Scab Management Keys: Sprayer coverage, orchard environment
As we descend into another spraying season, keep the following in mind. If you have problems with scab control, the least likely factor will be the efficacy of the fungicide you are using. Unfortunately, this is often the first thing most growers try to pin the blame on. Aside from the obvious issues of rainfall and pecan cultivar, the biggest differences I see in scab control result from sprayer coverage and the orchard environment itself.
Coverage is a well-traveled topic and most growers know what is required to obtain good coverage. You must drive slow (1.5 mph is preferable but certainly no faster than 2.0 mph). Regardless of how slow you drive, it has been proven time and time again that no air blast sprayer will get complete fungicide coverage up any higher than about 40 feet into the canopy. It may look like you’re getting spray to the top of a 65-foot pecan tree and, yes, you can see the plume of spray material go that high, but it is not covering and protecting the nuts sufficiently.
Many growers use a two-sided air blast sprayer to spray pecan trees. Spraying out of 2 sides has its advantages in allowing you to get over the orchard much quicker, but this comes at a cost. On a small tree of 30-40 feet, it won’t matter all that much. You get adequate coverage with 2-sided spraying up to this height.
By adding a volute to the sprayer, allowing you to direct all the spray to one side, you obtain better coverage over the whole tree, especially for old, mature trees. Dr. Clive Bock with USDA demonstrated this very well at the Georgia Pecan Growers Association Field Day last year by applying Surround, a kaolin clay-based product used for preventing sunscald on fruit and vegetable crops, to pecan trees with an air blast sprayer. Surround coats the surface of the spray target with a fine, chalky, white substance, which becomes highly visible. This allows you to see just how good your sprayer coverage is.
I know of many growers who choose to spray every other middle with a 2-sided sprayer each week. I’ve long been skeptical of this practice. The images of the trees at last year’s field day pretty well show that when you spray in this manner, you are covering about 1/3 of the tree with fungicide every other week while getting almost none to the other side of the tree. Therefore, in a wet year, scab can be expected to be a problem with this scenario of poor coverage.
If a grower is spraying large trees out of both sides of the sprayer, they are obviously not doing an adequate job. In order to get better spray coverage, there would basically be two options: 1) add a volute to the sprayer, and spray up one side of the tree and back down the other side on every spray. If you don’t have enough sprayers to get over the acreage in sufficient time, buy an additional sprayer. Or 2) hedge the trees to keep them at about a 40-feet height.
One common factor I see regarding production problems in pecan is a consideration of the site where the orchard was planted. Not long ago, I spoke with a grower who had been on a very good fungicide program. His scab control was excellent in most orchards but in a few orchards scab was a problem. As it turns out, most of those orchards in which scab was becoming a problem were smaller orchards of 30 acres or less and surrounded by woods. The surrounding woods hold moisture in the area and block wind, limiting airflow in the orchard. Sounds too simple to be the source of the problem, but I’ve seen it many times.
I consider the most often overlooked explanation for differences in scab control to be elevation. Scab-susceptible cultivars planted in low-lying areas often have serious problems with scab. This can be readily observed by comparing scab severity on trees on low and high ground within the same orchards. Invariably those trees at the bottom of a slope or in a low-lying hole will have more scab problems than those on the higher ground.
A scab-susceptible cultivar planted in a low-lying area surrounded by woods is a recipe for disease problems no matter what fungicide program you are using. Certain conditions may cause the climate in one zone within an area to differ from the surrounding areas. I believe this is one reason we see such a wide spectrum of disease control from one area to another with basically the same fungicides being used. The scientific term for this is “microclimate” and it varies greatly when comparing, for example, Albany, Georgia with Ft. Valley, or Waycross with Americus.
We often talk about the differences in the amount of rainfall in these locations. But, middle Georgia is on much higher ground than that of Southwest or Southeast Georgia. As a general rule, I would say that sites with an elevation of 300 feet or more will have an easier time producing pecans. I never realized how much difference there was in elevation from one location to another in South and Central Georgia.
Albany, GA (elev. 200 feet), Camilla, GA (elev. 177 feet), and Leesburg, GA (elev. 259 feet) all sit inside a bowl and are surrounded by counties with average elevations of around 300 feet or more. As you go northeast from this region, elevation increases to around 500 feet in Ft. Valley. Elevation is around 220 feet in Valdosta, GA and 112 feet in Blackshear, GA near Waycross, and goes back up to 280 feet as you move north into the Vidalia onion country, continuing to rise as you go north.
The dividing line in Georgia where scab management seems to get easier runs along U.S. Highway 280. Most locations above this line are 300 feet or more. Even with good elevation, an orchard surrounded by woods will tend to have more scab issues than one out in the open. I met with a grower a few years ago in middle Georgia who was having problems with scab control in an orchard growing beside a large block of hardwoods that sloped down to a creek. Air flow was an obvious problem. The decision was made to remove a portion of the hardwoods, opening up more room for sunlight and air movement in the pecan orchard. As a result, scab control improved dramatically. There was no need to undergo the time consuming and expensive task of changing over cultivars in the orchard or changing up the fungicide spray program. Simply taking the time to look around at the surroundings with an open mind provided the solution.
Production issues affected by elevation are not solely limited to problems with disease. Many of the quality problems we saw last year arose from heavy insect pressure in August, heavy cloud cover (limited sunlight) in September, and warm, wet harvest conditions. These same conditions occurred everywhere pecans are grown in the state. But, I routinely hear more complaints about quality from those areas with lower elevation or poor air flow, particularly on older trees. Such trees are more stressed in general. Add to this the factors mentioned above (insects, sunlight, etc.) and the resulting production issues (poor quality) become magnified.
That’s not to say pecans can’t be grown in areas with elevation below 300 feet. Growers in these areas just need to plant disease-resistant cultivars and be more aggressive in managing sunlight and air flow in these locations.