Xanthomonas campestris pv. juglandis overwinters in the walnut buds, catkins, leaves, and infected fruit still hanging on the tree, and twig lesions.[2] During this time, these buds may remain asymptomatic even just a few days before budburst. In the spring, when the buds begin to open, the bacteria invade and begin infecting the internal bud and developing fruit. Wind and rain splash play a large role in spreading this disease, as rain splash makes it easier for bacteria to move from infected plant tissue to healthy buds. Aerial dissemination of pollen from infected catkins have been recorded to infect the receiving bud/tree in France, but conflicting data has been shown for trees in California where researchers did not find a link between peak disease development and peak pollen shedding.[1] Once infected through the pistils, flowers, or fruit, symptoms of the disease may begin to take shape, such as dark green, circular water soaked areas on the leaves, developing lesions which grow in size, walnut shell and fruit stunting, and/or walnut shell staining.[1]
Lesions which appear on the leaves may grow larger and the center may turn black or translucent. This area may then ooze bacterial discharge and polysaccharides and further infect other tissue it may travel to by rain splash. The size of the lesions vary in young and more mature leaves. Young leaves have lesions from one to a few millimeters while older leaf lesions can range from a few millimeters to half of the leaf.[1]
Premature nut fall occurs when infection takes place before the formation and hardening of the shell tissue and is able to rot and shrivel the kernel. The tree then sheds the diseased nut