The endless quest: to match tree traits to today’s ever-changing economics. Industry knows that trees must be highly productive and disease resistant in order for growers to compete in the market.
The ongoing challenge of pecan scab for Southeast growers led to a resistance study for three pecan cultivars — Lakota, McMillan, and Excel — conducted by the University of Georgia (UGA). These trees were evaluated in the humid region of Georgia’s Coastal Plain and weren’t sprayed with fungicide.
Why these varieties?
“We wanted to look at numerous scab-resistant varieties, and we knew from observation that these three had high levels of scab resistance,” said Lenny Wells, professor of horticulture and Extension horticulture specialist with the University of Georgia. “These three varieties offered the best scab resistance we thought we could get some meaningful data from.”
The roughly half-acre trial block had multiple trees of each variety, and studies were conducted from 2018-2022. Disease ratings were conducted each year in mid-September, after pecan shell hardening.
The entire block had no fungicide sprays whatsoever.
Skipping fungicide sprays
No pecan scab was seen on any of the trees throughout the study, making all three varieties suitable for commercial production.
Should growers put in these varieties and not worry about fungicide? Not so fast,
said Wells.
While scab wasn’t observed, powdery mildew was found on all three cultivars, with McMillan demonstrating the most susceptibility to powdery mildew.
“We don’t recommend not spraying at all,” Wells said. “There are a lot of other secondary pathogens that can jump on the nuts — powdery mildew, zonate leaf spot, anthracnose, and others.”
But what is normally anywhere from 7-12 fungicide sprays in a season can be brought down to 2-3 sprays to take care of any secondary problems.
“It can also help with any chance for susceptibility development that may occur within the varieties — help keep that off a little further down the road,” Wells said.
Susceptibility development
It’s possible even these resistant varieties could lose that resistance, and this occurs relatively frequently among pecan varieties.
Desirable, the gold standard, didn’t scab when it first rose to pecan superstardom, and now it’s one of the worst varieties in terms of scab susceptibility.
“It can happen,” said Wells. “But then you have something like Elliot. It’s been around for over 100 years, and it’s still, for the most part, pretty well scab free.” In places where light scab may be seen on Elliot, it can be easily managed.
Scab does occasionally appear on McMillan and Excel, particularly in wet years and in orchards with poor air flow. To date, Wells hasn’t seen any scab on Lakota.
But where these varieties measure up in terms of pecan scab, they may fall
short elsewhere.
McMillan, for example, lacks the kernel attractiveness and quality that Lakota has, with a kernel percent in the low 50s, and is not as precocious. But it is a little more consistent.
For Excel, the biggest issue is the hard shell, and the thickness of the shell can make shelling difficult. It also has a 48-52% kernel.
Lakota has a high percent kernel at around 60%, has good scab resistance, and good yields, but it’s a dramatic alternate bearer. It loads very heavily, and growers must manage that crop. Lakota was mechanically fruit thinned during the study, which increased the following year’s production by 82% over the non-thinned trees.
Additionally, Lakota kernels can darken quicker. While this isn’t a sign that anything is wrong with the kernel itself, the attractiveness of the kernel doesn’t last as long as with some other varieties.
Wells suspects that Lakota will hold up similarly to Elliot.
“There’s some indication that there are varieties that have good, strong genetic resistance,” Wells said. “And that gives me some confidence.”
Main photo: Mature Lakota cluster.
Photos: Lenny Wells, University of Georgia
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