Cause

• Black walnut blight is caused by the bacterium Xanthomonas campestris pv. juglandis. The bacteria infect the buds and catkins, or male flower spikes, in the fall and reside in them until early spring growth. Then the disease spreads along the new shoots and developing nuts. It affects every green part of the tree, but its economic importance lies in the damage it does to the edible nut meat. Infected trees produce fewer nuts and lower-quality nuts than healthy trees.
  
  

Signs

• Dark spots on the buds and catkins in early spring are the first sign of black walnut blight. The disease can spread to new shoots where it encircles the twig and prevents further growth. Reddish spots on leaves and catkins eventually turn black and the infected parts die. Infection of the nuts starts with slimy dark spots on the outer husk that can penetrate beneath the shell to infect the meat. The infection spreads more rapidly during periods of heavy rain.
  
  

Non-pesticide Control

• Walnut blight can be controlled without the use of pesticides if the infection is treated early. Careful inspection of trees early in the growing season can reveal the presence of walnut blight on early buds and shoots. Pruning out infected parts can eliminate the infection on individual trees. Cutting out the infection on a badly infected tree or on hundreds of infected trees is time consuming and labor intensive, so proper pesticide use is often the best method of controlling the blight.
  
  

Pesticide Control

• Pesticides containing copper are among the most effective treatments for blight-infected walnut trees. Some strains of copper-resistant bacteria have emerged and it may be necessary to combine copper treatment with other pesticides such as Manex for effective blight control. It is best to treat trees when about 30 percent of buds have opened and then to repeat treatments every 7--10 days until all of the buds are open. Since prolonged rain spreads the infection, pesticide use can be reduced in drier years.
  
  

Safety Considerations

• When treating black walnut blight, always observe proper safety precautions. Wear appropriate safety equipment when using pruning equipment, climbing ladders or applying pesticides. Read directions and warning labels on pesticides and follow instructions carefully. Do not apply pesticides in higher concentrations or more frequently than labels recommend. Be aware of the "Restricted Entry Interval" indicated for the pesticide, which is the minimum amount of time that should elapse after application of the product before people are allowed back into the area for further work.