Materials and methods

Studies were conducted from 2018 to 2022 near Tifton, GA, USA. The orchard was located at lat. 31°51′N and long. 83°64′W. Orchard soils consisted of Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults). Trees were planted in 2008 and are spaced at 40 × 40 ft throughout the orchard. The orchard was managed under commercial conditions in accordance with the University of Georgia Cooperative Extension recommendations (Wells 2017). Vegetation-free strips 12 ft wide were maintained along each tree row using the herbicides glyphosate and indaziflam. Row middles consisted of Bermudagrass (Cynodon dactylon) sod. All trees were irrigated with microsprinklers at a rate of 15 gal/h. Microsprinklers were placed 2.5 ft from the tree trunk.

Four trees each of ‘Lakota’, ‘McMillan’, and ‘Excel’ pecan within the orchard were selected randomly for sampling. Trees within this orchard block received no fungicide applications. Cultivars were arranged in a randomized complete block design using four blocks, with each cultivar represented once per block. Single-tree plots were used with guard trees between treated trees. The same individual trees were evaluated throughout the course of the study.

Disease ratings were conducted on 11 Sep 2020, 10 Sep 2021, and 20 Sep 2022, after pecan shell hardening. Ten random terminals per tree were rated for incidence and severity of pecan scab and powdery mildew. Incidence is defined as the percentage of nuts with at least one pecan scab or powdery mildew lesion. Severity is defined as the percentage of the pecan shuck surface covered by pecan scab or powdery mildew. At harvest, nuts were shaken from the trees onto a tarp under each tree. All nuts were hand-harvested and weighed to determine yield. Nut weight and percent kernel were determined from 1-lb samples collected from each tree. Nut weight, percent kernel, and tree yield data were calculated for each cultivar by averaging data from the four trees within each cultivar in a given year. Rainfall data were obtained from a University of Georgia weather station located at the study site.

During 2020, four additional ‘Lakota’ trees from the same orchard were selected for evaluation of their response to mechanical fruit thinning. Trees were thinned mechanically for 8 s by trunk shaking using a tree shaker with a hydraulic shaker head (Orchard Machinery Corporation, Yuba City, CA, USA) to remove ∼30% to 40% of the fruit on each tree on 29 Jul 2020. Initial crop load for all trees was excessive in 2020, with ≥85% of terminals bearing fruit on each tree at the time of thinning. Fruit were in the late liquid endosperm stage and the ovule was fully expanded when fruit were removed.

Analysis of variance was used to determine differences between cultivars. All statistical analyses were performed using statistical software (SigmaPlot 14; Systat Software Inc., San Jose, CA, USA). All pairwise multiple comparison procedures were performed using Tukey’s least significant difference test (P < 0.05).

Results and discussion

‘Lakota’, ‘McMillan’, and ‘Excel’ demonstrated high resistance to pecan scab in 2020, 2021, and 2022. Although no formal disease ratings were taken in 2018 and 2019, scab lesions were not observed on pecan foliage or fruit in either year. These cultivars received no fungicide applications throughout the study. We observed no scab infections on any of these cultivars throughout this trial (Table 1). As an indication of disease pressure, ‘Desirable’ pecans in an adjacent orchard under a standard fungicide spray program demonstrated a significant amount of scab severity in 2020 (26%) and 2021 (49%) (Slade 2022). ‘Desirable’ is widely planted in the southeastern United States and is considered highly scab susceptible (Wells and Conner 2015).

Table 1.

Pecan nut disease ratings for pecan scab and powdery mildew in Tifton, GA, USA, from 2020 to 2022.

