UGA pecan specialist Lenny Wells recently updated us on the effect of pecan orchard air flow and elevation on scab control. See comments below.
“One of the most often over-looked factors 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.
Aside from air flow, elevation is an important factor to consider as well. Most growers know this but too often fail to accept it as a serious issue in their own orchards. 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′ 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. This map shows some of the variation in elevation across the Georgia pecan belt.
But, 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 are seeing have arisen 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 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.
Invariably I see more production problems each year in these same areas with low elevation or limited air flow due to the surroundings of the orchard. These situations lead to more stress on the tree which opens up the opportunity for a multitude of problems to arise, including the quality issues we see this year.
That’s not to say pecans can’t be grown in areas with elevation below 300′. Growers in these areas just need to plant disease resistant cultivars and be more aggressive in managing sunlight and air flow in these locations.”