BACTERIAL DISEASES OF WALNUT AND HAZELNUT AND GENETIC RESOURCES
In 2008, a global production rounding 1,724 and 1,053 million metric tones respectively for walnut and hazelnut has been estimated. Bacterial diseases are threatening these nut crops all over the World. Xhan- thomonas arboricola pv. juglandis (Xaj) is the agent of walnut bacteriosis, and has been associated also to brown apical necrosis. Several walnut genotypes have shown a range of tolerance to this disease in diverse countries. Deep bark canker (Brenneria rubifaciens) and Shallow bark canker (Brenneria nigrifluens) are the oth- er bacterial diseases affecting walnut. Being polyphenols involved in tolerance to bacterial diseases, it has been shown that the gene jrPPO1 is the sole polyphenol oxi- dase PPO gene in walnut able to encode a jrPPO en- zyme that is expressed in the leaves, hulls and flowers of walnut trees. It can also happen that some modifications in the anatomical traits of the epidermis of walnut prog- enies, such as hair density or wax thickness could act as shields against Xaj infection. These changes could ex- plain segregation of tolerance in walnut progenies. A range of tolerance to hazelnut blight, induced by Xan- tomonas campestris pv. corylina, has been observed in France in diverse nursery plants. Pseudomonas syringae pv. avellanae, observed first in northern Greece, is af- fecting hazelnuts also in Italy, where this disease it elicits is known as ‘moria’ (‘death’). No resistance or tolerance against ‘moria’ has been found.