The pathogene causing this disease is Glomerella cingalata. The fact that the same organism is responsible for both the rot and canker forms of the disease has been fully established. For example, the fungus may readily pass from the bark to the fruit, and this fact is highly important in its life-cycle. Following the seasonal development of the pathogene, it is to be noted that there are two chief sources of the inoculum in the spring; these are the mummies which hang to the tree through the winter, and the cankered areas in the limbs. From these sources conidia are washed by rains to the susceptible parts below, that is, young fruits and limbs. Insects appear to have little to do with the dissemination of the fungus. The best evidence that a canker or a mummy are sources of the inoculum, and that rain is the inoculating agent, is found in the fact that affected fruits usually map a pyramidal area at the apex of which is found a canker or a mummy. Furthermore, infections on fruits are frequently arranged in lines toward the calyx-end, which indicates that the spores were washed by water down the sides of the apple. The conidia soon germinate, their germtubes being capable of entering through the healthy surface as well as through wounds. Abrasions in the bark of various sorts furnish a channel of entrance into larger limbs. The fungus may be able to penetrate uninjured twigs and from these organs it spreads readily into the parent branch forming the canker previously described. Within a few hours after inoculation spots an inch in diameter will develop on the fruits; however, the rate of rotting depends on the temperature and the age and variety of the fruit concerned. As previously pointed out, the fungus develops fruiting pustules, acervuli, by the time the spot is an inch or less in diameter. Their general appearance and arrangement has also been described. Within the acervuli conidiophores arise, each stalk being capable of developing several conidia successively at the free outer end. These spores, as they are formed, pile up at the apex of the acervulus and in quantities are pink. These groups of conidia in the concentric or scattered acervuli produce a very striking appearance, which forms one of the most characteristic symptoms of the disease. When moist the conidia cohere in sticky masses for a time, but when wet are easily separated and washed to other points during the rains. Such conidia are in turn capable of inducing Bitter Rot and within three to seven days another crop of conidia is ready for dissemination. This process may be frequently repeated from June to October, if weather conditions are favorable. Many of the conidia are carried in rain drops by heavy winds to other apple-trees which thus become new centers of infection. Many conidia, too, are deposited on dead parts of a tree and there germinate, and establish the fungus for the winter. Accordingly it has been shown that the pathogene not only hibernates in the Bitter Rot mummies and cankers, but also in the Illinois Blister Cankers, dead tips of fruit-spurs, frost-injured portions of limbs, Fire Blight cankers, blotch and Black Rot cankers. In any or all of these places the fungus overwinters in the form of mycelium. In the spring, the parasite resumes activities by developing acervuli from which the first spring crop of conidia are produced. The fungus has a sexual stage, in which perithecia, asci and ascospores are borne, but it has not been shown that this stage is important in the life - cycle of the organism.
The history of Bitter Rot shows that it has always been spasmodic and erratic in its occurrence in different seasons. But the irregularity is not confined alone to different seasons, for it makes its appearance in different years at different dates. In different orchards it may occur at widely separated intervals during the same season. The fruits on the sunny side of a tree frequently are destroyed first. Sometimes the crop on the south side of a tree is destroyed while on the north side it escapes the disease. Likewise the fruit on the inside, lower branches, being well protected from the sun, often escapes, whereas apples exposed to the sun are ruined. Trees in the shaded mountain hollows of Virginia are said to show less rot than trees in situations more exposed to the sun.
All of these points indicate a decidedly close relationship between the fungus and the weather, especially temperature. The fact that the fungus is confined to the warmer states, that it appears chiefly in July and August, and that fruits exposed to the sun are destroyed first, point unmistakably to the fact that the pathogene is a hot weather fungus. Furthermore cold weather usually checks the disease - or may stop it altogether. A cold spring may retard the development of the fungus at that season, but there usually results a later attack on the fruit. In cool, dry summers Bitter Rot is sparingly present. Conditions most favorable for the development of the disease are warm, moist weather. A short series of hot, wet days in August may bring about a sudden and very destructive attack. Nights with a heavy dew, alternating with hot days, are usually followed by an extensive outbreak of the disease, even destroying the whole crop in three days.
Cankers, being a chief source of the inoculum, should be removed. Remove the whole limb except where the affected branch is a large and valuable one. Dead parts and cankers other than Bitter Rot lesions should be removed. All these operations should be performed in the winter since the cankers are not only more easily located when the leaves are off, but such work should be done before the conidia are disseminated in the summer.
Spraying has proved effective, 90 per cent Bitter Rot-free fruit being a possibility. Dormant spraying is said to have no value. Spray at least once before the buds open; make a second application by the middle of June. Succeeding applications must be made at intervals governed by the weather. Bordeaux mixture, 4-4-50, should be used; lime sulfur is not effective.
Schrenk, H. von, and Spaulding, P. The bitter rot of apples. U. S.
Agr. Dept. Plant Indus. Bur. Bul. 44: 1 - 54. 1903. (Extended bibliography.) Roberts, J. W. Sources of the early infections of apple bitter rot.
Journ. Agr. Research 4: 59-64. 1915. Scott, W. M. The control of apple bitter rot. U. S. Agr. Dept. Plant Indus. Bur. Bul. 93: 1-36. 1906. Burrill, T. J. Bitter rot of apples. Botanical investigations. Illinois Agr. Exp. Sta. Bul. 118: 554 - 608. 1907. Blair, J. C. Bitter rot of apples. Horticultural investigations.
Illinois Agr. Exp. Sta. Bul. 117: 482 - 551. 1907. Alwood, W. B. The bitter rot of apples. Virginia Agr. Exp. Sta.
Bul. 142: 252 - 279. 1902.