This section is from the book "The Gardener's Monthly And Horticulturist V26", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
We had supposed that this disease which takes the form of blistered and succulent blotches on the leaves, with a white mildewy substance beneath, was everywhere and generally familiar to peach cultivators. But specimens with inquiries as to the nature of these blisters come to us from different quarters, with the information that it was in those localities hitherto unknown. It is also very much worse in some parts of the country than in others. We have never seen it anywhere so destructive as in Canada, unless what we saw in California along the Stanislaus river was the effect of the curl, as we were told it was. Whole branches were dead, with the dry leaves attached to them. In Pennsylvania only a few of the earlier leaves are attacked; these fall off, but the shoots continue and make the new and healthy leaves necessary to health. The wood is weakened but not destroyed.
The disease is caused by the growth of a minute fungus parasite. Each species of fungus requires certain exact conditions of heat and moisture before it will germinate, and judging from the facts attested in these widely separated localities, we conclude that a comparatively low temperature is required by this one that produces the peach curl, and that when the weather gets very warm, or say to our eastern summer heat, this species will not develop. A steadily warm temperature will therefore be the best protection against the curl.