A Comparison of a Standard Radial Airflow and a Double Volute-Generated Focused Airflow Air Blast Sprayer to Control Scab in Tall Pecan Trees.

Pecan (Carya illinoinensis) is a valuable crop in the southern United States. Scab (caused by Venturia effusa) is a major biotic constraint to pecan production in the southeastern region and requires 10 to 15 or more fungicide applications for control. Spray application relies on large standard radial airflow air blast sprayers (SSs). Some SSs have been adapted to include a volute on one side to project spray higher into the canopy because of a decline in coverage with height. A grower-designed and engineered double volute-generated focused airflow air blast sprayer (DVS) was assessed for scab control and spray coverage compared with an SS. Over three seasons, on foliage and fruit, scab control was either equal to or superior using the DVS sprayer. On mature fruit, in 2017, scab severity on the control was 9.07%, on the DVS it was 0.19%, and on the SS it was 0.24%; in 2018 severity was 84.4, 18.4, and 29.1%, respectively; and in 2019 it was 32.7, 7.0, and 11.6%, respectively. There were no discernable gradients in scab severity with tree height with either sprayer in 2017, but in 2018 the difference was significant, with the DVS having a shallower gradient, and in 2019 the DVS slope was numerically less steep compared with the SS. Mature fruit weight was significantly greater on trees treated using the DVS compared with the SS in 2017 and 2018 but was not different in 2019. Spray coverage studies showed that the DVS sprayer had significantly more coverage at heights >13.8 m (up to 19.0 m), ranging from 18.4 to 14.1%, compared with coverage using the SS, which ranged from 7.9 to 2.9%. The slope in decline of spray coverage with height was significantly shallower with the DVS sprayer, and spray profiles reflected these differences. Disease control was improved overall when using the DVS, and the DVS provided greater spray coverage at heights >13.8 m when compared with the SS sprayer. The DVS design may offer economical, superior scab control in pecan orchards where trees are >13.8 m tall.

Effect of Tractor Speed and Spray Application Volume on Severity of Scab and Fruit Weight at Different Heights in the Canopy of Tall Pecan Trees.

Scab (caused by Venturia effusa) is the most important yield-limiting disease of pecan in the southeastern USA. On susceptible cultivars, the disease is managed using fungicides, but spray coverage is an issue in tall trees. In four experiments, we used an air-blast sprayer to compare scab severity on fruit at 5.0 to 15.0 m height in trees receiving the same dose of fungicide at 468, 935, and 1,871 liter/ha at 2.4 and 3.2 km/h (in two additional experiments fungicides were applied at 4.0 km/h at 470 liter/ha, 4.0 km/h at 940 liter/ha and 4.0 km/h at 1,100 liter/ha). An air-blast sprayer was used for the applications, which included typical recommended active ingredients (a.i.). Nozzles were selected to provide similar proportions of spray to the upper and lower canopy. The treatments (or subsets thereof) were repeated in 2015 to 2017 on cv. Schley and in 2017, 2019, and 2020 on cv. Desirable. All treatments reduced scab compared with the control. Overall, there was no consistent difference among the treatments for severity of scab on foliage, immature fruit, or mature fruit at any height in the canopy up to 15.0 m (maximum height sampled). Fungicide applied at 2.4 or 3.2 km/h at 470 liter/ha was as effective at reducing disease as were the higher volumes (sometimes more so). The scab epidemic severity affected control efficacy. Estimated cost and water savings based on faster speed and lower volume were considerable. These preliminary observations indicate no single volume or speed was consistently superior to control scab; this suggests that, in most seasons, low volumes (higher concentration of a.i.) may be similarly efficacious as high volumes (lower concentration of a.i.) for controlling scab in tall pecan trees and offer greater resource use efficiency.

Effect of Tractor Speed and Spray Application Volume on Spray Coverage at Different Heights in the Canopy of Tall Pecan Trees.

