, look for suddenly defoliated twigs and limbs. Worm masses should be spot sprayed with an insecticide. Bts (Bacillus thuringiensis) and Confirm (tubefenozide) are commonly used on walnut caterpillars. Other tree hosts in the woods near pecan orchards include hickory species and black walnut.

 

Past research has proven that early defoliation suppresses return bloom the following year. Early defoliation also reduces the available nonstructural carbohydrates, especially if the tree re-foliates in the fall. While defoliation is detrimental to crop production, it can usually be prevented or controlled by consistently scouting the trees for insects or mites, and by utilizing timely applications of pesticides in the orchard.

While some may think it is a little early to be thinking about this, spring is here and it can sometimes be difficult to locate the proper pesticide during the summer. Recognizing the pest and properly treating it may be the difference between having a crop to take care of or just going fishing.

Author Photo

Charlie Graham

Charles J. Graham is the Senior Pecan Specialist at the Noble Research Institute. Noble Research Institute, 2510 Sam Noble Parkway, Ardmore, OK 73401; E-MAIL: [email protected]