Mirko, the curling of the leaves may be a water stress issue so if the soil tends to be dry, make sure its getting enough water. Also, since we have had a relatively mild winter this year the leaves just now may be experiencing seasonal curling symptoms. According to the UCCE Orange County Master gardeners the leaves can cup downward in the late fall or early winter and appear normal in the spring http://mgorange.ucanr.edu/Edible_Plan...