Cause A graft-transmissible agent yet unknown. Also called stubborn, or Dilly disease, the latter for the Oregon town near which the disease was first found. Causes investigated include soilborne fungi, nematodes, and nutrition. The disease is similar to hazelnut yellows, caused by the clover yellow edge phytoplasma. The disease spreads very slowly in the field, possibly through root grafts. The cultivars Barcelona, Clackamas, and Du Chilly are the only cultivars known to be affected.
Symptoms Early symptoms include dull yellow-green, smaller than normal leaves. Leaves are very small and sparse in succeeding years. Shortened internodes and dwarfed foliage give branches a bottle-brush look. If trees set nuts they are often blanks. Roots lack fine root hairs. Symptoms may be mild to severe. Trees may have symptoms for years with occasional branch dieback. Trees eventually decline and die over decades.
Cultural control Remove affected trees.
Reference Jomantiene, R., Postman, J.D., Montano, H.G., Maas, J.L., Davis, R.E., and Johnson, K.B. 2000. First report of clover yellow edge Phytoplasma in Corylus (hazelnut). Plant Disease 84:102-102.