Submitted to: Southeastern Pecan Growers Meeting Proceedings
Publication Type: Other
Publication Acceptance Date: 2/23/2013
Publication Date: 2/23/2013
Citation: Bock, C.H. 2013. Disease and spray coverage in pecan trees. Southeastern Pecan Growers Meeting Proceedings. www.sepga.com/files/3413/6509/9081/Bock_SEPGA.pdf.
Interpretive Summary: Pecan scab is the major disease of pecan in the south-eastern US. New efficaceous fungicides are needed to manage the disease. The distribution of disease and the ability of fungicides to get spray into the canopy were studied. Leaf and nut samples showed more severe disease in the lower canopy of non-treated trees, but similar disease in the mid- and upper-canopy of both treated and non-treated trees. Spray from an airblast sprayer was shown to reach about the same height (10 m) as the point where scab severity wsa less on treated trees. Ground-based sprays do not provide adequate spray coverage in trees taller than 10-25 m. Mature pecan trees can be >30 m tall. Growers must consider alternatives to ground based sprays (perhaps aerial application) to control disease in tall pecan trees.
Technical Abstract: Pecan scab (Fusicladium effusum) is a destructive disease of pecan in the southeastern US. This study was conducted to investigate the vertical distribution of scab in tall pecan trees (14-16 m tall) in three experiments in 2010 and 2011. Although 2010 had average rainfall, a factor that drives scab epidemics, 2011 was a very dry year with a consequently low scab severity. A total of eight trees were included in each experiment, four were non-treated controls, and four were sprayed using a ground-based air-blast sprayer. Trees were assessed for foliar and fruit scab at 0-5.0, 5.0+-7.5, 7.5+-10.0, 10.0+-12.5 and 12.5+-15.0 m. Mixed model analysis showed main effects of height, fungicide treatment and height*treatment interactions in all three experiments, although on foliage the effects were less consistent (P-value = 0.003-0.8), perhaps due to delayed fungicide applications early in the season. However, fruit of non-treated treated trees had more sever scab low in the canopy compared to fungicide treated trees, with a consistent height*treatment interaction (P-value = <0.0001-0.04). Most often the severity of scab in the upper canopy was similar in trees on fungicide treated and non-treated trees suggesting that fungicide had less impact at heights =10.0 m compared to <10.0 m in the canopy. There was a consistent reduction in scab severity on foliage and on immature fruit in August due to fungicide treatment at heights =10.0 m, but above 10.0 m the effect was inconsistent, but late in the season (October) the fungicide treated trees showed lower scab severity throughout the canopy. A metallic tracer study using cerium (Ce) showed an exponential relationship between quantities of Ce recovered and sample height in the canopy, with the quantity of Ce at =10 m being statistically equal to background levels. The relationship between scab severity on fruit and sample height in the canopy of non-treated trees was most often described by a negative linear function (P-value = 0.004-0.5, R2 = 0.22-0.96), but there was no discernible relationship on fungicide treated trees as the severity of scab in the lower canopy was most often similar to that in the upper canopy (P-value = 0.2-1.0, R2 = 0.00-0.51). Gradients in fungicide coverage and scab severity have ramifications for scab management options and potentially for the development of fungicide resistance in F. effusum.