This peculiar and obscure raspberry disease is variously known as curl, Leaf Curl, yellows and the Marlboro disease. The term yellows is used to cover a variety of symptoms and is very loosely applied by the grower to any condition in which the leaves turn yellow. The name Marlboro disease is local in usage; it owes its origin to the fact that the Marlboro variety is very susceptible. By reason of its descriptive nature the name yellows seems most appropriate.
Raspberry yellows was first recorded from Minnesota in 1894. At present its geographical range is probably coincident with the plant affected, having been observed at one time or another in Ohio, New York, Connecticut, Massachusetts, Pennsylvania, Kansas, Michigan, Colorado, California and Washington. It is also said to occur in Canada. All who are acquainted with the disease concur in that it is very important. In certain localities it is regarded as the most prevalent and the most destructive of raspberry diseases. In many cases where the raspberry industry has been ruined the yellows disease has been held responsible for such conditions. Red raspberries stand alone in their susceptibility to injury from yellows. Only rarely are the black - caps and purple varieties affected appreciably. In order of their susceptibility red raspberries follow:
Cuthbert, Marlboro, Golden Queen, Early King and Herbert. The St. Regis is resistant. Occasionally other red varieties are affected. Often the Cuthbert and Marlboro are so injured that they are discarded from commercial plantations. Where the disease has wrought noticeable damage the original acreage has been reduced to one - fifth within a period of eight years. In such localities the annual loss is estimated at $200 per acre.
The true raspberry yellows (Fig. 118) has a striking and characteristic appearance and should never be confused with lack of thrift in plants due to cultural conditions, nor should it be confounded with Cane Blight. A withering or blighting of canes or leaves does not occur in the case of yellows. In general the disease does not appear until two years after planting; sometimes three years may elapse before it really attracts attention. Once affected, the plant continues to show signs of yellows annually to some extent. Cases are on record where recurrence of the disease has taken place for nine consecutive years.
Fig. 118. - Raspberry yellows; healthy plant on left, diseased on right.
Both healthy and diseased raspberry plants may occur in the same stool. In extended outbreaks all gradations between healthy and affected plants obtain. One of the more striking symptoms of yellows is that the plants are stunted, sickly and bushy in aspect (Fig. 118). In general the condition recalls that of peach yellows. The fruit-bearing laterals are dwarfed, often being but one-half their normal length. The leaves themselves present an unmistakable characteristic when affected with yellows (Fig. 118). They are abnormally small, and the margins of the upper leaves curl downward (Fig. 118). The tissue between the leaf-veins arches upward, and as a result the veins appear sunken. This uneven growth brings about a curling of the foliage (Fig. 118). In summer, the foliage acquires a mottled appearance (Fig. 118); at first a light coloration prevails, then it gradually changes to a darker green and yellow, and finally is reddish bronze. The colorations accompanying and symptomatic of yellows vary considerably with the soil and climatic conditions. The disease is more conspicuous in dry and hot weather. The berries on diseased plants usually become dry before maturity, or they ripen ten to fourteen days early. In case they dry up, the change may be gradual or sudden. If the fruit ripens prematurely, it is bitter, lighter in color, and smaller than normal. Not uncommonly the flowers develop prematurely on the tip of the current year's growth. From all appearances the root - system is normal.
As yet the causal nature of raspberry yellows is unknown. Various factors have been assigned in this connection, but always without proof. Among these may be noted fungi, insects, bacteria, poor drainage, lack of soil fertility and other similar factors. Authorities agree that fungi cannot be the cause. Likewise entomologists say that insects are not concerned. The red spider has been suspicioned, but conclusive evidence is lacking. If bacteria are the cause, they have not been seen. It is possible that they are present, but are so very small that they are easily overlooked. It has been noticed that the disease is more pronounced where plants were growing in soil which had a high water table. Plants growing on soil having compactness and lacking drainage are more liable to exhibit yellows than lighter well-drained soils. It has been suspicioned as having some connection with Crown Gall and Cane Blight, but careful observations do not substantiate this view.
No preventive measures are known, and little hope is in sight until the causal nature is understood. Fungicides have proved themselves worthless in controlling the disease. A few recommendations have been suggested, and the more important of these may be noted: (1) in planting red raspberries obtain plants from localities where raspberry yellows is known not to occur; (2) select varieties which have shown resistance in the neighborhood; (3) select for a plantation-site a light, or medium - heavy, adequately drained soil; (4) irrigation is helpful where practiced; (5) destroy diseased plants.
Stewart, F. C, and Eustace, H. J. Raspberry cane blight and raspberry yellows. II. Raspberry yellows. New York (Geneva) Agr. Exp. Sta. Bul. 226: 362 - 364. 1902.
Melchers, L. E. A preliminary report on raspberry curl or yellows. Ohio Naturalist, 14: 281 - 288. 1914.
Green, S. B. Leaf curl of raspberry. Minnesota Agr. Exp. Sta. Ann. rept. 1894:230. 1895.