Stony Pit of Pears

J. R. Kienholz.

Stony pit causes deformity and pitting of pear fruits. A roughened bark condition may accompany the pitting. Narrow yellow places along the vein-lets of the younger leaves appear in some varieties. Numerous "stone cells" beneath or surrounding each fruit pit and the hard structure of the fruit give the disease its name.

Research men have known of the existence of stony pit in Beurre Bose pears in Oregon and Washington since 1919 and in California since 1925.. Stony pit also occurs in New Zealand, Canada, England, and probably wherever Bose pears are grown. Surveys of orchards have disclosed that 1 to 70 of every 100 Bosc trees are affected; 10 to 80 percent of the cull Bosc fruits at the packing houses can be attributed to stony pit.

Stony pit was considered a form of drought spot before 1938, although various cultural practices failed to correct the trouble. Proof that the disease was caused by a virus was obtained in 1938, when previously healthy Bosc and Anjou trees produced pitted fruits after diseased Bosc buds had been inserted in their limbs in 1936. Because most of the commercial loss occurred in the Bosc variety up to 1950 and because the disease was prevalent only in Bosc, we believe that diseased trees originated largely in nursery plantings for which scion wood had been carelessly selected from diseased orchards.

Bosc is the only variety known to express complete symptoms of the disease fruit pitting, an oak-bark condition of the trunk and larger limbs, and a veinlet yellowing of younger leaves.

From 1 days to a month after petal fall, dark-green areas form just under the epidermis of diseased fruit. Lack of growth in those areas and the rapid development of surrounding healthy tissue result in deeply pitted or deformed fruit at maturity. Sometimes a greenish halo surrounds the pit; dark, sunken, circular areas, with raised centers of healthy tissue, may form.

The tissue at the base of the pits generally becomes lifeless. In severely infected fruits a concentration of the darkened spots occurs near or within the grit cell ring. The most striking feature of diseased fruit is the production of numerous sclerenchyma cells (grit cells) beneath or surrounding the pitted areas. Fruits bearing numerous pits become so gnarled and woody they can hardly be cut with a knife.

Sometimes only a few pits, or one, may form on a fruit, or pitting may be shallow and scattered on varieties like Anjou, Hardy, and Old Home. On those varieties the pitting generally is later in developing than on Bose; occasionally it appears only a month before harvest. Unless other tree symptoms are present, the mild fruit symptoms are easily confused with those caused by the tarnished plant bug (Lygus pratensis), boron deficiency, or cork spot.

A natural, angular, bark cracking occurs on healthy pear trees as they mature. A "measled" bark condition is associated with diseased Bosc trees after stony pit has been present in them for several years. Small pimples may appear on the bark of 1 or 2-year-old Bose twigs. Later in the season or the following year the epidermis cracks and the underlying tissue collapses. The cracking and shrinking occur in such a way that somewhat concentric, target-canker effects are produced on small limbs. Continued growth and cracking of the limb tissue causes older limbs to assume a ribbed appearance, sometimes called oak-bark. The trees reaching this stage of the disease have less foliage because the lateral buds fail to grow. Terminal shoots and fruit spurs bear most of the scant leaf surface. Such trees are more subject to winter killing than vigorous trees.

Oak-bark has been found only on scion wood of a top-worked tree. In others the rough bark may be present only on the trunk wood. The virus therefore may be carried in either unit, and although fruit symptoms appear after the second year from transmission of the virus to the healthy unit, bark symptoms appear only after several years of growth or not at all, depending on the varieties involved.

True leaf symptoms consist of narrow, chlorotic areas along the veinlets of the leaves. Varieties expressing this symptom usually begin showing the yellowing in younger leaves about June. The youngest two or three leaves on terminal shoots generally appear healthy, the three to five middle leaves show chlorotic veinlets, and the basal, dark-green leaves again appear healthy. As the middle leaves attain full maturity and take on the dark-green color, the virus symptoms gradually fade. An exception is the Forelle variety, in which leaf symptoms persist throughout the season.

A second, or false, leaf symptom consists of faint or definite mottling, usually toward the edges of the leaves. That may be the result, in diseased trees, of a partial plugging of the transporting tissues, so that leaves cannot obtain enough nutrients or water during critical stages of growth. This leaf symptom is not limited to stony pit trees and is no indication that the stony pit virus is present in the tree.

Each variety reacts differently to the stony pit virus. Bosc exhibits the complete symptoms on fruit, bark and leaves. Winter Nelis shows only fruit symptoms. Vicar of Wakefield expresses only leaf symptoms. Bartlett exhibits no symptom. It is known to carry the virus in a symptomless condition, however, for when healthy Bosc or Anjou are top-worked to infected Bartlett, the virus is transmitted to the scion varieties upon which typical pitting appears in the fruit.

Varieties of pears known to show fruit symptoms are Beurre d'Anjou, Beurre Bedford, Beurre Bosc, Beurre Clairgeau, Buerre Hardy, Cole Winter, Durandeau, Laxton's Superb, Old Home, Packham's Triumph, Patrick Barry, Pitmaston Duchess, Waite, and Winter Nelis.

Definite leaf symptoms have been found on Anjou (rarely), Bose, Doyenne du Comice, Forelle, Orel, Patten, some seedlings of Pyrus communis, and Vicar of Wakefield.

The oak-bark symptoms have appeared only on Bosc, although Anjou and Comice have sometimes formed a target-like cankering on trunks and scaffold limbs on trees known to have been infected for at least 10 years. It is possible that different strains of the virus exist which may cause mild leaf symptoms or severe fruit pitting on varieties not usually affected.

Graft inoculations have failed to yield definite stony pit symptoms in apples (Malus sylvestris, varieties Delicious and Yellow Newtown); flowering quince (Chaenomeles japonica); hawthorn (Crataegus douglasi); medlar (Mespilus germanica); mountain-ash (Sorbus sitchensis); quince (Cydonia oblonga); rose (Rosa multiflora); and serviceberry (Amelanchier florida).