It is evident that this disease has long been associated with the peach, this fruit-tree being a natural host for the causal organism. Furthermore the disease is distinctly one affecting the peach and its derivatives, such as the nectarine and peach-almond. It has rarely been found on other fruits. This trouble, known as peach Leaf Curl, curl, curly leaf and leaf - blister, affects both orchard trees and nursery stock, certain varieties showing more resistance than others. The Elberta and Carman are the most susceptible in New York State, while, in the nursery at least, the Richards is resistant. Seedling peaches show marked susceptibility, which fact indicates that the disease may have originated with the wild peach in central China.

The disease has long been known in England, where it was described as early as 1821 by an English gardener as blister. It has become widely distributed throughout the world wherever peaches are grown, being most serious near the seacoast or in the region of large interior lakes. In the United States and Canada it is very common and destructive in the peach - growing sections about the Great Lakes. Likewise in the Pacific Northwest it has been very troublesome in regions of considerable annual rainfall.

Peach Leaf Curl is regarded as the most serious fungous disease affecting this fruit in cooler climates. In warmer climates, on the other hand, Brown Rot assumes this role. The fruit being rarely attacked by the Leaf Curl pathogene, the losses involved are usually underestimated. The annual toll which American peach-growers give to the ravages of this pest is said to be three millions of dollars. The character of the losses is: (1) loss of leaves in the spring, followed by a forcing of a new crop of foliage later that year, which lowers the vitality of the tree; (2) partial or total failure of trees to set or hold a crop because of defoliation; (3) repeated loss of leaves for several seasons in succession, resulting in the death of the trees; (4) injury to trees by killing the twigs; (5) stunting of nursery stock due to the death of the shoot from the bud; curl - affected nursery buds never make good trees.

Symptoms

Most peach-growers are familiar with the symptoms of peach Leaf Curl, particularly in its later stages (Fig. 75). The first evidence of the disease, however, may be frequently overlooked. In the spring shortly after the leaves begin to unfold there is a puffing and folding of these organs. The leaf-blade becomes thickened and puckered along the midrib, causing the leaf to curl. The diseased portion becomes yellowish, with tints of red. The leaf is thickened, becomes brittle, and finally shows a characteristic silvery bloom over the upper surface (Fig. 75). Curling may be confined to a part of the blade, or the petiole, or may involve the whole leaf (Fig. 75). Affected leaves finally die and drop from the tree, in some cases the entire tree being defoliated. A new set of leaves is then formed from the dormant buds following defoliation. Affected twigs show a marked swelling and are stunted in length. Their color changes to pale-green and yellow. While the hypertrophy involves the current year's growth for the most part, sometimes the pathogene extends down the side of the previous season's growth, forming a swollen ridge. Badly infected twigs are usually killed, but the growing tip may develop a healthy shoot, leaving a swollen canker-like lesion at its base. An exudation of gum often accompanies the lesions on the twigs. The flowers and young fruits are often attacked, but because they soon drop away this symptom of curl is seldom observed. In general, the sickly yellow, curled foliage, and the final defoliation of the trees the latter part of June, followed by a refolia - tion, are the most striking symptoms of this disease.

Fig. 75. Leaf Curl of peach.

Cause

The peach Leaf Curl disease is caused by the fungus Exoascus deformans, so named because it forms its asci on the outside of the host, and because it deforms the leaves. The mycelium of the pathogene grows between the cells of the leaf, stimulating them to abnormal increase in size and number, and robbing the leaf of its green chlorophyl. When the spores are to be produced, the mycelium invades the cuticle of the leaf, and at once gives rise to sacs (asci) which bear from 3 to 8 ascospores. The presence of the asci on the upper surface of the leaf gives the leaf its silvery appearance. At maturity, the ascospores bud extensively within the ascus, giving rise to spores suggesting conidia. The ascus then empties its contents through a crack at the top. Thus far the life-history of Exoascus deformans is definitely known; but where these ascospores and their budded descendants go is unknown. The fate and habits of the fungus during the summer, fall and winter are not clearly understood. It has been held that the mycelium is perennial in the twigs, growing out into the leaves and young shoots in the early spring. But this is doubtful, since thorough spraying in the fall or before the buds open in the spring will control the disease. Since the fungicide cannot penetrate the twigs to kill the fungus, it can only be surmised that the inoculum comes from some external source. It is the opinion of most investigators that the fungus hibernates on the bud - scales in the form of spores (kind unknown), and that these spores germinate during the spring rains at a time when the buds swell, and that the germtube penetrates the very young leaf as it emerges from the bud. This opinion is based on the following circumstantial evidence: (1) lesions appear on leaves just as they are protruding from the buds; (2) leaves from buds sprayed before such buds swell show little or no infection during the season, while unsprayed buds on the same tree curl badly; (3) buds sprayed after they swell, and especially after rains, show curled leaves; (4) the disease occurs only during cold, wet springs.

