DISEASES. Possibly the present most serious limitation to commercial production of filberts in Virginia is the Filbert Blight or Black Knot (Cryptosporella anomala. (PK) Sacc.). While this fungus results in little damage to native species (C. americana) it does spread rapidly and with serious results to European varieties in the State. Possibly the seriousness of the disease has been lessened by the eradication of native hazel plants on roadsides, fence rows, and in the wild nearby, which serve as hosts for the disease.
It is present on the American hazel, but does little damage to the plant. The disease, however, as mentioned, is a serious menace to either European varieties or to the present hybrids resulting from C. americana x C. avellana. The control to date is to prune off and burn affected parts. Mr. George Slate has mentioned that Mr. S. H. Graham of Ithaca, New York, has a number of hybrids between C. americana and C. avellana that have been subjected to severe attacks of Filbert Blight and a few of these have to date escaped, although the others have been destroyed by blight.
The bacterial blight present on the Pacific Coast apparently does not occur in the East.
INSECTS. A second limitation is the problem of the attacks of insects. Dodge and Rickett (1948) report that Corylus may be affected by a leaf-damage from the feeding of leaf-hoppers (Phepsins ishida; P. tinctorius), which may involve less than half the leaf or may extend to the entire leaf. The first leaves to be infested are those next to the ground, which are affected early in July. Most of the damage ceases by the first week of August. Control is by spraying with nicotine sulphate and soap on the undersides of the leaves in late June or early July, repeating at the end of a week.
Certain nut weevils (Balaninus spp.) attack the native hazels, but Slate (1930) reports they do not attack the European filbert (C. avellana). Mr. Slate reports that in Geneva where nuts are carefully picked up they do not have much of a problem with weevils.
Dr. Crane reports that the Japanese beetle severely damages the filbert. While the Japanese beetle has not yet become widely established in Virginia, it undoubtedly will eventually become a problem throughout this state. The Japanese beetle can be destroyed by using four pounds of 50% wettable DDT or two pounds of actual DDT per 100 gallons. Such sprays should be applied as the Japanese beetles begin to cause injury, and usually two applications may be sufficient.
Mr. G. F. Gravatt has reported that his filbert plantings, surrounded on three sides by woods, are badly attacked by stink bugs that sting the nuts. DDT as suggested for Japanese beetles may also be used for stink bugs.
Another serious insect pest on hazelnut is the curculio. Clean cultivation has been reported as a supplementary measure for curculio control, as they depend, upon unbroken soil in the fall for their metamorphosis. Some hybrids are reported as being relatively immune to the attacks of curculio (Weschcke, 1946). Benezene hexachloride has shown promise with other plants in curculio control and may have possibilities on the filbert.
LACK OF HARDINESS. A third limitation has been lack of hardiness in the case of European varieties. With the European varieties the staminate or the pistillate flowers or both are likely to be killed by winter temperatures. In fact, occasional unduly low winter temperatures may kill the tree tops or even the tree trunks to the ground. The Winkler variety (C. americana) has been reported as more hardy in New York State than the Barcelona (C. avellana) or the Jones hybrids (C. americana x. C. avellana) (Ross Pier Wright, 1944).
Under western New York conditions, Slate (1930) reported that the blooming period starts about March 20 to 25 at Geneva, and lasts about a month. In central Virginia this may well be several weeks earlier. Slate (1930) also reports that the flowers in bloom will withstand considerable frost, and that even with temperatures of 16°F. during the blooming season, neither female nor male flowers, may be injured. Nevertheless, with filberts coming into bloom in late February to early March, they would be subjected to temperatures that might result in injury especially to the catkins.