Back to June 2018

Scab Battle Heats Up, Watch for These 3 Factors

Lenny Wells, Ph.D., discusses three factors to watch while managing pecan scab in your orchard.

Black splotches on pecan nutlets show evidence of pecan scab.

Pecan scab has infected these nutlets as you can see by the black splotches.

Planting resistant cultivars, planting on elevated land, and improving the orchard’s airflow are three key factors in managing pecan scab. However, for existing orchards, it is too late at this point in the season to do much about these variables. So, we are left with the most direct line of defense—fungicide application.

Regarding this route of protection, there are several nuances to keep in mind that will give you the best chance of winning the battle against scab. These include fungicide coverage, spray interval, and fungicide selection.

More often than not, when growers have been on a good fungicide schedule and still have a problem with scab, they immediately point the blame at the fungicide being sprayed. When I have been out to visit orchards in this situation, the problem has usually been shown to be more related to coverage than the fungicide used.

It has been demonstrated many times over that no matter what air blast sprayer you use, you will not get adequate coverage above a height of 40 feet without a volute on the sprayer. If you can see scab in the top of the tree while the bottom remains clean, you have a coverage problem.

Some growers in Georgia spray every other middle every other week (spraying out of both sides of the sprayer). I am not a fan of this method because it seems to me that on large, mature trees you are spraying half of the tree with half to two-thirds of the coverage every two weeks. The argument goes that you are still getting some fungicide to the opposite side of the tree so that you are putting something out there every week. However, the coverage you get to that opposite side is even less. So not only are you getting very poor coverage on the opposite side (less than half) but you are also exposing the trees to an inadequate amount of fungicide more often, which can be a recipe for resistance development.

There is really no question that the coverage you get spraying out of one side of the sprayer with a volute and spraying up one side of the row and down the other on every spray gives you the best coverage possible. A lot of growers spray out of both sides of the sprayer to get across all of their acreage at the appropriate time intervals. This may work in relatively dry years when scab pressure is minimal and for some cultivars with some degree of scab resistance, but if you are growing ‘Desirable’ or ‘Pawnee’ in the humid southeastern U.S., you will not get the best possible coverage (or control in wet years) unless you spray both sides of the row every time—spraying out of one side of the sprayer. For many growers this will require additional sprayers to get across their acreage, but that is the price you pay for growing these varieties in this climate.

In an effort to meet this need, growers often ask about the benefits of airplane sprays for large, old mature trees. Since we know fungicide coverage is poor at the tops of these trees, airplane sprays can be very useful in this situation.

If you spray with an airplane for your first spray before full canopy development, you get decent coverage. However, if you spray a tree with a fully-leafed out canopy using an airplane, you will only cover the top of the tree and barely penetrate the lower canopy.

I recommend using airplane sprays to supplement ground sprays in large, old mature trees at critical, heavy pressure times during nut scab.

Another coverage factor growers can control is tree height. Hedging creates more scab pressure on susceptible cultivars by stimulating more young, dense, vigorous growth. This growth creates an ideal microclimate for scab and provides plenty of young tissue for scab to infest, but at the same time, keeping the tree shorter and more compact allows you to get better fungicide coverage. So, if you are on a good scab program, hedging makes scab control much easier. Ultimately, the benefits of better coverage outweigh the cost of increased pressure. For this reason, I feel that hedging is the best way to manage susceptible cultivars like ‘Desirable’ and ‘Pawnee.’

Another factor, spray interval can greatly affect one’s chances in defeating scab. Once June arrives, we have now entered the most critical period for scab control. And we must stay on a tight spray interval.

Scab is driven primarily by weather. The more often it rains, the longer the shuck tissue stays wet, the more scab potential you have. If it turns dry, you can stretch out the interval on less susceptible cultivars like ‘Creek,’ ‘Sumner,’ or ‘Cape Fear,’ but on ‘Desirable’ and ‘Pawnee,’ or any highly susceptible cultivar do not go longer than two weeks between sprays from June 1 to mid-August. In the Southeast, there is often enough humidity at night or in the mornings and evenings to generate scab problems even during dry stretches. With more rainfall, you may need to tighten up to as much as seven to ten days.

Fungicide selection is also important, but it is probably the least important aspect of the three factors I have mentioned. That being said, there are differences in fungicide materials. Some are better suited for leaf scab, while others are better suited for nut scab.

Since we are in the nut scab period, I will focus on these more intensively. From June through mid-August you need Elast and one of the DMI/strobilurin mixes in your spray program on susceptible varieties. Without question, these are the best nut scab materials. The DMI/Strobi mixes include Absolute, Quadris Top (Amistar Top), Quilt, and Custodia. Tin should be tank-mixed with Elast unless you have tin resistance, in which case you should go with the full rate (48 ounces per acre) of Elast.

Once June arrives, I like to use two back-to-back sprays with Elast/tin (or the full rate of Elast) and then rotate to a DMI/Strobi mix. Most of the time Absolute is my go-to material in that rotation. If scab is present, especially with frequent rainfall, the extra cost of Quadris Top (Amistar Top) is justified. Save this for your most critical sprays with the most pressure.

Rotation of fungicide chemistries is critical to managing scab resistance development. We have a good variety of fungicide chemistries to utilize in managing scab. By using products like the strobilurins (Sovran, Abound, Headline etc.) and phosphites early in the season for leaf scab, you preserve your best nut scab materials (Elast/tin/DMI-Strobi mixes) for their best use when they are needed most.

University of Georgia plant pathologists recently changed their recommendation on the strobilurin fungicides such that strobilurins alone can be used for up to one-third of your sprays and DMI/Strobi mixes can be used for up to half of your total number of sprays as opposed to three. In addition, our pathologists have obtained good results with phosphites used alone at the 2-quart rate for leaf scab. These recommendations make it much easier to rotate fungicide chemistries appropriately. Don’t make more than two sprays back to back before rotating to a different chemistry.

We often get questions about the need for surfactants. Under most conditions they are not really necessary. However, with frequent rainfall, surfactants can make a difference with some fungicides in your scab control. Use them in high pressure situations when using the DMI/Strobi mixes or with straight DMI or strobilurin fungicides, but do not include them with Elast due to the potential for burn.

We are in the heat of the scab battle right now and these three factors may guide you in preparing your best defense. I hope these pointers can help you better manage this disease.

Author Photo

Lenny Wells

Lenny Wells is an Extension Pecan Specialist, University of Georgia, Tifton, Georgia.