Plant Disease Management Handbook

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.

Cause Phyllactinia guttata, a fungus that overwinters on fallen leaves and fallen infected twigs. Erysiphe corylacearum has been reported on Corylus californica. The conidia are readily wind disseminated. Chasmothecia develop within the external hyphae and form appendages late in their development.

Note: Apple mosaic virus (ApMV) and Tulare apple mosaic virus (TApMv) may produce irregular yellow line patterns, whitish vein banding, or irregular yellow flecks in hazelnut leaves. Symptoms might resemble herbicide injury and spread is very slow. Many cultivars show no symptoms. The disease has not been found in commercial orchards in the Pacific Northwest but is common in Europe. Does not seem to affect tree growth or yield.

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Algae, Lichens, and Mosses on Plants

Cause There are a number of fungi that can cause leaf spots on hazelnut. These are of minor importance in commercial production and are more common in native hazelnuts, hedgerows and abandoned orchards. Anthracnose is caused by Piggotia coryli and leaf blister (and leaf curl) by Taphrina coryli.

Cause "Mold" is defined as any visible growth of mold either on the outside or inside of the kernel. In practice, it means any white, fuzzy mycelial growth is classified as 'mold.' Many different fungi are easily isolated from the shell or kernel, with surface sterilization, with or without any specific symptoms, before or after harvest. In Oregon, Penicillium spp., Aspergillus sp., Cladosporium sp.

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Hazelnut Cultivar Susceptibility

Cause Unknown. No investigations have been reported. The cultivar Barcelona tends to show it more than other cultivars, but it is on fewer than 5% of the catkins. The cultivar Yamhill routinely has catkins with kinks and brown tips. The problem is widespread throughout the Willamette Valley in Oregon and has been found in British Columbia. No adverse yield effects have been linked to this problem. Although the big bud mite will blast open dormant buds, no mites have been associated with these deformed catkins.

Cause Unknown. Possibly a physiologic problem because no insects or pathogens have been associated with this problem. Severity of the problem changes from year to year.

Cause The official definition of blanks is a shell containing no kernel at all or a kernel filling less than one-fourth the capacity of the shell. Pollination stimulates the shell to develop, but the kernel, for unknown reasons, fails to develop normally. The kernel either fails to grow at all, or it starts to grow but then aborts. Pollination failure is not the cause; if pollination had failed, the shell would not have developed at all.