Abstract
We compared the growth and survival of young 'Hamlin' orange trees on five rootstocks (C22, C32, and C35 citranges, Cleopatra mandarin and Swingle citrumelo) that were planted within a declining mature citrus grove infested with the root weevil, Diaprepes abbreviatus. The study used a split plot experimental design and all young trees received chemical applications for pest control beginning at planting in September 2001 and continuing through 20 months of growth. Subsequently, half the trees received chemical treatments based on a pest-monitoring program whereas the others remained untreated. Over the next two years, treated trees had higher growth rates (except Cleopatra mandarin), larger tree canopies, and less tree decline than untreated trees. Trees on C32, C35, and Swingle citrumelo generally outperformed those on C22 and Cleopatra mandarin. Roots damaged by weevil larvae were often infected by the plant pathogen Phytophthora nicotianae, which appeared to be a major factor in tree decline. In the fall of 2004, the grove sustained three hurricanes that killed 18.6% of the young trees and caused major damage to an additional 27.2%. Differential storm damage occurred among rootstock and pesticide treatments at least partly because larger healthier trees suffered more broken branches and fatal trunk injury whereas smaller weaker trees had less damage overall but were more frequently uprooted. Despite hurricane damage, differences in canopy volume showed similar patterns in May 2004 and May 2005. This study demonstrated the importance of rootstock selection and effective pesticide use for maintaining citrus groves under stress from Diaprepes and Phytophthora.
