Phytophthora on Juglans spp. (walnuts)

The genus Juglans consists of about 21 species occurring over North and South America, Europe and Asia (http://www.discoverlife.org). Five main species can be considered as representative of the three main continental blocks since J. hindsii and J. nigra are from North America and they are included in the black walnut group, J. mandshurica and J. sieboldiana are from Asia and they are included in the Asian butternuts, while J. regia, known as common walnut or English (Persian) walnut, can be included within the Asian-European species. Juglans regia is the most widely cultivated walnut species worldwide, either for fruit or timber production. The origin of Persian (English) walnut is reported to extend from Asia over Easter Europe such as the Balkan and the Carpathians. Over 10% of potential walnut production is lost due to pests and diseases annually. For many of the major diseases, chemical forms of control are either unavailable or ineffective. Phytophthora root and crown rot can be considered as an increasing source of loss in the major walnut growing areas in Europe as well as in America. The incidence and severity of Phytophthora root and collar rots are closely linked to soil moisture. For some Phytophthora spp. and some rootstocks (i.e. J. regia) the duration of soil saturation dramatically affects the disease which severity increases with the duration of the saturation.