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Powdery mildew infections were evident in ‘Lakota’, ‘McMillan’, and ‘Excel’ each year of the study. Incidence and severity of powdery mildew was significantly greater (P < 0.001) in ‘McMillan’ than any other cultivar in 2020 and 2022 (Table 1). In 2021, powdery mildew was observed in all cultivars; however, we observed no significant differences in incidence or severity among cultivars, and severity remained relatively low (Table 1). The study site received 26.4 inches of rainfall from 1 Apr to 30 Sep 2020. However, the study site received 33 inches of rainfall during the same time period in 2021, facilitating conditions conducive to intensified scab pressure. In 2022, the study site received 24 inches of rainfall. The 2020 and 2022 growing seasons were slightly drier compared with 2021, generating a more conducive environment for powdery mildew growth (Brenneman and Bertrand 1989). This may explain the greater levels of powdery mildew observed on ‘McMillan’ in 2020 and 2022.

Pecan yield varied by cultivar from one year to the next (Table 2). ‘Lakota’ yield was greater (P < 0.05) than ‘McMillan’ in 2018, 2020, and 2022. ‘Excel’ yield was greater (P < 0.05) than that of the other two cultivars in 2019. During the course of the 5-year study, ‘Lakota’ and ‘Excel’ generated a greater nut yield per tree than ‘McMillan’. ‘Lakota’ nut production reached 159 and 137 lb/tree in 2020 and 2022, respectively. This is considered highly prolific for a pecan tree in this region at 12 to 14 years of age. Although prolificity is a desirable characteristic for commercial pecan cultivars, such heavy tree yields are commonly associated with severe alternate bearing, which was evident in ‘Lakota’ in 2019 and again in 2021, compared with ‘Lakota’ yields in 2018 and 2020 (Table 2).

Table 2.

Pecan yield, nut weight, and percent kernel from 2018 to 2022 for ‘Lakota’, ‘Excel’, and ‘McMillan’ pecan at Tifton, GA, USA.

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The alternate or irregular bearing tendency of ‘Lakota’ was also evident in the alternate bearing intensity index [I (Pearce and Dobersek-Urbanc 1967)], which was significantly (P ≤ 0.05) greater for ‘Lakota’ than for the other two cultivars from 2018 to 2019, from 2019 to 2020, and during the course of the 5-year study from 2018 to 2022 (Table 3). During this time, I averaged 0.51 for ‘Lakota’, 0.13 for ‘Excel’, and 0.22 for ‘McMillan’. Although ‘McMillan’ has been noted previously to be relatively consistent in its production (Wells and Conner 2015), ‘Excel’ has exhibited more severe alternate bearing tendencies as a mature tree than we observed in our study (Wells and Conner 2015). The discrepancy may be the result of the relatively young age of the trees in our study (10–15 years).

Table 3.

Alternate bearing intensity (I) of ‘Lakota’, ‘Excel’, and ‘McMillan’ pecan trees from 2018 to 2022 at Tifton, GA, USA.

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Individual nut weight differed by cultivar in 2020 and 2021 (Table 2). Across years, average nut weight ranged from 7.2 to 9.4 g/nut for ‘Lakota’, 8.1 to 11.1 g/nut for ‘Excel’, and 7.2 to 8.9 g/nut for ‘McMillan’. Nut weight for ‘Lakota’ was lowest in 2020 and 2022, the 2 years of greatest yield (Table 2), demonstrating the effect of excessive crop load on reduced nut weight.

Percent kernel was variable within and among cultivars throughout the study. ‘Lakota’ percent kernel ranged from 54% to 60%. Percent kernel ranged from 46% to 52% for ‘Excel’ and from 50% to 54% for ‘McMillan’. Percent kernel was significantly greater (P < 0.05) for ‘Lakota’ than for the other two cultivars in 4 of the 5 years of study and when averaged over the course of the study (Table 2). In 2020, percent kernel of both ‘Lakota’ and ‘McMillan’ was greater than that of ‘Excel’. The lowest percent kernel observed in ‘Lakota’ (54%) was observed in 2020, the year during which the greatest ‘Lakota’ nut yield per tree (159 lb) was observed (Table 2), demonstrating the potential effect of excessive cropping on percent kernel.

