Dead Arm, or side arm, is found on nearly all commercial varieties of grapes in the eastern states, although it is rare on the Delaware variety. The Pocklington, although not extensively grown, is apparently most susceptible.
The disease appears to be of American origin, and there is little doubt but that it is still confined to this country. It was unknown until 1909, when its prevalence and cause were discovered. Losses from Dead Arm are estimated at 1 to 5 per cent annually for New York vineyards, while the total for all states concerned is unquestionably of considerable importance.
The arms, trunks, green shoots, petioles, peduncles, leaf-veins and fruits are affected. As the name indicates, the disease exhibits itself chiefly through the death of the arms (Fig. 67). Frequently, however, the whole vine dies, in which case suckers usually grow up at the base. The affected vines may die at any time. However, the majority die in winter. Occasionally longitudinally ribbed excrescences develop on the trunk or arm of diseased vines. These outgrowths are not fleshy, nor are they hard, and they, should not be confused with Crown Gall (see page 253) and outgrowths following winter injury. Internal symptoms are characterized by a dry Heart Rot of the trunks in affected portions.
Fig. 67. - Dead Arm of the grape - vine, general appearance.
In June or early July the leaves show a peculiar yellowing, dwarfing and curling. Later, the discoloration disappears, although the other abnormalities persist. Such vines, while apparently on the road to recovery, are found to be dead or are considerably weakened the following year. The vines may die during the summer and as a result the leaves wilt. Many vines freeze to death as a result of a weak growth the previous season. On the green shoots, petioles, peduncles and leaf-veins small reddish brown or black spots are produced (Fig. 68). These may be deep, in which case V-shaped slits are noticeable; or they are more superficial, and are so numerous as to coat the affected part for some distance. When these lesions are older, they show either as reddish elevations or as a longitudinal cracking. Berries affected with the Dead Arm disease exhibit a rotting (Fig. 69) very similar to that in the case of the Black Rot disease. They shrivel to a mummy as in Black Rot, but have a slightly more grayish aspect and the pustules are less numerous and more scattered in Dead Arm than in Black Rot.
Fig. 68. - Dead Arm on grape - canes.
It has been experimentally demonstrated that the fungus Cryptosporella Viticola causes Dead Arm. From vines which were diseased the previous year pycnospores ooze forth, during wet weather, about the time the buds burst in the spring. These spores are spattered promiscu-ouslv, some of them falling on young shoots, either those directly beneath the old infected cane or those some few feet away. After about thirty days symptoms of Dead Arm begin to appear. During this period the spores of the fungus have germinated, the germtube has penetrated the part concerned, and infection has resulted. Days of mist and fog favor spore-germination, the process requiring not more than twenty - four hours. The manner in which the fungus gets into the vine is unknown.
In a great many cases the mycelium developed from the germtube may grow from affected canes into the permanent wood of the spur, arm or trunk, although in cases of less severe infection this does not take place. The growth of the mycelium in the canes is very slow, but it gradually gets into the arms and trunks. When affected canes are saved for bearing wood, the fungus spreads into the arm. As a rule the mycelium does not extend into the roots. Pycnidia are developed abundantly on affected canes and on green and ripening berries (Fig. 70). They appear on the former early in the spring, and may be found on canes severely infected the previous year or occasionally on the current year's growth. The pycnidia mature shortly after the buds burst in the spring, and the pycnospores ooze out in long, reddish yellow coils, each containing several thousand spores. Rain favors this exudation. Perithecia are developed on a stroma beneath the bark, but these bodies do not seem to be important in the life - history of the fungus. They are rarely found, having been reported from but three southern states.
Fig. 69. Rot of grapes caused by the Dead Arm pathogene.
It has been shown that the transmission of the fungus through nursery - stock is of no little consequence. Badly affected cuttings do not root. Cuttings which show but few lesions will probably make vines which might grow for several years without exhibiting marked evidence of the disease. It has been shown further that infections may result from the pruning tools; in cutting through an affected vine bits of the fungus, which cling to the tools, are deposited in the cut next made on another vine. Control.
In attempting to control Dead Arm the following points should be remembered:
Fig. 70. - Dead Arm; fruiting bodies of the pathogene.
(1) Vines which exhibit a dwarfed, yellowed foliage during the early part of the season (in June) may be regarded with suspicion; it is probably a case of Dead Arm. Diseased plants are thus located.
(2) About this time the ground is being cultivated frequently; vines may then be marked and at trimming time may be removed. In two or three seasons all affected plants may be eliminated.
(3) The disease affects the heart-wood. If vines show this, they should be removed and the cut should be made at some point below the last indications of Dry Rot. Often this is at the ground, in which case renewals come from below the surface.
(4) Renewals must be carefully inspected, for they may become infected from some outside source.
(5) At trimming, reddish elevated lesions should be watched for. More than passing attention must be given when Dead Arm is present in the vineyard.
(6) Vines may be brought quickly into profitable bearing by the renewal system.
(7) It has been shown that the fungus may be carried by pruning tools.
Healthy vines may be protected from new infections by the application of bordeaux mixture. This should be done at the time recommended for the first Black Rot spraying, that is, when the shoots are eight to ten inches long. The application should be made every year, regardless of the weather. For while practically no infections occur in certain years, yet such conditions cannot be safely forecasted by the grower. Nurserymen should take cuttings from vineyards which receive this early application or which are known to be free from Dead Arm.
Reddick, Donald. Dead Arm disease of grapes. New York (Geneva)
Agr. Exp. Sta. Bul. 389: 463 - 490. 1914. Reddick, Donald. Necrosis of the grape vine. Cornell Univ. Agr.
Exp. Sta. Bul. 263: 323 - 343. 1909.
Gregory, C. T. A rot of grapes caused by Cryptosporella viticola.
Phytopath. 3: 20-23. 1913. Selby, A. D., and Van Hook, J. M. Dying of bearing grape - vines.
Ohio Agr. Exp. Sta. Circ. 64: 1-6. 1907. Shear, C. L. The ascogenous form of the fungus causing Dead Arm of the grape. Phytopath. 1: 116 - 119. 1911.