Xanthomonas arboricola pv. juglandis
The bacterial disease Xanthomonas arboricola pv. juglandis, is called walnut blight. The bacterium overwinters in infected buds and catkins. Buds with the highest bacterial populations are the ones most likely to develop blight. During early spring growth, bacteria spread along developing shoots and nuts. There seems to be very little secondary spread to other shoots and trees by raindrip. This results in local infection centers within a tree or orchard. Frequent, prolonged rain, just before and during bloom and for about 2 weeks after, result in severe blight outbreaks within these local infection centers. This is when nuts are most susceptible.
On leaves, infection appears first as reddish brown spots, on the stems as black, slightly depressed spots often girdling the shoots. Young, infected leaf and catkin buds turn dark brown or black and soon die. The disease is serious on nuts, where it causes black slimy spots of varying sizes. The organism penetrates the husk, the shell, and occasionally the edible meat. Late-season infection produces black rings on husks.
The bacterial disease is favoured by warm, moderate seasons with temperatures of 10-28°C, light and frequently rainfalls with heavy winds and dews. Local dispersial is possible by rainsplash.