The purpose of this project is to investigate the relationship between damage by citrus leafminer (CLM) and incidence of citrus canker in Florida to clarify the importance of CLM control in reducing incidence of canker. CLM has been shown to increase incidence of canker in India and Brazil. Damage by CLM to the cuticle of leaves exposes the mesophyll of the leaf lamina to infection. Mesophyll exposed through the formation of leaf mines resists callus formation needed for protection and remains callus-free for about 14 days compared to 1-2 days for mechanically-induced wounds. The canker bacterium [Xanthomonas citri subsp. citri (Xcc)] spreads naturally via wind-blown rain. Unless Xcc comes in contact with mesophyll tissues exposed by wounds during the short time period prior to callus formation, the only infection that occurs is through stomatal infection. However, when CLM wounds are prevalent, inoculum-laden rain simply coming to rest on a CLM mine easily passes through the damaged cuticle and comes in direct contact with the highly susceptible, exposed and non-callused mesophyll cells. Many cultivars that have some resistance to canker become more susceptible if damaged by CLM. Compared to inoculum levels associated with the relatively few stomatal infections that normally occur, Xcc infections in CLM mines can produce large inoculum levels. We sampled flush shoots from eight mature groves for CLM injury and canker in the Indian River citrus area during 2008. None of the groves were subjected to canker management. Two large blocks of grapefruit, a block of lemon, and a bock of orange trees were sampled in St Lucie County. One large block of grapefruit and one of Minneola trees were sampled in Martin County. One large block of grapefruit and one of orange trees were sampled in Palm Beach County. All of the blocks were sampled once during July or August. One of the blocks in St Lucie County was also sampled during May. Sample dates were timed so that flush shoots sampled were 6 to 8 weeks old. Two hundred flush shoots were collected from each grove and examined for CLM damage and canker. Among all leaves observed over 200 flush shoots per grove in July and August, 50 to 64% of the leaves had CLM injury in each Martin County grove and in each of the three St Lucie County groves. Around 20% of the leaves had mines in the Palm Beach County grapefruit grove and the St Lucie County orange grove. Only 1% of the leaves from the Minneola trees had mines. With respect to the grapefruit grove sampled in May, 4% of the leaves were damaged by CLM. Canker was observed only on leaves from grapefruit (in spite of each block of non-grapefruit trees sampled being in somewhat close proximity to infected grapefruit). The percentage of flush shoots with canker in the different grapefruit blocks ranged from 1.5% to 9.5%. Among flush shoots with canker, canker was directly associated with CLM injury on an average of 73.1% of the flush shoots. An average of 9.3% shoots had canker but no CLM injury. These data indicated that CLM caused a 7 to 8 fold increase in the percentage of flush shoots infected. About 27% of flush shoots with both canker and CLM injury had some canker lesions that were not directly associated with leafminer injury. Among leaves with canker, the number of lesions per leaf averaged 11.2 – the number of lesions associated with mines averaged 8.1 and the number not associated with mines averaged 1.8. The results support that CLM exacerbates citrus canker in Florida. Canker levels in the groves were relatively low during 2008, which may have been an atypical year for canker. Depending on weather patterns, a 7 to 8 fold increase in the incidence of flush shoots infected by canker might be economically important during some years but not others.