Grape - growers in certain parts of America have known this trouble for at least fifty years. It has been reported especially from Connecticut, central and western New York, and Michigan. What appears to be the same disease has been observed in Australia (Queensland) and in many parts of France. In France there appear to be two forms of shelling: one in which there is a failure of the flowers to set fruit in some or all parts of the cluster, known to the French as coulure; the second form, in which the fruits fall or shell. The latter type is the more important in this country; the losses at times being serious. Fifty per cent of the fruit may drop when affected with shelling, or rattling.
Affected grapes fall two or three weeks before maturity. Those diseased berries, particularly of the green varieties, exhibit a peculiar though indistinct mottling of the surface. The skin becomes abnormally thick, and the whole berry is harder than healthy berries of the same age. The interior of such a fruit shows a brown zone just beneath the skin. The taste of shelling berries is noticeably insipid as compared to the tart, astringent flavor of the healthy, unripe berries. Shelling grapes separate easily from the stem, leaving the latter as if cut with a knife; no such phenomenon occurs with unaffected berries. Generally those berries at the lower end or at the extremity of the shoulder are first to fall from the bunch. Shelling is not always accompanied by foliage discolorations, nor is a browning of the leaves a certain indication of the disease.
The cause of grape-shelling is obscure, although a great many suggested causes have been eliminated by close students of the trouble. Among the excluded primary causal factors may be noted: certain fungi, insects, lack of phosphoric-acid and meteorological conditions. Those agencies which are thought to increase or favor shelling are: a weakening of the vines due to overbearing, heavy vegetative growth, excessive nitrogen supply emphasized by over-tillage, prolonged drought or excessive rains followed by drought, and a poorly developed root - system. One authority concludes that a lack of potash is in many cases the primary cause. There is considerable evidence, however, that this is not a factor. The agencies already listed as favoring the trouble may be regarded as exciting causes.
Since the main cause of shelling is apparently deficient nourishment, steps should be taken to correct such conditions. The soil should receive attention; potash should be applied where needed. Cultivation of poorly-nourished vines aggravates shelling; proper cultivation is essential. Care should be taken to properly handle vines that overbear; such plants need an adequate food - supply. In France it has been advised that shoots be pinched or cut off after the development of six to eight leaves where the grapes are borne; or twelve to fifteen on those shoots not carrying fruit. This diverts the elaborated food from the shoots to the berries. This procedure is said to give satisfactory results in France.
Sturgis, W. C. Notes on the so-called "shelling" of grapes. Connecticut Agr. Exp. Sta. Rept. 1896: 278 - 281. 1897.
Rainsford, E. H. Coulure, or non-setting of grapes. Queensland Agr. Jour. 10: 41 - 42. 1902.
Clinton, G. P. Report of the botanist for 1906. I. Notes on fungous diseases, etc., for 1906. Grape, Vitis sps. Shelling and rot. Connecticut Agr. Exp. Sta. Rept. 1906: 315 - 316. 1907.
Lodeman, E. G. Some grape troubles of western New York. Shelling or rattling. Cornell Univ. Agr. Exp. Sta. Bul. 76:413 - 440. 1894.