Inheritance of Resistance to Elsinoe fawcettii (Citrus Scab) in Citrus

The fresh citrus industry in Florida is currently under strong pressure from diseases such as citrus greening (HLB) and canker.  While Citrus scab does not pose as severe a threat to the citrus industry as HLB, it does represent a challenge to fresh fruit producers. Citrus scab disease, characterized by the production of hardened scab like lesions on the leaves, fruit and stems of susceptible cultivars, is caused by the fungus Elsinoe fawcettii.  This disease has a significant impact on the external quality of fresh fruit cultivars such as ‘Temple’, ‘Murcott’, and ‘Grapefruit’.  ‘Clementine’ a popular seedless mandarin grown in Mediterranean climates is highly susceptible to the disease and cannot be cultivated commercially in Florida. Current control methods consist of fungicides, which represent an additional cost to growers attempting to produce fresh market fruit. E. fawcettii has also been reported to develop genetic resistance to fungicides. Therefore, the development of high quality mandarin cultivars with genetic resistance to scab disease would represent a significant contribution to the viability of fresh market citrus production in Florida.  Segregating F1 populations of ‘Fallglo’ X sour orange, pummelo X sour orange, and ‘Minneola’ X sour orange were generated to study the inheritance of citrus scab resistance. Trees were planted at Gainesville, Florida in 2006 at a spacing of 0.8 meters within the row and 4.6 meters between rows.  Trees were irrigated with overhead irrigation as needed. Populations were evaluated twice per year for two years using a 0 to 3 disease rating scale. Evaluation scores were determined based on scab lesions on the leaves, fruit and stems of the entire viewable plant under field infection conditions. Preliminary data showing the segregation of the resistance phenotype and initial conclusions on the inheritance mechanism of this phenotype will be presented.