Field evaluations have shown that Satsuma mandarin (Citrus unshiu) ‘Okitsu’ is one of the mandarin cultivars that shows substantial resistance to Xanthomonas citri subsp. citri (X. citri), the causal agent of citrus bacterial canker disease. However, the mechanisms underlying this resistance are not well understood. In this study, we have shown that ‘Okitsu’ leaves are nevertheless susceptible to X. citri infection during a period of their development; however, this period is shorter than that seen in the susceptible mandarin ‘Clemenules’ (C. clementina). Under controlled growth conditions, the resistance of ‘Okitsu’ to X. citri was associated with the age of the leaf and was evident in spray-inoculated plants but not in those inoculated by infiltration. Furthermore, X. citri showed reduced attachment and biofilm formation in ‘Okitsu’ leaves compared with ‘Clemenules’. Taken together, our data suggest that structural features of the ‘Okitsu’ leaf surface, such as the physical properties of the cuticle, are involved in the resistance to X. citri.

The aim of this study was to characterize the phenology of different sweet orange, tangerines and tangerine hybrid varieties growing under the temperate climate conditions of Santa Fe Province, Argentina. Phenological stages were observed weekly during five consecutive years using a BBCH (Biologische Bundesanstalt, Bundessortenamt and Chemical industry) scale adapted for Citrus trees. All varieties showed a winter rest period from June to August. ‘New Hall’ and ‘Navelina’ varieties were the first to reach sprouting stage, whereas ‘Okitsu’ was the last. Inception of flowering occurred from August 13th to September 6th; and full bloom from September 12th to October 2nd. Fruit harvest started with the ‘Okitsu’ cultivar in March, and continued over a 7-month period. Interannual variation for inception of sprouting was high (44 days), and sprouting was correlated with both thermal accumulation (above 13ºC) and the amount of solar radiation measured during July (p<0.0001; r2=0.79). Navel oranges and the ‘Murcott’ hybrid bloomed 5–15 days earlier than other varieties, increasing probability of damage by late frosts.

ABSTRACT:The crop load level of an apple (Malus × domestica Borkh.) tree impacts fruit yield and quality parameters, tree vigor and biennial bearing. The optimal crop load is that which allows for consistent annual cropping and fruit quality acceptable to the market. We evaluated the effect of crop load on yield and fruit quality of two low-chill apples cv. 'Caricia' and 'Eva', growing in a mild winter area. During 2010 and 2011 crop load was manually adjusted from 2 or 3 to 17 fruits cm −2 of trunk cross-sectional area (TCSA). Fruit yield was positively related to crop load in both cultivars but mean fruit weight diminished as the crop load increased. For both cultivars, the production of non-commercial and small-sized fruit increased, whereas production of middlesized fruit diminished as the fruit load increased. Shoot length was not affected by crop load in 'Eva' whereas it was reduced in 'Caricia'. Red skin color (RSC %) had a quadratic response to crop load in 'Caricia'. On the other hand, the RSC % of 'Eva' fruit was adjusted to a negative logarithmic model as an effect of crop load increment. No biennial bearing was observed in either cultivar. This research study suggests that the maximum limit of crop load for both cultivars is 7 fruits cm −2 of TCSA, and the lower limit of crop load was 3 fruits cm −2 of TCSA for 'Eva' and 5 fruits cm −2 of TCSA for 'Caricia'.

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