It is probable that the pathogene causing White Rot is native to America, although it was first found in Italy about 1878. In 1885 it was observed in France, and in 1887 considerable alarm was aroused because of White Rot. That same year the disease was first noted in America, having been found in southwestern Missouri and neighboring territory. Since its discovery in the United States the disease has appeared in Ohio and New York and the grape-growing regions of the Southwest. In recent years White Rot has been found in Algeria and Hungary.
White Rot is regarded as one of the less important of the grape diseases in America. It is not often serious, although it is said to do damage in some parts of Ohio. The amount of losses from White Rot is decidedly greater in European vineyards than in those of this country.
White Rot affects the fruit, young shoots, stems of berries, and rarely the foliage.
Berries may or may not be directly affected. Generally the disease appears first on the fruit-stalks, as a result of which the berries wither and become dry. Berries affected directly show Brown Rot lesions. The fruit becomes abnormally juicy, shrivels, and brownish pustules appear over the surface. Berries may be affected after maturity. In severe cases all fruits in a single bunch may be diseased. Eventually a dry mummy is produced, but it neither blackens nor shrivels into prominent ridges as in the case of Black Rot.
Cankers are formed on the peduncles. These are brownish, depressed areas which may enlarge continuously until the part is girdled. This results in a withering of the berry, as already noted.
The causal fungus is referred to as Coniothyrium diplodiella. It is claimed by certain French authorities that at least one other form (sexual) exists in that country, and therefore the proper name should be Charrina diplodiella.
The mycelium of the fungus is abundant in the lesions, particularly those on the berries. Sometimes the seeds are affected. In the peduncles the mycelium causes the death of the tissues as a result of which the berries wither. About the time grapes begin to ripen pycnidia are produced. These arise as a result of a special growth of the mycelium; a cushion is developed in which a cavity bearing spores is finally formed. These fruiting bodies lie beneath the cuticle until maturity, when they burst forth, first appearing as shining, rosy points, then white and ultimately brown in color. Although perithecia have been reported in France, they are unknown in America.
It is doubtful whether special treatment for White Rot is ever necessary. The disease should be controlled by the Black Rot spray - schedule; at least there is nothing to indicate the contrary.
Quaintance, A. L., and Shear, C. L. Insect and fungous enemies of the grape east of the Rocky Mountains. White Rot. U. S. Agr. Dept. Farmers' Bul. 284: 36. 1907.
Scribner, F. L. White rot. U. S. Agr. Comm. Rept. 1887: 325 - 326.1888.