Annual growth rings and the impact of Benlate 50 DF fungicide on citrus trees in seasonally dry tropical plantations of northern Costa Rica

Abstract

Tree ring scientists have generally ignored the use of dendrochronological techniques to assess growth impacts of cultural treatments for the major horticultural trees in plantations. In this study, we investigated differences in radial growth between orange ( Citrus) trees, a non-native subtropical species, growing in plantations in northern Costa Rica and southern Nicaragua that were either treated or untreated with the fungicide Benlate 50 DF (benomyl), which has potential phytotoxic effects. This region of Central America experiences prolonged and severe drought from January through April each year, during which an average of only 10% (about 260 mm) of the annual rainfall occurs. Basal cross-sections taken from the citrus trees reveal distinct annual tree rings that closely match the ages of the trees. The rings are reasonable concentric and have good circuit uniformity. An analysis of 111 tree cores indicates that Benlate 50 DF treated trees had significantly slower radial growth during their first 6 years (averaging 4.54 mm/year) than untreated trees (averaging 10.76–11.84 mm/year). After year 6, the treated trees had a similar growth rate (11.27 mm/year) to the untreated trees. Four cores taken from each of 10 citrus trees at cardinal directions around the stem reveal that the mean growth rate from any one aspect did not differ significantly from the mean growth per tree. Further analysis of these cores indicates that taking one or two random cores per tree produced a similar average growth calculation. The results of this study suggest that young citrus trees produce distinct annual tree rings in a seasonally dry tropical environment, that trees treated with Benlate 50 DF had slower radial growth during their first 6 years, and that, with proper replication, taking one core per tree was adequate to assess radial growth. But this latter point needs to be determined by researchers on a case-by-case and species-by-species basis. The approach used in this study to determine growth impacts following chemical application may have widespread application for other agricultural practices in a variety of horticultural woody plants worldwide.