From the above data it may be said that, theoretically, some sort of spores are lodged by the wind or rains among the hairs of the bud - scales during the late summer, and that these spores remain there dormant until conditions favorable to infection arise the following spring.

The effect of the environment upon this disease is very marked. This is noticed in a general way in connection with the geographical range of the disease, which is most common and severe in the neighborhood of large bodies of water. The combination of conditions most favorable to an epiphytotic of peach Leaf Curl is a cold wet-period following warm spring weather. Warm weather starts the buds; a cold wet spell immediately following results in a saturation of the leaf-tissue with water, due to a lowering of the temperature and to the high humidity of the atmosphere. The buds are retarded, their cells gorged with water, and their walls distended, while the damp atmosphere permits spore - germination and infection. As already noted, curl is most severe near large bodies of water; this is doubtless due to the increased humidity of the air in these localities, and to reduced temperature in the early spring as evidenced by the more frequent occurrence of fogs in such regions. Heavy dews can exert but little influence on curl, since the moisture and temperature factors are not sufficiently pronounced nor of adequate duration to effect a response on the part of the host and the parasite. Rainfall seems to have considerable influence on the disease. In the Pacific Northwest, for example, little or no curl exists east of the Cascade Mountains, where the annual rainfall is light; on the other hand, west of these mountains the rainfall is heavy and curl is very destructive. In these two regions the temperature is approximately the same.

Control

The time and thoroughness of the application of remedial measures are important points in the control of peach Leaf Curl. It is imperative that such work be done before the buds swell in the spring. Conjectures with reference to the winter and spring activities of the pathogene lead one to infer that spraying should prevent the disease. It has been shown that the application of almost any common fungicide ordinarily controls the trouble satisfactorily. One spraying is sufficient and may be made in the fall, or in the spring before the buds swell. Never spray more than once, as it is a waste of time and materials: and do not spray for curl after the leaves are expanded, for such an operation is useless and dangerous. Spray thoroughly, making sure to coat every bud. If the season is favorable to curl, the unsprayed buds will show the disease. Ordinarily lime sulfur solution of standard Baume test, 32°, at a strength of 1 to 15 or 1 to 20, is effective. If San Jose scale must be combated, both troubles may be controlled by using the fungicide at scale strength, 1 to 8. Scale can be controlled as effectively by spraying in the fall as in the spring. Bordeaux mixture of any strength, or copper sulfate 2 pounds to 50 gallons of water, are also effective in controlling peach Leaf Curl, and may be used when scale is not a factor to be considered.

References

Reddick, D., and Toan, L. A. Fall spraying for peach leaf cuil.

Cornell Univ. Agr. Exp. Sta. Circ. 31: 65 - 73. 1915. Pierce, N. B. Peach leaf curl, its nature and treatment. U. S. Agr.

Dept. Veg. Phys. and Path. Div. Bul. 20: 1 - 204. 1900.

Wallace, E., and Whetzel, H. H. Peach leaf curl. Cornell Univ.

Agr. Exp. Sta. Bul. 276:155 - 178. 1910. Knowles, Etta. The curl of the peach leaves; a study of the abnormal structure induced by Exoascus deformans. Bot. Gaz. 12: 216218. 1887. Duggar, B. M. Peach leaf curl. Cornell Univ. Agr. Exp. Sta. Bul.

164: 371 - 388. 1899.