Thompson et al. (2008) suggested that the alternate bearing tendency of ‘Lakota’ appeared to be at least the result, in part, of large cluster size and high percent fruiting shoots, and that it may be possible to reduce the effects of alternate bearing by midsummer fruit thinning. Mechanical fruit thinning alleviated the alternate bearing tendency of ‘Lakota’ during our study (Table 4). Nonthinned ‘Lakota’ trees produced 159 lb/tree in 2020, whereas mechanically thinned trees produced 110 lb/tree. Return yields in 2021 were significantly greater (P < 0.05) at 155 lb/tree for 2020 thinned trees compared with 85 lb/tree for nonthinned trees, and I was 0.17 for mechanically thinned trees and 0.30 for nonthinned trees. Nut weight was unaffected by mechanical thinning in 2020, possibly because nut sizing was nearly complete by the time thinning took place in late July. Nut weight was reduced in 2021 for trees crop-thinned in 2020, which is likely the result of the heavier return crop on 2020 crop-thinned trees (Table 4). Percent kernel was increased by crop thinning in 2020. Similar to the effect of crop thinning on return crop nut size, percent kernel in 2021 was reduced on trees fruit-thinned mechanically the previous year (Table 4). These results suggest that mechanical fruit thinning can reduce the alternate bearing tendency of ‘Lakota’ pecan trees. Similar results have been demonstrated previously for other cultivars (Reid et al. 1993; Smith and Gallot 1990; Smith et al. 1993; Wells et al. 2009). Based on the heavy return crop of thinned trees, annual fruit thinning may be required to achieve the full benefit of crop load management in ‘Lakota’.

Table 4.

In-shell pecan tree yield, nut weight, and percent kernel of ‘Lakota’ pecan in mechanically fruit-thinned and nonthinned control treatments from 2020 to 2021 at Tifton, GA, USA.

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Our study demonstrates that commercial yields of certain pecan cultivars can be grown without an extensive fungicide spray program in the southeastern United States. Pecan scab was not observed on any cultivar evaluated throughout the study. Although this occurred without use of fungicides, it remains advisable to use a minimal fungicide program, even on scab-resistant cultivars, to reduce the potential for emergence of secondary pathogens such as powdery mildew. Current resistance of pecan cultivars to pecan scab is no guarantee of prolonged resistance to the disease. Pecan scab has the ability to persist and adapt to new cultivars over time (Conner 2002). Previously unsusceptible cultivars such as ‘Desirable’ have lost their scab resistance with time as they became extensively planted. Others, such as ‘Elliot’, have maintained a high degree of resistance to pecan scab for nearly 100 years (Sparks 1992). Planting resistant cultivars is still considered the most desirable practice for controlling pecan scab (Bock et al. 2016). Lovell et al. (2021) identified genes within the 1.41-Mb region of the ‘Lakota’ genome with homologs found in other species known to be involved in the acquisition of nutrients, plant development, and defense responses. In addition, 46 orthogroups were identified within the pan-genome database of the 1.41-Mb region, of which eight were specific to ‘Lakota’. This suggests a potential genetic basis for disease resistance in ‘Lakota’, which may help to prolong its disease resistance.

Based on current data, all three pecan cultivars evaluated during our study are suitable for commercial production; however, ‘Lakota’ and ‘Excel’ performed best under these conditions in terms of nut yield. Scab was not observed on any of the cultivars throughout the study. Powdery mildew was observed on all cultivars, but was significantly greater on ‘McMillan’ in 2020 and 2022 as a result of drier conditions in those years. There was no consistent difference in nut weight among cultivars. ‘Lakota’ has the greatest commercial market potential in terms of percent kernel, averaging 58% kernel compared with 49% and 52%, respectively, for ‘Excel’ and ‘McMillan’. However, in years of excessive cropping, percent kernel of ‘Lakota’ is reduced significantly. Mechanical fruit thinning increased the following year’s production of ‘Lakota’ by 82% over nonthinned trees. Therefore, some form of crop load management such as mechanical fruit thinning is necessary to maintain high kernel quality in ‘Lakota’.

Units

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