Both currants and gooseberries are commonly affected by this Leaf Spot. In certain regions, for example New York, the black varieties of currants, such as the Naples, are said to be more resistant than the red and white currants. On the latter kinds the foliage is more liable to drop when affected by this trouble.
The disease was first given special attention in Massachusetts in 1886; five years later it was an object of study in Iowa. It was rather common in New York in 1899 and 1900, but the damage wrought was not serious. The disease ordinarily occurs wherever the currant and gooseberry are grown. In some seasons nearly complete defoliation occurs. This loss of foliage interferes with the maturing of both the wood and buds, and as a result the succeeding crop is injured. Perhaps the cumulative effects constitute .the worst feature. Symptoms.
During the month of June, lesions appear as small brown spots (Fig. 53) on the upper and lower surfaces of the leaves.
The spots may number few or several to a leaf. Each diseased area enlarges until it has reached a diameter of about one-eighth of an inch, and the center becomes pale with small black fruiting bodies of the pathogene, while the border remains brown (Fig. 53). Older lesions are frequently confluent. From the last of June to August the affected leaves turn vel-low and fall prematurely. This happens on red and white currants particularly. The Leaf Spot lesions are distinguished from those of the anthrac - nose by their larger size, sharp outline, and pale, dead center. Cause.
The causal organism, Mycosphcefella Grossularice, is a fungus which has been known since 1842. It was first described from Europe. When the affected leaves fall to the ground, the fungus is carried with them. It begins to develop perithecia, but these do not reach maturity before winter comes. With the advent of spring these bodies complete their growth. Within each many ascospores are formed. These are discharged with force into the air, are caught by the air currents, and are carried to the young leaves. The ascospores germinate and their germ-tubes enter the leaves, wherein mycelium is developed. In a short time the leaf-tissue is killed in local areas and the result is a spot. Finally asexual, summer, fruiting bodies are formed. These are black, spherical structures, called pycnidia, occupying the central portion of the lesion. At maturity each pycnid - ium is filled with long, curved pycnospores. These ooze out and are scattered promiscuously. Some perchance fall on currant or gooseberry leaves; they germinate and cause spots. Some time during the summer (June to August) the affected leaves fall, and preparations for the winter are again made by the fungus. Damp cloudy weather favors the fullest development of Mycosphcerella Grossularice.
Fig. 53. - Mycosphaerella Leaf Spot on currant.
The spraying schedule for currant anthracnose is reliable for the control of this Leaf Spot. Five to seven applications of lime sulfur used at the rate of one gallon of lime sulfur solution (commercial concentrate) to fifty gallons of water are effective. Finely ground sulfur (dust), ninety parts, and powdered lead arsenate, ten parts, has been shown to be satisfactory. The spray or dust should be applied: (1) when the first leaf-clusters appear; (2) subsequently every two weeks until the first of August. (See Control of Currant Anthrac - nose, page 208.)
Stewart, V. B. Dusting nursery stock for the control of leaf diseases.
Experiment for the control of the leaf spots of currants. Cornell Univ. Agr. Exp. Sta. Circ. 32: 8-9. 1916. Stewart, V. B. Some important leaf diseases of nursery stock. Septoria Leaf Spot of currants and gooseberries. Cornell Univ. Agr.
Exp. Sta. Bul. 358: 198-200. 1915. Pammel, L. H. Spot diseases of currants and gooseberries. Iowa Agr. Exp. Sta. Bul. 13: 87 - 70. 1891. Pammel, L. H. Treatment of currants and cherries to prevent spot diseases. Experiments with currant spot diseases. Iowa Agr.
Exp. Sta. Bul. 30: 289 - 291. 1895.
Stewart, F. C, and Blodgett, F. H. A fruit disease survey of the Hudson Valley in 1899. Currant diseases. Leaf spot. New York (Geneva) Agr. Exp. Sta. Bul. 167: 291 - 292. 1899.
Stone, R. E. The perfect stage of Septoria Ribis. Phytopath. 6: 109. 1916.