Scab (caused by Venturia effusa) is the most important disease of pecan in the southeastern U.S.A. The yield losses in susceptible cultivars, combined with costs of control, amount to tens of millions of dollars annually. It is known that fungicide coverage from air-blast sprayers declines with height in the canopy, and conversely, disease severity increases. But how application volume (liter/ha) and speed (km/h) affect spray coverage at different heights is unknown. Coverage was quantified using Kromekote cards (CTI Paper USA, Sun Prairie, WI) and Vision Pink dye (GarrCo Products, Converse, IN) at heights of 5.0, 7.5, 10.0, 12.5, and 15.0 m in pecan canopies. An orchard air-blast sprayer was operated at 2.4 and 3.2 km/h applying 468, 935, or 1,871 liters/ha. Nozzles were selected to provide proportionally similar volumes to the upper and lower canopy positions at set speeds. Speeds tested did not affect spray coverage consistently. However, greater volumes resulted in significantly greater spray coverage, but most of that increase was at heights ≤12.5 m. Although there were significant differences among volumes applied at 12.5 m, differences were numerically small. Card orientation had a profound effect on spray coverage at heights ≤12.5 m, with most spray being detected on the cards facing horizontally downward, and least on those facing vertically backward. The study demonstrates that higher volumes result in more coverage, but the effect declines rapidly with height. If disease control achieved with 470 liters/ha is no different (or is more efficacious) compared with >470 liters/ha and is the same at higher speeds (3.2 km/h), savings may be possible in terms of operating time and equipment costs.

A Comparison of Ground-Based Air-Blast Sprayer and Aircraft Application of Fungicides to Manage Scab in Tall Pecan Trees.

Pecan scab (caused by Venturia effusa) is a destructive disease of pecan in the southeastern United States. Susceptible cultivars must be sprayed with fungicide every 10 to 21 days to ensure yield and kernel quality. Fungicide is most often applied using large orchard air-blast sprayers. Pecan trees grow tall, and air-blast sprays result in a gradient in spray deposition and consequently of scab. Aerial fungicide application is also practiced. Disease distribution and spray deposition of the two methods have not been compared but will provide information aiding decisions on spray application methods. We compared air-blast, aerial, and air-blast + aerial applications for efficacy controlling scab at five heights in the canopy of 25-m cultivar Schley pecan trees. There was a negative relationship between scab severity and height in control trees, a positive linear relationship with height in air-blast treated trees, and a generally negative linear relationship between scab severity and height in aerially treated trees. Air-blast + aerial treatments resulted in low severity of scab at all heights. Spray deposition on water-sensitive cards indicated a declining gradient with height using an air-blast sprayer, whereas aerial applications resulted in a low deposition at all sample heights. Air-blast sprays tended to result in less good control at heights >12.5 m, and aerially treated trees at ≤7.5 m. The results provide insight into the efficacy and advantages of these methods for applying fungicide to control scab in tall pecan trees; further research is needed to better understand the impact of frequency and timing of these two methods.

Population Genetic Structure of Venturia effusa, Cause of Pecan Scab, in the Southeastern United States.

Venturia effusa is the most important pathogen of pecan in the southeastern United States. Little information exists on the population biology and genetic diversity of the pathogen. A hierarchical sampling of 784 isolates from 63 trees in 11 pecan orchards in the southeastern United States were screened against a set of 30 previously characterized microsatellite markers. Populations were collected from Georgia (n = 2), Florida (n = 1), Alabama (n = 2), Mississippi (n = 1), Louisiana (n = 1), Illinois (n = 1), Oklahoma (n = 1), Texas (n = 1), and Kansas (n = 1). Clonality was low in all orchard populations (≤10.1% of isolates), and there were consistently high levels of genotypic diversity (Shannon-Weiner's index = 3.49 to 4.59) and gene diversity (Nei's measure = 0.513 to 0.713). Analysis of molecular variance showed that, although 81% of genetic diversity occurred at the scale of the individual tree, 16% occurred between orchards and only 3% between trees within orchards. All populations could be differentiated from each other (P = 0.01), and various cluster analyses indicated that some populations were more closely related compared with other pairs of populations. This is indicative of some limited population differentiation in V. effusa in the southeastern United States. Bayesian and nearest-neighbor methods suggested eight clusters, with orchards from Georgia and Florida being grouped together. A minimum spanning tree of all 784 isolates also indicated some isolate identification with source population. Linkage disequilibrium was detected in all but one population (Kansas), although 8 of the 11 populations had <20% of loci at disequilibrium. A Mantel test demonstrated a relationship between physical and genetic distance between populations (Z = 11.9, r = 0.559, P = 0.001). None of the populations were at mutation-drift equilibrium. All but 3 of the 11 populations had a deficiency of gene diversity compared with that expected at mutation-drift equilibrium (indicating population expansion); the remaining populations had an excess of gene diversity compared with that expected at mutation-drift equilibrium (indicating a recent bottleneck). These observations are consistent with the known history of pecan and pecan scab, which is that V. effusa became an issue on cultivated pecan in the last approximately 120 years (recent population expansion). Recently reported mating type genes and the sexual stage of this fungus may help explain the observed population characteristics, which bear a strong resemblance to those of other well-characterized sexually reproducing ascomycete pathogens.

