Walnut Anthracnose Walnut anthracnose is common on black walnut in Nebraska in areas where the weather is often warm and humid. It is caused by the fungus Gnomonia leptostyla . Spotting, yellowing, and premature leaflet drop caused by walnut anthracnose. Symptoms of walnut anthracnose begin to appear in late spring on leaves that have reached full size. Extensive leaf spotting causes yellowing, curling, and premature defoliation.
Lesions appear also on the nut husks as dark sunken spots.
Affected nuts may have reduced meat quality and may drop prematurely. The severity of walnut anthracnose tends to increase as the summer progresses, especially if rainfall is frequent. However, the disease usually does not significantly affect tree growth since most of the year's growth is complete by the time the symptoms become severe. Disease Development Anthracnose fungi overwinter in twigs and small branches on the tree and in leaves on the ground. The fungi are active during periods of mild winter weather, forming cankers and causing twig dieback.