Although apple- and pear scab are very similar in all respects, they are not the same disease. The general opinion prevails among growers that because pear scab looks like apple scab, and because the two diseases bear similar names and are controlled in approximately the same way, the one is identical with the other. But the causal fungus in the one case is specifically different from that in the other, and therefore a scabby pear - tree is in no way dangerous to an apple tree and vice versa.
Pear scab is perhaps as well known to growers of this fruit as any other disease except Fire Blight. It has a wide range, but is controlled with less difficulty than Fire Blight, and consequently is not nearly so greatly feared. On the other hand, it does considerable damage. For example, it is estimated that pear-growers in the State of Ohio lost in 1905 at least $50,000 through this disease alone. In practically all regions where it occurs it is second to Fire Blight in importance. In California losses are put at 25 to 100 per cent in unsprayed orchards. In all carefully sprayed orchards the losses are much less, and often not appreciable. Damage is wrought in nurseries as well as orchards.
The quality of the fruit, rather than the quantity, is affected. The taste is not altered noticeably, but the appearance and keeping qualities, as well as canning and drying qualities, are considerably impaired. Affected fruit sometimes falls and is then worthless. When blossom - pedicels are affected, a loss of the set of fruit results. This happens frequently.
Not all varieties of pears suffer alike. Scab is notably common on the Flemish Beauty, Winter Nelis and Easter Beurre. The Duchess, Seckel and Summer Doyenne are among the other susceptible varieties. Many others are at times scabby but not so frequently nor to such a marked extent as in the case of those varieties already mentioned. The Kieffer shows considerable resistance in some parts of the country.
Pear scab occurs practically everywhere the fruit is grown. It was found in Belgium in 1832, but was not reported as doing great damage until 1875. Elsewhere in Europe scab prevails commonly on the pear. In the United States outbreaks occur in New England, the Middle West and along the Pacific Coast. It was particularly destructive in 1906 over the country generally; in 1898 it was epiphytotic in Ohio; in 1902, 1910, 1912 and 1915 it was unusually prevalent in New York; while it prevailed extensively in California in 1904, 1905 and 1906.
The disease is found on the fruit, leaves and twigs. On the fruit (Fig. 92) the spots are at first olivaceous, velvety and circular in form. Later in the season this velvety aspect becomes corky and the skin is cracked, sometimes in a T - shape fashion (Fig. 92). At times growth of the young fruit is halted, and so when such pears are mature they are considerably distorted. Affected fruit may drop when the size of a cherry.
Fig. 92. - Pear scab.
Lesions also occur on the fruit-pedicels. On the leaves the disease exhibits itself as spots very much like those on the fruit. Lesions on the foliage are abundant and conspicuous on the lower surface. This is particularly true on certain varieties, as, for example, Winter Nelis. Pear scab is common on the twigs. In this respect the disease displays a difference from apple scab; the latter affects twigs rarely in comparison to pear scab. Young twigs are affected chiefly, but the lesions are not so conspicuous as on the fruit and leaves. The spots suggest the appearance of a scale insect. On one- and two-year-old twigs the affected areas are velvety and enduring, whereas those on older twigs are soon lost to view, the bark being sloughed off and replaced by healthy tissue. Not infrequently the blossom stalks are affected, in which case a dark - brown spot is produced and the young fruit falls; thus the young fruit fails to set.
The scab spots already described are composed chiefly of mycelium of the fungus Venturia Pyrina. Numerous radiating and branching hyphae make up the velvety layer visible to the naked eye. Before the fungus completes its growth on the various organs attacked, dense erect conidiophores arise from the mycelium. Within a short time after their development numerous conidia are formed. These spores are carried to other susceptible organs of the pear where new spots are initiated. Spores which by any chance fall on the apple do not produce the disease. Those which fall on the pear begin their development by germinating. Mycelium is soon formed and a scab spot is perceptible. If a young fruit be attacked, the cells beneath the lesion cease growth, as evidenced by the dwarfing of the fruit. This indicates strongly that there is a drain on the cells in the infected region. The fruit reacts against the fungus by forming cork, which is most evident in old scab spots where the fungus has ceased vigorous development. However, much of the food for the fungus no doubt comes from the attacked cuticle.
Infection of the various parts continues throughout the growing season. In the fall the affected leaves drop to the ground, carrying the fungus with them. Soon the mycelium changes its superficial habit and permeates the entire leaf. Before cold weather, perithecia begin their development within the old dead leaf-tissues. These bodies remain immature until spring, when they resume growth. This begins at least by the time the pear-tree starts into growth. By the time the blossom-buds are showing white the perithecia contain mature asco-spores which are discharged during periods of moisture. Their ejection is accomplished with force enough to carry them into the air, where they are easily caught by the wind and are blown to the opening buds. The germination of these spores finally results in infection. The period over which ascospores are discharged is not definitely known, but reasoning from the known facts concerning the apple scab fungus the period probably extends over several days.
The pear scab fungus also passes the winter on the twigs. The mycelium and conidia remain alive from autumn until spring, when new infections are initiated. Twigs are commonly affected, so that this method of hibernation is unquestionably of considerable importance. It is not known nor believed to be true that the fungus winters over on fallen fruit.
Pear scab, although found everywhere, can be controlled effectively. That it is not held in check in many orchards where spraying for it is done is no indication that it cannot be prevented. Time and thoroughness of the applications of fungicides are of prime importance.
The number of sprayings will depend somewhat on the history of the management of the orchard in the past. If the orchard has been well sprayed, three applications will suffice under ordinary conditions. If spraying has been neglected in , the past, then more than three applications will be necessary on account of the constant source of trouble on the twigs. A dormant spraying is advised for the far West. This is done late in the winter to kill the fungus on the twigs. A second dormant spraying is recommended in California, to be made just before the buds swell. Other applications follow: (1) a few days before the blossoms open; blossom-buds exposed and separated; (2) when the blossoms have for the most part fallen; (3) two weeks after the second application. In the East the two dormant sprayings advised for California conditions are not practiced for scab, although many pear orchards are sprayed just before the buds swell for blister-mite. The value of this application for scab needs investigation. Lime sulfur 1-50, or bordeaux mixture 3-3 - 50, may be used; the latter is much less desirable on account of the russeting which it produces on the fruit. Since the fungus hibernates in the twigs and fallen leaves, it is advised that in pruning and cultivation attention be given respectively to the removal of badly infected twigs and to the burial of old leaves.
Smith, R. E. Pear scab. California Agr. Exp. Sta. Bul. 163: 3 - 18.
1905. Clinton, G. P. Report of the botanist. Pear. Scab. Connecticut Agr. Exp. Sta. Rept. 1904: 323-324. 1905. Wallace, E. Lime sulfur as a summer spray. Control of pear scab.
Cornell Univ. Agr. Exp. Sta. Bul. 289: 160-162. 1911. Barrus, M. F. Diseases of pears. Pear scab. In The Fruit Industry in New York State. New York Agr. Dept. Bul. 79: 1048 - 1049.
Cornell Univ. Agr. Exp. Bul. 145: 616 - 620. 1898. Jackson, H. S. Diseases of pomaceous fruits. Pear diseases. Scab.
Oregon Crop Pest and Hort. Bienn. rept. 1911-1912:246 - 247.