The disease under consideration is generally known as anthracnose, which name was given it in 1887. Prior to that date it was called cane rust. The first account of the disease comes from Illinois dated 1882. Complaints were made about 1885 to 1887 of the serious injury which anthracnose was doing in Illinois, Wisconsin, Missouri, Texas and New Jersey. The sources and number of these complaints indicated that the disease was then widespread and destructive. More recent records of the trouble show that it occurs in the North Central States, the North Atlantic Division and the Western Division where the output in quarts is greatest.
Raspberry anthracnose (Fig. 119) also affects the blackberry (see page 161). The raspberry, however, suffers more from this disease than the blackberry. All above-ground parts are injured more or less. Canes are sometimes girdled and are therefore killed. Leaves are about one - half their normal size and distorted when affected by anthracnose. Fruits, if formed at all, may not reach their full development, but they ripen prematurely and are therefore worthless. If young canes are not killed the first year, the formation of fruit for the next year is prevented.
It is difficult to estimate the amount of the losses incurred from anthracnose because of the nature of the losses. It has been estimated in Missouri that 10 to 12 per cent of the entire crop is injured. In 1904 in one county in Nebraska 33 per cent of the crop was injured, and in 1907 one - half the crop in Wisconsin is thought to have suffered. But no records based on carefully selected data are available which will indicate the annual dollar loss. In many plantations everywhere the disease is enphytotic, the amount of the injury being about the same from year to year and in no case disastrous or fatal to the affected plants.
Some varieties suffer more than others. In general, black raspberries are more susceptible than red varieties. Among the red raspberries the Antwerp is injured to a considerable degree, and the spots are very conspicuous thereon. The Cuthbert, on the other hand, is but slightly affected. Among the black varieties the Cumberland is susceptible.
Although anthracnose is preeminently a disease of the canes, no part above the ground is free from the attacks of the patho-gene. The disease first appears on the canes when they are from six to ten inches high. Both fruiting and non - fruiting canes (suckers) are affected. On the suckers the disease shows more commonly at the base. In any case the larger and older lesions are found toward the base of the cane. The course of development appears to take place from below upwards. Younger and smaller lesions always are to be found toward the tip of the cane. The spots (Fig. 119) are at first small, purplish and variously scattered. Their form is elliptical. As the lesions become older they increase in size and the color, particularly at the center, becomes grayish white (Fig. 119). The margin remains purplish. It is slightly raised, and thus the healthy and diseased tissues are sharply separated (Fig. 119). In more advanced stages the affected areas coalesce and the cane appears irregularly blotched. These blotches are often an inch and a half long and sometimes girdle the cane. The affected cane attempts to heal the wound so that old spots have a rough, scabby aspect. Sometimes canes are severely cracked (Fig. 119), in which cases the pith is exposed. Often the affected areas present an appearance of large irregular cankers in which the surface is sunken and the bark split lengthwise. Canes thus affected cannot function properly, and as a result they become sickly. Their leaves are dwarfed and the fruit, if formed at all, never reaches full maturity, but ripens prematurely or dries up.
The petioles of the oldest leaves are attacked soon after the canes are affected. Purplish spots as described for the canes above are developed. The affected tissue enlarges along the petiole, finally reaching the leaf proper. On the leaf whitish, blisterlike lesions appear. The leaves then become distorted, their edges rolling in toward the midrib. Spots on the leaf-blade are similar to those on the canes, although smaller; they are one-twenty-fifth of an inch or less in diameter. They are closely distributed, but rarely coalesce. Frequently the diseased area, in which most of the tissue is dead, becomes separated from the surrounding healthy tissue and the leaf shows a shot-hole effect. Leaves on the laterals of new canes do not ordinarily become diseased. Anthracnose shows on the fruit. The upper drupels are more commonly attacked. Many fruits are affected while still green in color and sometimes before they are larger than a pea. A small brown speck develops on the end, but soon the entire surface is involved. As the fruit matures the surface of the lesion becomes depressed. If affected while small, the drupels remain firm and finally become dry. Fruit - pedicels may be attacked, in which case the fruit dries up and dies.
Fig. 119. anthracnose on canes of black raspberry.
The anthracnose lesions just described are caused by the fungus Glceosporium venetum. Details of its full life-history are as yet unpublished, but the facts as generally known will be discussed. The winter is spent in the canes. It is very likely that in addition to mycelium, immature fruiting structures are concerned in this connection. In the spring conidia are formed on the old diseased areas by the fungus. These spores are disseminated about the time the plants are from six to ten inches high. Apparently the exact origin of the conidia is not generally known. The perithecial stage has recently been described, but as yet is not named. The ascospores developed within the perithecia are shot forcibly into the air and later germinate by budding. These buds are identical with the conidia; they may be referred to as such. Conidia probably also are developed in acervuli. These spores attack and their germtubes penetrate only the tender growing parts of the cane, as evidenced by the smaller and younger spots at the tip and the older ones at the base (see Symptoms). Following the germination and penetration processes mycelium ramifies in the cortex of the cane where the cells are killed. Subsequently conidial fruit - bodies are developed from which conidia are liberated. These initiate secondary infections on the canes, leaves and fruits. These secondary infections continue throughout the summer. In the winter the fungus again remains dormant in the canes.
Diseased canes should be eradicated before the spring season. It is preferable that this be done at the end of the fruiting-season. Short rotation should be practiced. Ordinarily it will not pay to keep a plantation of black raspberries after it has produced its third crop. Clean, anthracnose - free plants should be set. All weeds should be kept down; this operation tends to reduce the relative humidity about the canes; moisture favors the fungus.
It is possible to keep a plantation relatively free from the disease by the use of bordeaux mixture 4-4-50 in the spring. The applications should be made as follows: (1) before the leaves appear; (2) when the leaves are well developed and by the time the shoots are six inches high; (3) just before the blossoms appear. Whether subsequent applications are needed will depend on the weather and the severity of the disease. Later sprayings are made at ten - day intervals.
Lawrence, W. H. Anthracnose of the blackberry and raspberry.
Washington Agr. Exp. Sta. Bul. 97: 3 - 18. 1910. Jackson, H. S. Anthracnose of raspberry, blackberry, loganberry, etc.
Oregon Crop Pest and Hort. Bienn. Rept. 1911-1912: 261 - 263.
1913. Scribner, F. L. Anthracnose of the raspberry and blackberry. U. S.
Agr. Comm. Rept. 1887: 357-361. 1888. Paddock, W. Anthracnose of the black raspberry. New York (Geneva) Agr. Exp. Sta. Bul. 124: 261-274. 1897. Detmers, Freda. Anthracnose of raspberry and blackberry. Ohio Agr. Exp. Sta. Bul. 4 (no. 6): 124 - 126. 1891. Burkholder, W. H. The perfect stage of the fungus of raspberry anthracnose. Phytopath. 4: 407. 1914.