This section is from the book "The Gardener's Monthly And Horticulturist V18", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The grape vine seems to afford special inducements for the growth of fungi; Curtis, in his list, enumerating no less than eighteen species which grow upon it. "The grape disease, properly speaking, that which proved so disastrous at different times to the vines in Europe and Madeira, is caused .by a fungus to which Berkeley has given the name of Oidium Tuckeri." It is a form of a fungus which has not yet been recognized in its perfect state, and is supposed to occur to some extent on this side of the water; but as many fungi have this conidial form, and some so nearly identical with it, and also growing on the grape, there may be some doubt as to our vines ever being attacked by the true grape disease of Europe.
The disease which most interests the grape growers of America, is the fungus called Pero-nospora viticola by Berkeley and Curtis. It is quite common, appearing on the under surface of the leaves about the first of August, and continues to flourish until the dying leaf will nourish it no longer. It can be most easily seen on the smooth leaves of Vitis cordifolia, as small frost-like spots, which rapidly spread, and soon cover the whole leaf, frequently extending down the petiole to the stem. This fungus, like the potato rot and other closely related species, thrives best in moist, warm weather.
Under the microscope, the tissue of the grape leaf is seen to contain an abundance of minute threads, which force their way in all directions between the cells of the leaf, thrusting their suckers into the cells to rob them of their contents. When the time for fruiting comes, some of these threads push through the stomata of the leaf-branch considerably, in a definite and peculiar manner, bearing the conidial spores on their tips.
* An abstract of an article on this subject by Dr. W. G. Far-low, in the last issue of the Bulletin of the Bussey Institution.
Under the head of the germination of these asexual bodies, Dr. Farlow has several interesting experiments. They germinate equally well in the dark as in the light. Those sown in the morning germinate more quickly and abundantly than those sown in the afternoon. It was not possible to keep the conidia which were produced in the night until the afternoon, as they generally fell from their attachments in the morning, and began to germinate. In all cases the germination took place with surprising regularity. At the end of an hour the conidia were slightly swollen, and their contents had begun to segment, each segment having a light-colored nucleus. At the expiration of an hour and a quarter, the segments had resolved themselves into a number of oval bodies, which collected at the distal end of the conidia, and which, before long, succeeded in rupturing the cell wall and making their escape from the mother cell. They passed out rather slowly, usually one at a time, and paused for a moment in front of the opening, where they remained as if not yet quite free from one another. In a short time each segment began to extricate itself from the common mass, moved more and more actively, and finally darted off with great rapidity, a full-fledged zoospore, furnished with two cilia.
The average number of these rapidly moving spores is from six to eight. Their movement gradually grows slower, and in fifteen to twenty minutes come to rest. Soon an outgrowth proceeds from one side, and rapidly develops into a new plant.
The oospores, which are the result of sexual action, are found in autumn within the tissue of the shrivelled leaves as spherical, thick-walled bodies. They escape by the breaking up of the dry leaf, or through the decay of the surrounding tissue.
The fungus under consideration is common on Vitis aestivalis, - Michx., V. labrusca L., and their cultivated varieties; V. cordifolia, - Michx., V. vulpinata, and nearly all varieties of cultivated grapes. The statement is often made that it does not grow on V. vinifera. By carefully conducted experiments in the laboratory, Dr.
Farlow found this mildew could be made to grow on the leaves of this European species, and with the usual luxuriance.
It is quite gratifying to know that so common a disease as this one upon the grape does no real damage, but on the contrary is beneficial to the grape crop. It makes its appearance late in the season, when the large leaves have finished their work. By shrivelling up the leaves, the Peronospora enables the sun to reach the grapes without loss to the vines, as is shown by the fact that the vines continue to live on from year to year without apparent injury.
In Europe, where the "winters are warmer, the springs earlier, and the summers much moister than here, it is quite possible that the advent of the Peronospora, by reason of the greater warmth and moisture, would be some weeks earlier than here, before the vine had attained its growth, and at a time when the leaves are needed for the work of absorption and assimilation." Should this be the case, the disease which with us gives little cause for alarm, might prove very disastrous in the vast vineyards of Europe.