Severity of Scab and its Effects on Fruit Weight in Mechanically Hedge-Pruned and Topped Pecan Trees.

Scab is the most damaging disease of pecan in the southeastern United States. Pecan trees can attain 44 m in height, so managing disease in the upper canopy is a problem. Fungicide is ordinarily applied using ground-based air-blast sprayers. Although mechanical hedge-pruning and topping of pecan is done for several reasons, improved management of scab is an important reason in the humid, wet Southeast. Resulting shoot growth on cut limbs of susceptible cultivars could lead to more severe scab. In three experiments over three years, we explored the effect of hedge-pruning trees to ∼12 to 14 m compared with non-hedge-pruned trees. All trees received fungicide treatments (air-blast sprays and ≤3 aerial applications). Hedge-pruning either had no effect, or increased or decreased scab severity only slightly on leaflets, immature, or mature fruit (a -9.95 to +14.63% difference in scab severity compared with the control). However, height in the canopy invariably had a large and significant effect on scab severity, and amounted to a 0.05 to 73.77% difference in severity between the lowest and highest sample in the canopy. Fruit weight depended on sample height, with fruit most often weighing less when collected at greater sample heights. A robust relationship between fruit weight and scab severity was found at the highest sample heights where scab was also most often severe (R2 = 0.21 to 0.67, P < 0.0001). Hedge-pruning and topping pecan tree canopies to manage tree size will enable better fungicide coverage, reducing risk of a scab epidemic as more of the canopy is assured efficacious fungicide spray coverage.

Scab Susceptibility of a Provenance Collection of Pecan in Three Different Seasons in the Southeastern United States.

Pecan scab (caused by Fusicladium effusum) is the most economically destructive disease of pecan in the Southeast United States. Wet, humid conditions typical of the Southeast are known to provide conditions conducive to epidemics. A provenance collection of pecan from 19 locations representing the native range of the tree is located in Byron, Georgia, and was assessed for pecan scab severity in 1998, 2013, and 2014. There were significant differences among the 19 provenances (F = 5.6 to 62.5, P < 0.0001). Provenances from wetter locations (generally north of Texas) had the greatest proportion of scab resistant trees, while provenances from the drier southern areas (Texas and Mexico) tended to be the most susceptible to scab. The association with rainfall was borne out by correlation analysis (r = -0.625 to -0.823 [P < 0.0001 to 0.004]). Other factors consistently associated with scab severity included leaflet tilt and droop angle (r = -0.533 to -0.883 [P < 0.0001 to 0.02]). Multiple regression analysis demonstrated that leaflet droop angle was a particularly good predictor of provenance susceptibility. Leaflet characteristics vary with provenance location, and whether there is a direct relationship between scab severity and leaflet characteristics is not established. Estimates of heritability were not entirely consistent among years, but different methods were used to assess scab severity in 1998 (a 1 to 5 category scale) compared with 2013 and 2014 (the percent ratio scale). Despite using different methods, there was generally good agreement among years in regard to severity of disease on individual trees. In conclusion, trees from more northern populations (in areas with greater annual rainfall) are most likely to provide valuable and diverse sources of resistance to scab. The provenance collection contains a range of scab-resistant genotypes from diverse locations that can contribute to genetic improvement regarding scab resistance.

The Effect of Sample Height on Spray Coverage in Mature Pecan Trees.

