This disease does its greatest damage in the central Mississippi Valley and is as well known in that section as Bitter Rot. Unlike Bitter Rot, however, apple blotch is less sporadic in its nature, appearing from year to year and being gradual in its destruction. Over a long period, apple blotch, because of its annual recurrence, is in the end more destructive than Bitter Rot and apple scab combined. This is particularly true in the Middle West and Southwest in such states as Arkansas, Kansas and the southern portions of Missouri, Illinois and Indiana. In northwestern Arkansas and southern Missouri 75 per cent of the apple-crop is commonly affected. It is almost as serious in the other regions indicated. In Benton County, Arkansas, the loss to apple - growers from blotch on the fruit in 1906 amounted to $950,000. The loss was about the same in 1907. In addition to rendering fruit unfit for market, the twigs are badly cankered and the badly affected foliage drops prematurely. The greatest loss occurs on unsprayed susceptible varieties. Among commercial apples which suffer most may be noted the Ben Davis, Missouri and Limbertwig; other varieties but slightly less susceptible are Smith Cider, Maiden Blush and others. The York Imperial, Winesap, Grimes and Jonathan are only slightly injured by blotch. In the East, blotch is rarely destructive and has never been reported from northeastern United States.
Apple blotch, also known as the star-fungus, fruit blotch, cancer, Leaf Spot, and incorrectly as black scab and late scab, first attracted attention from Maryland and Texas in 1897. A few years later the same disease was reported from Illinois and in 1903 it was of serious extent in Missouri. It is said that the pathogene invaded Kansas about 1905. Symptoms.
The disease first shows on the fruit (Fig. 14) about six or eight weeks after the blossoms fall. It is then evidenced by a very small, inconspicuous light-brown speck, which under slight magnification has the appearance of a stellate collection of brown fibers just beneath the epidermis. The spots enlarge radially, attaining a diameter of about one-fourth of an inch, and becoming darker in color. The advancing margin is irregular and has a fringed appearance (Fig. 14). The lesions show some interesting variations with varieties. Certain spots may or may not show a fringed margin, and may first be noticed as a dark, slightly sunken area. Not infrequently, the first indication of blotch is in the form of dark-brown, irregular, umbonate elevations. Such a condition occurs on the Ben Davis. About the margin of older, sunken spots is sometimes seen a reddish border. On very young apples the points of infection show as small water-soaked areas, and in wet weather there may be a yellowish, gummy exudate. On some varieties at least, like the Ben Davis, the skin ultimately cracks (Fig. 15). The crevices, although usually about one - half of an inch long, may girdle the fruit and extend to the core. Often these cracks intersect, forming a cross (Fig. 15). Within a few days after the blotches become visible, black pimples are developed on the lesions (shown in Fig. 15). These are the fruiting bodies of the pathogene and they may number from three to several on each diseased area. They are either scattered promiscuously over the lesion, or are arranged around the margin. A large percentage of affected fruit drops prematurely.
Fig. 14. - Apple blotch, early stage.
Apple blotch also affects the fruit spurs, twigs and rapidly growing shoots, showing itself in the form of characteristic cankers (Fig. 16). Larger limbs are not commonly attacked. On the fruit - spurs the cankers are at first purplish or blackish. The center turns brown with age, the margin remaining unchanged in color. The resulting lesion is small and rather inconspicuous, with a crack along the margin. On water sprouts, and on other rapidly growing shoots, the cankers have much the same appearance as just described, but are longer, often measuring an inch or more in length, and sometimes girdling the stem. Longitudinal cracks appear not only along the edge but also through the cankered area, eventually giving the surface a roughened aspect. This is especially noticeable on cankers two or three years old. The diseased area may extend along the margin at several points, resulting in a lobed or somewhat concentric appearance. Cankers may or may not surround the limb; in the latter case the margin is marked by a crevice beneath which originates a callus.
Fig. 15. - Apple blotch, late stage.
Affected leaves show spots within two months after the petals have fallen. The lesions are irregular in outline, light-brown to yellowish or whitish in color, and measure one-sixteenth of an inch, or less, in diameter. The spots are scattered indiscriminately over the leaf, occurring frequently on the veins, midrib and petiole. On account of their minuteness, several lesions may appear on a leaf without attracting attention, and perhaps without great injury. In more severe cases the diseased leaf is more noticeable and such foliage may be dropped prematurely. The leaf - petioles are attacked, in which case the leaves, instead of falling, turn brown, die and hang on the tree.
Fig. 16. - Apple blotch cankers.
The fungus causing the various lesions just described has been known since 1895, at which time it was found on the leaves of wild crab; it was then named Phyllosticta solitaria E. & E. Two years later it was discovered on the apple, but not until 1907 was it known that the fungus on the apple was the same as that previously found on crab.
The pathogene hibernates in the cankers as mycelium. In the spring about the time the petals fall pycnidia appear along the margin of the lesion. Within three to six weeks, spores ooze forth from the pycnidia in great quantities, this period of spore - discharge extending from about the middle or last of May until the end of August. It is thought that wind, possibly insects, and certainly rain, act in carrying the spores from their spring quarters to the leaves, fruits and other twigs. Although the spores are disseminated as early as the middle of May, first signs of the disease do not ordinarily appear before some time in June. This means that approximately one month is consumed by the pathogene in getting established in its new environment. During this time the spores germinate, which process requires from twelve to eighteen hours; and assuming that moisture is present at the time of inoculation, then the remaining time is spent in developing mycelium and producing an injury of sufficient extent to become evident to the naked eye. The fungus grows superficially on the fruit, and in many cases the effect is not of serious nature beyond that of marring the appearance. On the other hand the attacks are often of a more detrimental character. Early in the development of lesions on the leaves, fruits and twigs, pycnidia appear. Spores are developed within these pycnidia on the fruits and twigs, although on the leaves the fruiting bodies seem to be sterile. However, the spores produced on the fruit and twigs are readily spread to other fruits, leaves and twigs. This phenomenon of secondary spread of the fungus appears to decline by the first of July, and no spores are disseminated after the last of August. After this season of the year the history of the pathogene on the leaves and fruits is unknown; but it is believed that these organs do not carry the fungus through the winter and hence are not sources of the first inoculum in the spring. . It is on the twigs that the organism hibernates, a fact of great importance in dealing with the pathogene.
Since the disease characteristically occurs annually, control measures should be applied each year. There are two lines of remedial procedure: the removal of the cankered twigs and the protection of the susceptible parts by spraying. Careful pruning will remove a large portion of the diseased twigs, which are the source of trouble. Their removal is a valuable operation supplemental to the application of a protective spray. Spraying must be done before inoculation takes place; this, as has been seen, occurs within a month after the petals fall. The number of applications depends on the nature of the weather. In the Middle West and Southwest the schedule is as follows: First application, use bordeaux mixture, 3-4-50, three weeks after the blossoms drop. Lime sulfur should be substituted for bordeaux mixture in wet weather, since the latter produces injury to the fruit and foliage under such conditions. Second application should be made two to four weeks after the first. A third application is recommended ten weeks after the petals fall. The second and third applications correspond to those made for Bitter Rot, so that one course of spraying will suffice for both diseases.
Scott, W. M., and Rorer, J. B. Apple blotch, a serious disease of southern orchards. U. S. Agr. Dept. Plant Indus. Bur. Bul. 144: 1 - 28. 1909.
Lewis, D. E. The control of apple blotch. Kansas Agr. Exp. Sta. Bul. 196: 520 - 574. 1913.