Cherries, without preventive or remedial intervention, are at the mercy of two or three fungus diseases and sometimes several others are virulent, depending upon locality, season, weather and variety. One of these diseases, brown-rot, in spite of the great advances in plant pathology of recent years, is almost beyond the control of preventive or remedial measures. Happily, all the others yield better to treatment. Brown-rot1(Sclerotiniafructigena (Persoon) Schroeter), sometimes known as fruit-mold or ripe-rot, very frequently attacks flowers and shoots but is most conspicuous on the ripe or ripening cherries where its presence is quickly detected by a dark discoloration of the skin which is afterwards partly or wholly covered with pustule-like aggregations of gray spores. The decayed fruits usually fall to the ground but sometimes hang to the tree, becoming shriveled mummies, each mummy being a storehouse of fungus threads and spores from which infestation spreads to the next crop. The disease, in some seasons, like a withering blight, attacks twigs, flowers and leaves early in the spring doing great damage to the young growth and often wholly preventing the setting of fruit. The rot spreads with surprising rapidity on the fruits in warm, damp weather either before the fruit is picked or in baskets while being shipped or stored. Preventive remedies have so far met with but indifferent success; probably the best method of control is to destroy the mummy-like fruits and all other sources of infection either by picking them from the trees, or much better by plowing them under deeply. Varieties of cherries show various degrees of susceptibility to brown-rot. All Sweet Cherries are more subject to the disease than the Sour sorts. But with either of the two species there are great variations in the susceptibility of the varietal hosts - a matter specially noted in a later chapter in the discussion of varieties. [This is done variety-by-variety, there is not a comparison table (but I plan to make one from this book and more modern sources...A.S.C.)]
[See also Brown Rot entries in the Plums of New York & The Peaches of New York. ]
Another serious disease of the cherry, and probably the most striking one in appearance, is the black-knot2 (Plowrightia morbosa (Schweinitz) Saccardo), characterized by wart-like excrescences on shoots and branches.
Black-knot looks more like the work of an insect than a fungus and was long supposed to be such even by those who were studying the trouble. The knots begin to form early in the summer and are of characteristic color and texture -dark green, soft and velvety, but in the fall, as the fungus ripens, the color changes to coal-black and the knots become hard and more or less brittle. The excrescences usually form on one side of a twig or branch so that death seldom follows quickly. The disease attacks both wild and cultivated plants in every part of this continent where cherries are grown but is epidemic only in the East, the cherry regions of the West being practically free from the disease. Up to the present time the fungus has not been found elsewhere than in America. Happily, black-knot may be controlled by cutting out the diseased wood. To completely eradicate the fungus, if it is especially virulent, however, the orchard must be gone over several times during a season. In New York the removal of black-knot is ordered by law, the results showing that when the law is obeyed, especially if there be hearty co-operation among growers, eradication is usually possible. Sweet Cherries are much less attacked by black-knot than the Sour sorts but the differences in immunity between varieties in either of the two species are not very marked - at least such is the case on the grounds of this Station where the disease is always present and is often very prevalent.
Cherry leaves are often covered with a grayish powder which in severe cases causes them to curl and crinkle and sometimes to drop. This powdery substance consists of the spore-bearing organs of a mildew7 (Podosphaera oxyacanthae De Bary). Powdery mildew is much more common on nursery stock than on fruiting trees and in New York is a serious pest on young cherry trees. In the nursery, injury may be prevented by the use of copper sprays or lime and sulphur, either of which is also an efficient preventive in the orchard but the mildew is seldom prevalent enough on orchard plants to require treatment. [Sources of genetic resistance]
Old cherry trees are often attacked by a fleshy fungus or "toadstool (Polyporus sulphureus (Bulliard) Fries)10. This fungus is said to be worldwide in its distribution and to occur upon a large variety of trees. It is very striking in appearance, the clusters appearing during late summer or early autumn in large, shelving branches, the sporophores fleshy and of cheese-like consistency when young but becoming hard and woody with age. At first the "toadstools "are all yellow but later only the under surfaces are yellow while the upper surface is orange-red. The plants are more or less odoriferous, the odor increasing with age. Happily, the fungus is not very virulent but is often the cause of decay in the tree-trunk - the brown-rot of the wood of this and other orchard and forest plants. In localities where the fungus thrives it may usually be controlled by covering all wounds with tar or other antiseptic materials.
