Minireview 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. Xhanthomonas 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 other bacterial diseases affecting walnut. Being polyphenols involved in tolerance to bacterial diseases, it has been shown that the gene jrPPO1 is the sole polyphenol oxidase PPO gene in walnut able to encode a jrPPO enzyme 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 progenies, such as hair density or wax thickness could act as shields against Xaj infection. These changes could explain segregation of tolerance in walnut progenies. A range of tolerance to hazelnut blight, induced by Xantomonas campestris pv. corylina, has been observed in France in diverse nursery plants. Pseudomonas syringae pv. avellanae, observed first in northern Greece, is affecting hazelnuts also in Italy, where this disease it elicits is known as ‘moria’ (‘death’). No resistance or tolerance against ‘moria’ has been found.