walnut husk fly

Symptoms

Black sticky staining on outer nut husks (the diagnostic visual symptom), husks remain stuck to nuts at harvest instead of opening cleanly, kernels inside stained brown with off-flavor, larvae visible inside soft rotted husk tissue, infested nuts unmarketable.

Easily confused with

• walnut blight

  How to tell them apart: Xanthomonas arboricola pv. juglandis (walnut blight) is a bacterial disease: look for small, oily, water-soaked black spots that start on the husk surface, leaves, catkins and young shoots, often with a translucent yellow halo, and that appear early in the season after wet spring weather. The blackened husk tissue stays firm and sunken with no internal tunnels, and you may see dark angular leaf spots and dead catkins on the same tree. Rhagoletis completa (walnut husk fly) is an insect: damage appears later in summer/autumn as the husk turns soft, mushy and slimy-black, and cutting it open reveals white legless maggots and brown rotting pulp inside, often with the husk stuck tightly to a stained shell. The decisive clue is internal: blight = firm sunken surface lesions with no larvae; husk fly = soft rotting interior packed with maggots.