Disorders of Cranberries

Herbert F. Bergman.

Fruit rots cause an annual loss of 10 to 15 percent in our crop of cranberries, which are grown extensively in Massachusetts, Wisconsin, and New Jersey and less widely in Washington, Oregon, Maine, and Rhode Island. The value of the crop is 10 million to 20 million dollars a year.

Two or three decades ago the losses went as high as 25 percent, but they have been reduced by changes in cultural practices, improvements in fungicides, greater efficiency in handling and storing, and the development of the canning industry, which offers an immediate and profitable outlet for the berries that although sound at harvest probably would develop excessive decay if they were stored and shipped as fresh fruit.

Directly affecting the abundance of rot are the amount and frequency of rain and the temperature and humidity. Humidity and temperature (to a lesser extent) are affected by local conditions. A local condition that bears on humidity is the density of vine growth. Excessive growth prevents the evaporation of water among and under the vines after rain, fog, or dew. Thus the atmosphere may remain saturated or nearly saturated for hours after the air and soil surface under the thinner vines have dried. Poor drainage also keeps the soil wet and makes a high humidity under the vines.

The application of nitrogenous fertilizers, late holding of the winter flood (particularly holding until July or holding late in successive years), sanding too frequently, or too heavy applications of sand on bogs with dense vine growth promote excessive growth especially on bogs with peat or muck bottom. Excessive use of water (such as too frequent flooding and holding water high in the ditches) also tends to increase the amount of rot because the surface of the soil is kept wet and the humidity of the air under the vines is increased.

To find out the relationship between weather in Massachusetts and the ability of cranberries to remain in good condition, extensive studies were made by Dr. Neil E. Stevens, formerly of the Department of Agriculture. He began the work with Dr. C. L. Shear in 1915. Although he worked on all phases of investigation pertaining to cranberries, his chief interests were weather in relation to keeping quality, the occurrence and spread of the false blossom disease, and the effect of acidity or alkalinity of flooding water on the productivity of cranberry vines. Dr. Shear, also of the Department of Agriculture, went to New Jersey in 1901, at the request of the American Cranberry Growers' Association, and began studies which were continued until 1907. He made extensive observations in the field, collected specimens in New Jersey and Massachusetts, and made laboratory studies of the various diseases of cranberries. He also conducted experiments in New Jersey on methods of controlling fruit rot. As a result of the work, most of the present knowledge as to the identity and life histories of the fungi causing fruit rot as well as of those causing diseases of the vines was obtained.

Records over a long period show that the keeping quality of berries from individual bogs and from an entire region varies greatly from year to year.

An apparent correlation was found in Massachusetts and New Jersey between exceptionally large crops and unusually poor keeping quality. Keeping quality was correlated with rainfall, but the correlation between keeping quality and the number of rainy days in July and August, when the fruit is developing, was closer than the correlation between keeping quality and total amount of rain. The amount of rain in September has no bearing on keeping quality.

The correlation between keeping quality and temperatures during May and June was very close. The basis used in comparing temperatures in different years was the total of "day degrees" above 50 F. Fifty was subtracted from the mean temperature of each day and the remainders were added together. Fifty was chosen as a base point because many fungi begin active growth on cranberries at about that temperature.

During the years of the study, an unusual amount of rot occurred in berries of the Early Black variety whenever the May and June temperatures were unusually high. Moreover, those were the only years in which an unusual amount of decay is known to have occurred in Early Black. In years in which May and June temperatures were lower than normal, the keeping quality of Early Black was better.

Early Black and Howes, the oldest of the cultivated varieties, are the principal ones grown in Massachusetts, where about 65 percent of the entire crop of the United States is produced. McFarlin is grown to a limited extent. Early Black and Howes are grown extensively in New Jersey. The principal variety in Wisconsin is Searles, and McFarlin is next in importance. McFarlin is the most important variety in Washington and Oregon, but Searles also is grown to a considerable extent.

The keeping quality of berries of the Howes variety also was poor in all years in which that of Early Black was poor. The keeping quality of Howes, however, is less definitely correlated with temperature during the growing season than that of Early Black.

Field observations indicate that in determining the keeping quality of the general crop, temperatures during May and June are the most important; distribution of rainfall and temperatures during July and August come next; and the size of the crop, third.

The keeping quality of cranberry crops in Massachusetts and New Jersey vary to some extent independently of each other, although it has been observed that when the keeping quality of the crop in either State has been conspicuously poor that of the crop in the other State also has been below normal.

