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United States Department of Agriculture

Agricultural Research Service

Research Project: PECAN CULTIVATION AND DISEASE MANAGEMENT

Location: Fruit and Nut Research

Title: Vertical distribution of scab in large pecan trees

Authors
item Bock, Clive
item Wood, Bruce

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 3, 2012
Publication Date: May 1, 2013
Citation: Bock, C.H., Wood, B.W. 2013. Vertical distribution of scab in large pecan trees. Plant Disease. 97:626-634.

Interpretive Summary: Pecan scab is the most destructive disease of pecan in the southeastern US and some other parts of the world, and infection can result in severe yield loss. There is no information on disease distribution within canopies of large, mature pecan trees. This study aimed to investigate vertical disease distribution on foliage and fruit within canopies. Four large (14-16 m tall), 30-y old trees of a susceptible cultivar (‘Desirable’) were assessed for foliage and fruit disease at five separate heights (0-5, 5.0+-7.5, 7.5+-10, 10+-12.5 and 12.5+ m) on the east and west side of each tree. Leaf and nut samples were collected at random from each height on each side. Lesion counts and visual estimates of percent area infected were made on leaflets and fruit. There was a linear relationship between lesion counts and estimates of the percent area infected. There was also a linear relationship between fruit disease severity and height. Pecan fruit were heavier higher in the tree, and whereas individual fruit weight was positively related to tree height, there was a negative linear relationship between individual fruit weight and scab severity. Linear regression analysis to explore the effect of disease severity on fruit weight at each height on each tree demonstrated that, in most samples of individual fruit at each height, more severe scab resulted in smaller fruit. The mean scab severity on fruit followed a characteristic trend from east to west with maximum severity at the lowest heights and minimum severity at the apex of the tree (up to a five-fold increase in disease between 0-5 m and 12.5+ m). In contrast, the largest fruit were consistently found at the apex of the tree, with an up to four-fold increase in weight between fruit at 12.5+ m compared to 0-5 m. Knowledge of the distribution of disease in tall pecan tree canopies can help provide an informed basis for applying fungicide treatments to minimize disease.

Technical Abstract: Pecan scab (caused by Fusicladium effusum) is a destructive disease of pecan (Carya illinoensis) grown in humid environments, such as the southeastern US. The disease can cause severe yield loss, and although much is known about the processes of dispersal and infection, there is no information on disease distribution within canopies of large, mature pecan trees. This study aimed to investigate vertical disease distribution on foliage and fruit within canopies. Four large (14-16 m tall), 30-yr old trees of a susceptible cultivar (‘Desirable’) were assessed for foliage and fruit disease at five separate heights (0-5, 5.0+-7.5, 7.5+-10, 10+-12.5 and 12.5+ m) on the east and west side of each tree. Two leaf samples (4 June and 19 July) and one nut sample (12 August) were taken. Ten leaves and five fruit were collected at random from each height on each side. Lesion counts and visual estimates of percent area infected were made on individual leaflets and fruit. Linear regression analysis between lesion counts and estimates of the percent area infected showed that estimates of percent area were superior. There was a linear relationship between fruit disease severity and height on both sides of all four trees (P = <0.0001, R2 = 0.46-0.82), with the exception of the west side of Tree-1. There was no consistent effect of height on foliar disease, but very low disease severity (<5% area infected) might have precluded meaningful leaf analysis. Pecan fruit were heavier higher in the tree (P = <0.0001, R2 = 0.49-0.75). Whereas individual fruit weight was positively related to tree height, there was a negative linear relationship between individual fruit weight and scab severity on all sides of all trees (P = 0.0003-<0.0001, R2 = 0.46-0.82), except the west side of Tree-1. Linear regression analysis to explore the effect of disease severity on fruit weight at each height on each tree demonstrated that, in most samples of individual fruit at each height, more severe scab resulted in smaller fruit (P = 0.0358-0.0002, R2 = 0.44-0.83). The mean scab severity on fruit followed a characteristic trend from east to west with maximum severity at the lowest heights and minimum severity at the apex of the tree (there was up to a five-fold increase in disease between 0-5 m and 12.5+ m). In contrast, the largest fruit were consistently found at the apex of the tree, with an up to four-fold increase in weight between fruit at 12.5+ m compared to 0-5 m. Knowledge of the distribution of disease in tall pecan tree canopies can help provide an informed basis for applying fungicide treatments to minimize disease.

Last Modified: 7/23/2014