Pecan scab (caused by Fusicladium effusum) is the most damaging disease of pecan in the southeastern United States. Large air-blast sprayers for orchards are used to apply fungicide to control the disease but little quantitative information exists on the spray coverage achieved in the canopy of these trees. A series of experiments using water-sensitive spray cards to record spray coverage (percent area) at different heights and locations up to 15 m in the canopy of pecan trees showed a significantly greater percentage of card area covered at the lowest sample height when compared with the highest sample height. At the lowest height (5 m), spray coverage on individual cards ranged from 4.7 to 73.5% and, at the highest sample height (15 m), spray coverage ranged from 0.02 to 9.5%. In general, there was little significant difference in spray card coverage up to at least 10 m but, at 12.5 and 15 m, there was significantly less spray coverage compared with the coverage at 5 m. Regression analysis indicated a consistent linear relationship between sample height in the tree and the percent area covered. When spray cards were positioned at different heights without possible interference from pecan limbs and foliage, similar effects of sample height on spray coverage were noted. Wind speed measurements showed that air movement declined rapidly with distance from the sprayer fan. Whereas, at 2 m from the fan, wind speeds were approximately 26 m s-1, by 10 m, speeds had declined to 2 to 4 m s-1. At distances >12 m, wind speed was approaching ambient air movement of about 1 to 3 m s-1. Although aerial application resulted in numerically greater spray coverage at sample heights >10 m, it was not significant even though a weak linear relationship (R2 = 0.21 to 0.25) suggested an effect of height. Characterizing and understanding pesticide spray coverage in pecan will allow us to discern limits imposed by existing technology, and provide the basis for improving spray application methods (or tree management) for more efficacious disease control.

Comparative impact of artificial selection for fungicide resistance on Beauveria bassiana and Metarhizium brunneum.

Hypocreales fungi such as Beauveria bassiana (Balsamo) Vuillemin and Metarhizium brunneum Petch can be negatively affected by fungicides thereby reducing their biocontrol potential. In a previous study, we demonstrated enhanced fungicide resistance in B. bassiana through artificial selection. However, it is not clear if the enhanced resistance was because of improved germination, vegetative growth, or both. Additionally, the enhanced fungicide resistance has only been demonstrated in B. bassiana, and therefore it is of interest to investigate the potential to enhance resistance in other fungi. Thus, the objectives in this study were to determine the potential to enhance fungicide resistance in M. brunneum through artificial selection, and investigate if selection is based on germination, vegetative growth, or both in B. bassiana and M. brunneum. Selection for resistance to fenbuconazole, and triphenyltin hydroxide was assessed through inhibition evaluations on solid media, and germination and mycelial growth in liquid media. Increased resistance after selection was observed for all fungicide-fungus combinations on solid and or liquid media. Selection resulted in increased resistance to fenbuconazole in both fungi in solid and liquid media; in liquid culture fungicide resistance in B. bassiana was manifested by increased germination and mycelial growth, whereas in M. brunneum fungicide resistance concerned only mycelial growth. Selection for resistance to triphenyltin hydroxide varied in the different media. For B. bassiana, triphenyltin hydroxide resistance was enhanced on solid media but not in liquid, whereas enhanced resistance of M. brunneum was detected in both media. Fungicide sensitivity and selection potential differs based on the medium and fungal species. Selection for fungicide resistance, had negative effects on other beneficial traits when fungicide pressure was removed, for example, some selected populations showed decreased germination or growth, relative to their nonselected control populations. Additionally, reduced virulence to the greater wax moth, Galleria mellonella (L.), was observed in all fungal populations that were exposed to fungicide resistance regimes. We conclude that it is possible to use genetic selection to enhance fungicide resistance in B. bassiana and M. brunneum, but before use the resulting populations should be screened for inadvertent negative impacts on beneficial traits.

The potential for enhanced fungicide resistance in Beauveria bassiana through strain discovery and artificial selection.

Our objectives were to determine the (1) natural variation in fungicide resistance among Beauveria bassiana strains, (2) potential to increase fungicide resistance in B. bassiana through artificial selection, and (3) stability of virulence in selected B. bassiana strains. Fungicides included dodine, fenbuconazole, and triphenyltin hydroxide, which are commonly used in pecan and other horticultural crops. Comparison of seven B. bassiana strains indicated some are substantially more resistant to fungicides than others; a commercial strain (GHA) was less resistant than all wild strains isolated from pecan orchards. Artificial selection resulted in enhanced fungicide resistance in the GHA strain but not in a mixed wild strain. Removal of selection pressure for three passages did not reduce the enhanced fungicide resistance. Sub-culturing with exposure to fungicides did not affect the GHA strain's virulence to pecan weevil, Curculio caryae, larvae, whereas fungicide exposure increased virulence in a mixed wild population of B. bassiana.