After symptoms appear the tree remains infected for the rest of its life. The severity of fruit pitting may vary somewhat from season to season, but it never disappears. Occasionally an individual leader branch may bear severely pitted fruits for years, while the other fruit on the tree appears healthy. The virus seems to become more potent in rapidly growing young trees and milder in very old trees that make little terminal growth. Dehorning, or cutting off diseased limbs, is no cure for stony pit. In fact, the fruit and leaf symptoms usually appear it, more severe form after such treatment.

Severely infected Bosc trees, with oak-bark symptoms and sparse foliage, should be eliminated from the orchard. as they are no longer profitable. When it was discovered that Bartlett showed no symptoms, even though infected by the stony pit virus, it was suggested this variety might be top-worked to less severely diseased Bose trees. Many growers followed this suggestion, despite the hazards of leaving diseased but symptomless varieties in the orchard.

S. G. Babson, of Parkdale, Oreg., has top-worked more than too pear trees affected with stony pit to the Bartlett variety. In no case have stony pit symptoms appeared in the Bartlett fruits on those trees in 15 seasons. His method of top-working has been to place 50 buds or more in the smaller limbs, eventually retaining only the framework of the diseased trees. To change a variety completely over to Bartlett has sometimes required two additional years of supplemental budding to complete changing limbs missed in the first budding.

Studies of the production of such top-worked trees, made by G. G. Brown, of the Hood River Branch Station of Oregon State College, indicated full production was usually attained during the third or fourth year. The average production of the top-worked trees after the fourth year was higher than unworked Bartlett trees of the same age: Mr. Babson obtained in 4 years what would have taken 12 to 14 years to accomplish by replanting healthy trees. As the trees were bearing full crops during the high-price cycle of the war, the gain was amply justified from an immediate economic standpoint. But because of greater mortality of the trees to winter injury and because a virus reservoir remains in such trees as a source to infect healthy trees in the orchard, the advisability of top-working diseased trees can be seriously questioned as a sound practice in maintaining a productive orchard.

FEW NEW PLANTINGS of Bosc pears have been made in the Pacific Coast States. Many healthy and diseased Bosc trees have been top-grafted to Bartlett. Infections resulting from top-working diseased Bosc to susceptible varieties have occasionally been observed in the varieties Anjou, Hardy, and Winter Nelis. Natural infections on those varieties were largely a curiosity up to 1945. About that year, fruit growers from various districts began calling attention to the pitted fruits on an occasional Anjou pear tree in their orchards. Since Anjou is one of the leading varieties of winter pears grown in the Pacific Coast States, the increase of stony pit on the variety presents a real threat.

Pear trees in three orchards at Hood River have been mapped periodically since 1936. One orchard, planted originally to Bosc pears in 1913, developed considerable stony pit before 1936. Most of the trees were subsequently top-worked to either Anjou or Bartlett. In this top-worked block of trees, 44 percent of the Anjous developed stony pit and 15 percent of the unworked Bose became diseased. None of the Bartletts top-worked to previously diseased Bosc showed symptoms of stony Pit. Severe winter damage in 1948-1949 killed many of the top-worked trees, or parts of them, so that the original orchard contained many inter-planted young trees in 1952. An adjacent part of the same orchard, planted originally to Anjous and Bartletts, remained free from stony pit until about 1945. By 1949 eight Anjou trees were diseased; 16 Anjou trees showed stony Pit symptoms in 1951. Nine percent of the original Anjous became diseased after 1945. Chances for new infections in the orchard were above average because of the presence of many diseased trees nearby.

The two other orchards mapped probably represent more usual conditions. Stony pit was not found in the two orchards, which had 858 and 669 pear trees, before 1940. Only one Bosc tree existed in the orchards, and even though it remained healthy until 1950, it was removed. No known source of the stony pit virus is present within one-half mile of the two orchards. In 1949, three trees, or 0.6 percent, of the Anjous had developed stony pit; that increased to 1.7 percent by 1951 in the one orchard. In the second orchard 1.1 percent of the Anjous were diseased in 1949; that increased to 2 percent by 1951.

We know of no insect vector responsible for spreading the stony pit virus, although several suspected species of leafhoppers need to be investigated. The point to be emphasized is that stony pit is now common enough in the Anjou variety that the practice of cutting scion wood from random trees in an orchard presents a hazard in the development of disease-free nursery stock for the pear industry. Nurserymen should select mother trees during the growing season that are known to be free from disease and true to type from which to select their scion wood. Fruit growers should eliminate all virus-infected trees from their orchards whenever it is practical to do so. The chances of infecting healthy trees increases greatly as the virus source becomes more common. A season extremely favorable for the spread of the stony pit virus could be disastrous to the individual grower or to a whole district.

J. R. KIENHOLZ, a pathologist in the division of fruit and nut crops and diseases of the Bureau of Plant Industry, Soils, and Agricultural Engineering, is a native of Washington. His advanced training was obtained at Washington State, Michigan State, and Oregon State Colleges.