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Stephen H. Futch, James H. Graham, and Larry W. Duncan

Florida citrus groves of sweet orange (Citrus sinensis), tangerines (Citrus reticulata), and grapefruit (Citrus paradisi) experience an annual tree loss of 3% to 4% due to various causes of tree decline. Commonly used tree removal methods in Florida include “pushing,” which lifts most of the root system completely out of the soil, or “clipping,” which shears the tree off above the soil line leaving the tree stump and root system in place. Several operational and economic advantages and disadvantages exist for both tree removal systems. There are also potential problems with citrus resets that can occur due to foot rot (Phytophthora nicotianae) and citrus nematodes (Tylenchulus semipenetrans) that remain in the soil after tree removal. To investigate reset tree performance after “pushing” versus “clipping,” a study was conducted in three groves representative of three production regions in Florida to compare the impact of tree removal method on the pest/pathogen status and growth of resets over a period of 4 years. Based on the findings, tree removal by “pushing” or “clipping” appears to have minimal effect on subsequent pest and pathogen status and performance of citrus resets. Therefore, the method of tree removal should depend primarily on operational and economic considerations.

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Peng Ling, Fred G. Gmitter Jr., Larry W. Duncan, and S. Y. Xiao

A family of 63 citrus intergenemic backcross hybrids was used for this study. The parents and hybrids were multiplied by rooted cuttings, with 6 uniform replicates selected per hybrid, and each plant was inoculated with citrus nematodes (Tylenchulus semipenetrans) 5 times over 2 mo. The number of nematode female larvae per gram of fine fresh root was determind 2 mo after the last inoculation. The phenotypic variation of the hybrids was continuous and wide-ranged, from 8.0 females· g-1 of root tissue (resistant parent Swingle citrumelo=15.6) to 620.0 females· g-1 of root tissue (susceptible parent LB 6-2=540.5). Bulked segregant analysis (BSA), using RAPD fragments, was conducted with 2 DNA bulks of individuals from the extremes of the phenotypic distribution. Three hundred twenty primers were screened and 5 were found to generate repeatedly single RAPD fragments specific to the resistant bulk. The segregation of resistance-associated fragments among the individuals was examined, and the linkage between these markers and potential nematode resistance loci was estimated.