It is in the humid regions of the Appalachian Mountain valleys that Hypochnus Leaf Blight is most destructive. The disease is known, however, from Maine to Florida, and is most common in North Carolina, Florida, Alabama, Georgia and West Virginia. The pathogene was probably introduced from Brazil, and is known to attack the apple, pear, quince, snowball and lilac.
The chief damage is wrought through the loss of leaves. Affected trees are devitalized, and while twigs are killed, this is an indirect injury through the effects of the disease on the foliage. The disease is, then, principally a Leaf Blight. From a distance the symptoms are similar in appearance to those of Fire Blight, but Hypochnus Leaf Blight is clearly distinguished as follows: (1) affected leaves droop and are matted together, a character not exhibited by Fire Blight; (2) small sclerotia are present on twigs adjacent to diseased foliage (Fig. 36); these sclerotia are white when young, but are cinnamon-brown in color when mature; they are roundish or oblong and measure one - eighth of an inch or less in diameter; (3) accompanying the sclerotia are found rhizomorphic strands extending lengthwise of the twigs and petioles; (4) in later stages the leaves fall.
Fig. 36. - Hypochnus Leaf Blight pathogene; sclerotia on apple - twig.
The causal fungus, Hypochnus ochroleuca, hibernates as sclerotia on or near the terminal buds. As new twigs develop in the spring the fungus renews its activities by sending out rhizomorphs from the sclerotia. By this means the pathogene spreads over the lower surface of the leaves, causing them to droop and die, and finally to fall prematurely. In some cases the fruit is an object of attack. The rhizomorphs follow the petioles and main veins of the leaves, finally separating into single mycelial threads. Sometimes mycelial wefts are found on the lower sides of the leaves; these are always connected with the strands or rhizomorphs on the petioles and the strands in turn are in organic connection with the sclerotia on the twigs. Occasionally the weft of mycelium on the lower surface of the leaves assumes a more even texture and ultimately becomes the fruiting layer of the fungus. Here the weft consists of a very closely woven mass of hyphse. Erect hyphal threads become basidia, each with four spores. The spores are capable of reproducing the structures of the pathogene: the mycelium, sclerotia, rhizomorphs, and finally the basidia. This fruiting stage, however, is rare, and the dissemination of the fungus is efficiently accomplished by the sclerotia and rhizomorphic strands. Throughout the summer the mycelium spreads from one leaf to another, fastening them together in a mat. This matting of leaves constitutes one of the peculiar and striking symptoms of Hypochnus Leaf Blight. By midsummer the rhizomorphs form new sclerotia on or near the uppermost buds, where the winter is again passed. The rule seems to be that sclerotia develop in greatest abundance on the distal buds of the twigs where they can be of maximum use in perpetuating the fungus. Sclerotia occasionally develop on the fruit, but in general they are more prevalent on the lower, shaded side of the foliage.
Spraying to kill the sclerotia is the only known remedial measure. Special fungicides and special applications are not regarded as necessary. Bordeaux mixture applied before the buds open, and again before the blossoms open, is recommended.
Stevens, F. L., and Hall, J. G. Hypochnose of pomaceous fruits.
North Carolina Agr. Exp. Sta. Rept. 32: 76-85. 1909. (See also Ann. Myc. 7: 49 - 59. 1909.) Smith, R. I., and Stevens, F. L. Insect and fungous diseases of apple and pear. Hypochnose. North Carolina Agr. Exp. Sta. Bul.
206: 90 - 94. 1910.