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Seedlessness is an important trait in the evaluation of commercial mandarin for fresh consumption. However, in the last decade, the presence of seeds in fruit of cultivars considered as seedless has become a problem in different citrus-growing areas because the commercial value is depreciated. Seeds have appeared concomitantly with the introduction of new cultivars that appear to be cross-compatible. To overcome this problem, different strategies have been explored, but a definitive answer is still elusive. The search for alternatives contrasts with how little is known about the basis of the problem: the pollen-pistil incompatibility reaction in mandarin, and the intercompatibility relationship between different cultivars. In this work, we characterized the pollen-pistil incompatibility in the two commonly grown mandarin cultivars Fortune and Nova, and evaluated the intercompatibility relationship between six cultivars with different genetic origins; these cultivars included Fortune (Citrus clementina Hort. ex Tan. × Citrus reticulata Blanco), Nova [(Citrus paradisi Macf. × C. reticulata) × C. clementina), Comune Clementine (C. clementina), Avana apireno (C. reticulata), Primosole (Citrus unshiu Marcov. × C. reticulata), and Simeto (C. unshiu × Citrus deliciosa Ten.). Following the controlled hand pollination in the field, we evaluated pollen tube performance after self- and cross-pollination, as well as intercultivar compatibility by observing pollen tube growth. The results show the self-incompatibility of these cultivars with the pollen tube having been arrested in the style, which explains their seedless condition when planted in solid blocks. The study of intercompatibility indicates a different pollen tube behavior depending on the genotype, as well as on the cross combinations. These results provide a basis to evaluate self- and intercompatibility in citrus, and the effect of close planting of some cultivars.

Mal secco, caused by the fungus Plenodomus tracheiphilus, is a xylem disease that is a limiting factor for lemon production in the Mediterranean. Resistance or field tolerance are major goals for lemon breeders; however, there is scant information regarding the heritability of mal secco resistance in breeding populations. As with other vascular diseases, phenotyping is the bottleneck for ascertaining resistance and susceptibility, and a validated protocol for greenhouse phenotyping would be valuable to accelerate the selection of tolerant trees before field evaluation. We report phenotyping of 148 hybrids of Khasi papeda (Citrus latipes; tolerant to mal secco) × lemon (susceptible to the disease) in field and greenhouse conditions. Field evaluation was performed on all hybrids for 2 to 3 consecutive years on trees subjected to high natural-pathogen pressure. Detection of the fungal infection was performed by visual observation and real-time polymerase chain reaction (PCR). The first infections occurred ≈6 months after planting, but 2 years of observations were needed for a reliable estimation of susceptibility. The spread of the disease did not occur uniformly throughout the plot, with patterns of spread within rows, probably resulting from infections from plant to plant. The possible errors in the estimation of susceptibility as a result of the uneven distribution of infections in the plot were reduced by using more than one replicate tree per hybrid. The correlation between phenotyping scores and cycle threshold values was weak (r = –0.48, P < 0.001). Three years after planting, hybrids clustered into three groups—susceptible, tolerant, and intermediate—based on symptom progression. A subset of 65 self-rooted hybrids was also subjected to stem inoculation in an unheated greenhouse, with two to seven biological replicates per hybrid. Three months after inoculation, the samples were monitored for symptoms appearance and subjected to real-time PCR pathogen quantification. We observed a weak (r = 0.41) but significant (P < 0.001) correlation between phenotypes in the field and the greenhouse, indicating that, in our conditions, field evaluation remains the best method for phenotyping. However, artificial inoculations might help to discard the highly susceptible hybrids before field evaluation.