At least two other fleshy fungi have been found injuring cherries. These are Clitocybe parasitica Wilcox11and Armillaria mellea Vahl.,12 the latter the honey agaric, more or less abundant in both Europe and America. Both are associated with and are probably a cause of the root-rot of the cherry and other orchard fruits. Neither is a common enough pest in this country, however, to receive extensive description in texts on diseases of plants. Control measures are different in localities where fungi occur, consisting in the main of getting rid of stumps and roots in orchard lands and planting to field crops before using for orchard purposes. Infected trees should be removed or isolated by trenching about them.
All stone-fruits suffer more or less from an excessive flow of gum. The name gummosis13 is generally applied to these troubles. Gumming is much more prevalent in the far West than in the East but is to be found wherever stone-fruits are grown. This excessive gumming is a secondary effect of injuries caused by fungi, bacteria, insects, frost, sun scald, and mechanical agencies. There is a good deal of difference in the susceptibilities of varieties and species to this trouble, the Sweet Cherry suffering much more than the Sour sorts and varieties of other species having hard wood suffering less than those having softer wood. There is less gummosis, too, on trees in soils favoring the maturity of wood; under conditions where sun and frost are not injurious; and, obviously, in orchards where by good care the primary causes of the diseases are kept out.
A number of diseases of the trunk arise from mechanical injuries from wind, sun, frost and hail. Few, indeed, are the fruit-growers whose trees are not occasionally damaged in one way or another in the vicissitudes of a trying climate. Very often these mechanical injuries are followed by fungal parasites or insects so as to make it difficult to distinguish the primary from the secondary trouble, There is a wide difference in the susceptibility of Prunus avium and Prunus cerasus to such injuries, the Sweet Cherry, with its softer wood, being much more easily injured by any and all stresses of weather than the Sour Cherry. In the main the elements cannot be combated but low heading of the trees is a preventive from sunscald, at least, and sometimes may have a favorable effect in preventing wind and frost injuries.
1 Smith, E.F. Peach Rot and Peach Blight,
Journ. Myc. 5:123-134. 1889. Quaintance, A. L. The Brown Rot, etc., Ga.
Sta. Bul. 50:237-269, figs. 1-9. 1900.
2 Parlow, W. G. The Black Knot, Bulletin Bussey Institution 440-453. 1876. Halsted, B. D. Destroy the Black Knot, etc., N. J. Sta. Bul. 78: 1-14. 1891.
3 Duggar, B.M. Fungous Diseases of Plants 185, fig. 68. 1909.
4 Higgins, B.B. Contributions to the Life History and Physiology of Cylindrosporium on Stone Fruits. Am. Jorn. Bot. 1: 145-17.3. 1914.
5 Aderhold, R. Mycosphaerella cerasella n. spec., die Perithecienform von Cercospara cerasella Sacc. und ihre Entwicklung, Ber. d. deut. Bot. Ges. 18:246-249. 1900.
6 Duggar, B.M. Fungous Diseases of Plants 314. 1909. Pierce, N.B. A Disease of Almond Trees. Jour. Myc. 7:66-67, Pls. 11-14. 1892.
7 Duggar, B.M. Fungous Diseases of Plants 226. 1909.
8 Smith, E.P. and Townsend, C. O. A Plant Tumor of Bacterial Origin, Science 25:671-673-1907.
Tourney, J.W. Cause and Nature of Crown Gall, Ariz. Sta. Bul. 33: 1-64, figs. 1-31. 1900. Hedgcock, G. C. Crown Gall, etc., U.S. Dept. Agr. Bur. Pl. Ind. Bul. 90:15-17, Pls. 3-5. 1906.
9 Scribner, P. L. Leaf Rust of the Cherry, etc., U. S. Dept. Agr. RPt- 353-355, Pl. 3, 1887.
10 Atkinson, Geo.P. Studies of Some Shade Tree and Timber Destroying Fungi. Cor. Agl. Exp. Sta. Bul. 193:208-214. 1901, Schrenk, H. von. Div. Veg. Phys. and Path., U.S. Dept. Agl. 25:40-52, Pls. 11 (in part), 13. 1900.
11 Wilcox, E.M. A Rhizomorphic Root-Rot of Fruit Trees. Okla. Agr. Exp. Sta. Bul. 49:1-32, Pls. 1-11. 1901.
12 Duggar, B.M. Fungous Diseases of Plants 473. 1909.
13 Hedrick, U.P. Gumming of the Prune Tree, Ore. Sta. Bul. 45:68-72. 1897.