Apple Brown Rot is at times erroneously called Black Rot; likewise Black Rot is often referred to as Brown Rot. The two diseases should not be confused. Apple Brown Rot is much less common than Black Rot in America, while in Europe Brown Rot is by far the more common. In fact, Brown Rot is one of the most serious of apple rots in Europe. In the United States the disease occurs to some extent in several different states, including North Carolina, West Virginia, Virginia, New York, Minnesota, Arkansas, Nebraska, New Mexico and Missouri. It probably occurs in other apple states.
Varieties do not all suffer alike. The disease is more common on summer varieties like the Yellow Transparent and Chenango. In Kentucky, the Genet is injured more than other varieties. In England, a similar disease affects the limbs, forming cankers.
The first indication of the disease may appear while the fruit still hangs on the tree, and the symptoms consist in the development of a smooth, brownish discoloration in the skin. The rotted area increases in size, the general form being retained. The pathogene often comes to the surface and shows itself as grayish tufts (Fig. 37); these may be arranged in concentric circles. The rotting of the fruit is finally complete (Fig. 37). In many cases the affected fruit becomes jet black and the skin assumes an ebony aspect. Fruits showing such symptoms usually exhibit no external signs of fruiting bodies of the causal pathogene. The conditions that determine whether an apple affected with the Brown Rot disease will remain brown or will become black, as already described, are not well understood. The appearance of these various characters has been explained in relation to weather conditions as follows: - (a) if the weather is warm, and the atmosphere has a high relative humidity, the affected fruits become brown, and grayish tufts make an early appearance; (b) if the relative humidity is reduced, the fruit is at first brown, then black, and the tufts are rare or absent; (c) if the air is dry and cool, the affected fruit is black and no grayish tufts develop. In regions where dry air prevails, this type of the Brown Rot disease is the more common. Likewise under conditions of storage approximating those last enumerated, the Brown Rot mummies are shiny and jet black. In any case the interior of the affected fruit is brown and soft. It has been stated elsewhere (page 140) that Brown Rot and Black Rot are sometimes confused. The following external characters serve to distinguish the two diseases:~ (1) Apples affected with Black Rot are at first brown, and sometimes remain so, but more often become black:, and the surface is dotted with minute black pustules. These characters are sufficient to distinguish Brown Rot from Black Rot. (2) Apples affected with Black Rot shrivel rapidly, become greatly reduced in size and are considerably wrinkled. Apples affected with Brown Rot do not shrivel greatly, are not appreciably reduced in size, and are much less wrinkled. (Figs. 10, 11 and 37.)
Fig. 37. - Brown Rot on apple.
The apple Brown Rot pathogene, Sclerotinia cinerea, is a fungus which has been confused with Sclerotinia fructigena, the organism causing Brown Rot of pome-fruits in Europe. Both species may attack both stone- and pome-fruits, but S. fructigena invades chiefly pome - fruits in Europe, while S. cinerea is the more common form in America. It is doubted that the European S. fructigena occurs in this country. The habits and structures of the two organisms are very similar.
Hibernation occurs chiefly as mycelium in the hanging mummies. In the spring conidia are liberated from tufts developed from the over-wintering mycelium. As the fruits come to maturity the conidia, falling into wounds on the fruit, germinate, and finally cause Brown Rot. In the autumn many affected fruits fall to the ground; others hang on the tree over winter. It is the latter class which plays the important role in carrying the fungus from fall to spring. The history of the fallen mummies is not known; whether a sexual stage ultimately develops from them has not been definitely shown. It is reasonable to assume, however, that such is the case, judging from the habits of the fungus on stone - fruits.
Careful remedial measures have not been determined for American conditions. The suggestion is made on good authority that spraying for apple scab will help to control the apple Brown Rot. Store the fruits in a dry, well - ventilated, and clean house at the customary low temperatures.
Heald, F. D. The Black Rot of apples due to Sclerotinia fructigena.
Nebraska Agr. Exp. Sta. Rept. 19: 82 - 91. 1906. Clinton, G. P. Apple rots in Illinois. Brown rot. Illinois Agr. Exp.
Sta. Bul. 69:190. 1902.