The keeping quality of the cranberry crops of Wisconsin and of the Washington-Oregon areas also varies from year to year, depending on weather conditions during the growing season. In the Washington-Oregon areas rainy weather during the picking season is apparently the most important factor in determining the amount of rot that will occur.

EIGHT SPECIES of fungi are known to be important causes of fruit rot: Guignardia vaccinii, Acanthorhyncus vaccinii, Glomerella vaccinii, Godronia cassandrae, Diaporthe vaccinii, Sporonema oxycocci, Pestalotia vaccinii, and Ceuthospora lunata.

The rots caused by most of them are practically the same in external appearance. The fungi causing them can be identified only by making cultures from the rotten berries and identifying the fungi in the cultures.

The fungi grow, in culture, at widely varying rates at different temperatures. Godronia and Sporonema grow slightly at 32 F., and grow most rapidly at about 59 and 68 , respectively. Diaporthe, Glomerella, and Guignardia begin to grow between 35 and 40 , and their rate of growth increases rapidly above 50 . They grow well at temperatures between 59 and 86 and make their best growth at 82 , 77 , and 77 , respectively. Acanthorhyncus, however, begins to grow only at temperatures of 60 to 70 and makes its best growth at about 85 .

Because temperatures in New Jersey during the growing season are higher than in other cranberry-growing regions, field rot occurs on the bogs more abundantly there and often has caused the loss of 50 to 75 percent of the crop in unsprayed bogs. Many bogs in New Jersey, however, produce berries of good keeping quality; most of the rot there is due to fungi that grow best at high temperatures. Guignardia is the most important; Acanthorhyncus is next; a small part is caused by Glomerella.

Temperatures during the growing season in Massachusetts usually are high enough to permit the development of field rot. It usually occurs, however, only on a few bogs, but on them sometimes causes the loss of 15 to 40 percent of the crop. In years when conditions are more favorable for rot, it is found on a greater number of bogs and becomes more serious on the few bogs where it occurs yearly.

When field rot occurs, Glomerella generally is the principal cause, particularly in early varieties. Some seasons, however, the temperatures may be higher or lower than normal, and are higher on some bogs than on others. In years in which temperatures during the growing season are above normal, and because of differences in local conditions on some bogs, or parts of them, Guignardia, Sporonema, and Diaporthe are sometimes more important as causes of rot than Glomerella. Acanthorhyncus very rarely occurs on Massachusetts bogs.

Field rot is negligible in Wisconsin. It occurs only occasionally in Washington and Oregon. Most of the rot there is caused by Godronia and occurs during storage. In Wisconsin, Godronia is the only fungus that causes more than a negligible amount of rot, but in Washington and Oregon a considerable part of it is caused by Ceuthospora. Occasionally serious loss due to field rot caused by the cotton-ball fungus, Sclerotinia oxycocci, occurs.

The crucial period seems to be near and during flowering, when the fungi causing fruit rot gain entrance into the berries. The fungi apparently are always present on bogs although not always in sufficient numbers to cause a noticeable amount of rot.

Mature fruiting bodies, ready to discharge spores, often are found on old leaves, stems, and dried berries on bogs in Massachusetts within 2 weeks after the winter flood has been drawn off and are abundantly present in May and June. The fruiting bodies discharge spores during and after rain and periods of fog and when the vines are wet with dew. Wind and water may carry the spores to blossoms and leaves. Whenever enough moisture is present, the spores germinate; germ tubes from them grow into the ovaries of flowers and cause rot later.

Spores may also be carried to the leaves where they germinate, the germ tubes entering the leaves, where the fungus continues to grow and later forms fruiting bodies on the surface. Spores are discharged from the fruiting bodies before and during the flowering period of the following year to initiate infection for that season.

THE USE OF FUNGICIDES greatly reduces the amount of rot before and after harvest. Spraying is most necessary in New Jersey and has been generally practiced there since the early 1900's. Spraying has been necessary on only comparatively few bogs in Massachusetts. Spraying generally is not necessary in Wisconsin. In Washington and Oregon, the weather during harvest has a much greater influence on the amount of rot than Spraying does.

Most generally used in New Jersey until 1945 was bordeaux mixture 8-8-100, with 2 pounds of rosin fish-oil soap added. Four or five applications of the spray were made. A 10-4 100 bordeaux, with 1 pound of rosin fish-oil soap, has been used in Massachusetts. Only two applications are necessary, one at the beginning of the blossoming period and the second near the end.