This section is from the book "The Gardener's Monthly And Horticulturist V20", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
There are few cultivators of the Peach who have not been annoyed by the disease known in this country as the curl and in Europe as the blister. About the time the leaves are fully developed in Spring they begin to twist and crimp and shrivel, either in whole or in part, and when the disease is very bad an occasional tree will lose almost the entire foliage - a second crop of leaves usually quickly replacing those which arc destroyed.
If an examination be made when the distortions begin to appear, they will often be found infested with aphides, and a certain proportion of curl in the leaf is undoubtedly caused by the attacks of these insects. The injury they accomplish is usually very early in the season, and they generally disappear entirely before the disease has reached its maximum. The leaves they attack often assume a reddish hue, and seem favorite places for the fungus named at the head of this article, - which is the principal cause of curl, - to commence its growth. It is often difficult to say precisely how much is due to the insect and how much to the fungus on the leaves which have suffered from both, but as the season advances the fungus assumes entire possession of the field. Its speres lodge on the Surface of the leaf and immediately produce small short-jointed and irregularly shaped threads or mycelium that penetrate the substance of the leaf, between the cells of the parenchyma, which is also stimulated to abnormal growth, causing the thickening wrinkling and ultimate blanching of the leaf, which are the symptoms of the disease in question.
For some time nothing can be found on the surface of the leaves, but at length a whitish filmy mould may be seen in places by the naked eye, which is a mat of fruit-bearing branches of the mycelium in question, and shortly after this is produced the disease usually disappears.
The fruit is composed of small sacs terminating the short erect branches that rise from the leaf; generally they contain eight transparent spores, and the growth of the parasite is promoted by warm moist weather.
There are several species of this genus of fungi, all of which are parasitic on living plants. Asconiyces Juglandis lives on Walnuts, A. bul-latus attacts Pear leaves. The latter was described and figured in Vol. IX, Journal Horticultural Society, of London.
Peach curl seems to have been first alluded to by Berkeley in his Introduction to Cryptogamic Botany, and named and figured by him in his Outlines of British Fungology. It is also described and figured by W. G. Smith in the Gardener's Chronicle of July, 1875; the figure must, however, be considered rather as diagramatic as regards our American form. Smee, in My Garden, copies Berkeley's figures, but says he has not been able to see the fungus, and believes "the aphis is constantly present, and the fungus is but rarely so. However it may be in England, in this country the reverse is true, although it is often necessary to use chemical re-agents to detect the creeping mycelium in sections of the leaf before the fruiting branches appear.
Except when trees are old or feeble, we have rarely seen sufficient injury by Peach curl to render serious efforts for its destruction advisable. Undoubtedly, if a remedy were indispensable, it might be found in some of the preparations of sulphur applied by such methods as are used in extirpating mildew from foreign Grape vines.