The late varieties of cranberries suffer from a disease known as rot or scald. The term scald has been in general use for a long time, and it owes its origin to a belief of the growers that the softening of diseased fruit was actually a scalding caused by heat from the sun when the berries were wet. The name scald as used by many growers embraces at least three diseases of the fruit: scald, rot and anthracnose. The discussion here concerns only the true scald.
Growers have known this troublesome disease for several years. It was the subject of discussions at the early meetings of the New Jersey Cranberry Growers' Association which was organized in 1869 (now known as the American Cranberry Growers' Association). Scald is more prevalent in the East and Southeast than in the Middle West, although it is well known to Wisconsin cranberry - growers. It also occurs in Canada and in Europe.
The amount of damage done by this disease varies considerably with the season. In some years seventy-five per cent of the crop is destroyed. In New Jersey the annual loss to cranberry-growers on account of scald is estimated at about one - third of the crop.
The berries, flowers, leaves, stems and roots are affected. The first evidence of scald appears as a minute, light-colored, watery area on the surface of a berry. This enlarges until the whole berry is involved, softens, and turns brown (Fig. 52). It is thought that in the East affected berries do not turn brown, and that this character is influenced by the soil and climatic conditions. Sometimes several spots develop on a single berry. Finally, either the whole affected fruit assumes a scalded or cooked appearance; or, in case only a portion decays, the berry shows a concave surface on the affected side. The interior of scalded berries is soft and watery. It is sometimes difficult to say whether a berry is affected with scald, for although it appears healthy it may still be diseased. Berries when affected before they are one-half grown usually hang shriveled, and covered with black dots - the fruiting bodies of the causal fungus. Dark concentric rings often show; however, this is not a peculiarity of scald, for it does not occur constantly, and furthermore o it is an accompanying characteristic of cranberry rot (see page 201).
Fig. 52. - Cranberry - scald.
When the flowers are affected, they suddenly shrivel and die. This effect is commonly known as blast.
Leaves are not usually affected, but do not always escape. Brown spots, irregular in outline, are produced at times. Within such areas the black fruiting bodies of the pathogene may be found. Ultimately the leaves turn yellow and fall.
Cranberry-scald is caused by the fungus Guignardia Vac - cinii. It was formerly held that it is caused by too much acid in the soil. It was also believed that excessive heat and drought are contributing factors which induced fermentation in the fruit. A condition similar to scald is sometimes induced when berries are flooded and kept covered with water for a half day or more during hot weather.
The causal fungus, Guignardia Vaccinii, is found generally in the cranberry sections of the country. It was once thought that it lived perennially in the stems, and that it entered the fruit therefrom. But now this is regarded as a false idea. The fungus winters in the old fallen leaves. In the spring pycnospores initiate the first infections. The time and manner of this process is unknown. But it seems very likely that it occurs very early in the growing - season, soon after the water is removed from the bog, and while the berries and leaves are quite young. Assuming that pycnospores from old leaves come to lie on the susceptible parts, then, under conditions favorable to the process of germination, a germtube is developed from each spore. The organ inoculated is penetrated and a mycelium is developed. It appears that after the fungus has entered its host it may remain inactive for some time, during which period there is no external evidence of the disease or the fungus. Thus affected berries may pass unnoticed as healthy fruit. The conditions affecting the length of this period of inactivity are not certainly known. With the resumption of growth the tissue in the region of the mycelium is killed, and the lesions described above are produced. The fungus fruiting bodies, pycnidia, break through the epidermis, and expose their tips. Each pycnidium contains, at maturity, many pycnospores which coil out in a gelatinous tendril. A second type of fruiting body, perithecia, is sometimes developed, but these occur less frequently, and are not regarded as important in the dissemination of the fungus.
The conditions favoring the fungus are not unusual. Warm, wet weather furnishes the best conditions for maximum development. If berries are kept at a high temperature after picking, the disease is greatly increased.
It has been shown that bordeaux mixture, 5-5-50, when thoroughly applied, gives satisfactory results. Resin-fish-oil soap at the rate of four pounds to fifty gallons is added as a sticker. Five applications are recommended. In general, not more than fifteen days should elapse between any two applications. The first should be made early in June; the second later in June when the blossoms are ready to open; third, as soon as the plants have passed the height of their blooming period; subsequent applications at fifteen - day intervals.
The evidence at hand indicates strongly that regulation of the water-supply is a very important factor in the control of all cranberry diseases. The amount needed varies with the type of soil and the contour of the land. The supply should be controllable. It should be constant; fluctuations should be avoided during the growing - season. Keeping the plants continuously moist, but not wet, is recommended.
Bog sanitation should be practiced. Dead leaves and vines should be destroyed. This must be done before the spores are discharged, to be effective - at least within a week after the water has been withdrawn from the bogs in the spring.
It is advised that plants which seem less subject to the disease be selected for propagation.
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