Occurrence and importance
Stony pit, a virus disease, has been known to occur in the Bosc variety of pear in Washington and Oregon since 1919, and in California since 1925. The disease also occurs in Canada, England, New Zealand, Australia, South Africa, and perhaps wherever Bosc pears are grown. In the United States, however, the disease is not known outside the Pacific Coast area.
Stony pit causes serious losses in pears of the Bosc variety. In the past it was estimated that from 10 to 80 percent of the Bosc pears culled at the packing house were rejected because of that disease.
Stony pit is also found in pears of the varieties Winter Nelis, Anjou, Hardy, and Forelle; but losses in these are generally negligible. Bartlett is a symptomless carrier of the virus, but Waite, thought to be a cross of Bartlett, is susceptible.
Pears with stony pit are rarely seen on the market.
Typical stony pit symptoms are strikingly different from most other diseases of pears. On Bosc and to a considerable extent on Anjou, the pits are deep with a very narrow base giving the appearance of cone-shaped pits (top photo). Fruits may develop a single pit or numerous ones. Those observed on the market had only a few pits. The pits may extend 1/4 inch or deeper into the flesh. At the base of each pit, the stone cells have fused into a stone-like mass (bottom photo) that is extremely difficult to cut. If an attempt is made to cut down through a pit, the stone is pressed deeper into the flesh instead of being cut apart. The color of the pitted tissues about the stone is brown, but there is not extensive discoloration of the surrounding tissues. In the orchard, seriously affected fruits may be found that are curved and malformed. These fruits, described as being woody, are tough and difficult to cut.
As in most diseases, there is a range of symptoms associated with the pits. A single fruit may show many typical pits, but it may also show some that developed later or for some reason are not typical in depth and external appearance. In these the stone cells are less fused, and the mass is smaller and not as hard. Although typical pits have been seen on Anjou on the market, this variety tends to be less affected than Bosc. In general there is a tendency for pits to be shallow, scattered, and fewer on Anjou, Hardy, and Old Home as compared to those on Bosc.
Stony pit is caused by a virus, which can be transferred from one susceptible variety or tree to another by grafting. If diseased Bosc buds are placed in healthy Anjou trees, most fruits on those trees become pitted by the end of the second season. Such control of the disease as is possible depends on the use of budwood from healthy trees only and the removal of severely affected trees. The Bartlett variety is a symptomless carrier of the virus, and its use is possible where the disease prevails. Less severely affected trees of susceptible varieties may be topworked to Bartlett if compatible. A record should be kept of such top-worked trees, since they form a virus reservoir that is always potentially dangerous.
For control of stony pit, follow the recommendations of the State agricultural extension service or experiment station.