THE VINE PLAGUE.
Characteristics.
—The first, or at least the most characteristic signs of this disease appear especially after a summer rain, or after the first fall rain. The leaves then become spotted with yellow. The following season these yellow spots appear as if fused together, and many leaves become entirely yellow, except the veins, which stand out bright green. Some leaves are invaded by the yellow from the edges, while the veins as before remain green. These yellow spots soon turn brown, the leaves dry up and curl slightly backwards and finally fall off, leaving the canes bare. During the very first appearance of the vine plague, many leaves turn brown and dry up in certain spots in the vineyard without the previous appearance of any yellow spots. The drying of the leaves proceeds either from the center of the spots, or from the margin of the leaves, destroying both the leaves and their veins. Later on in the fall a new crop of leaves appear, but these leaves are small or very small, bright green and sickly, and do not continue to develop after they have reached a certain size, different in different vines. In red varieties of grapes, the yellow spots in the leaves gradually turn red or claret colored, often resembling the most beautiful autumn leaves. In districts where the disease is common, these leaves are generally known as calico leaves on account of their peculiar markings.
The canes do not attain their regular growth, and fail to mature in the fall, or mature only in spots, the balance of the wood remaining dull green. The inner parts of the canes are, as a rule, more mature than the tips. Very often only one or two joints nearest the stem mature, and in bad cases no part of the canes mature, but at the advent of the rain turn black and die. Late in the fall the tips of the green canes turn black, dry up and snap off like glass when touched. The pith turns in the older canes dark brown, dries up prematurely and dies, while in very young canes the pith remains watery like a semi-transparent jelly.
Many vines have no mature wood when the leaves have fallen in the autumn, while others again have some. While the spotted leaves may appear all over the vineyard, the diseased canes appear on vines in spots, these spots in the vineyard growing larger year after year. A dead vine may be seen in the midst of healthy ones, while a healthy vine, on the other hand, may remain in the midst of dead ones. It takes generally several years to kill the vines, and some varieties are hardier than others. Some Muscats may succumb in one year, while some will last for three years or more. The roots remain alive and healthy longer than any other part, and, when the top of the vine has already died, it is common to see the root send up a healthy sucker, which, however, in its turn, will become diseased and die. It is likely that the vines in some districts will suffer more than in others, and in places the vines may not become seriously injured by the disease.
The berries on badly diseased vines do not develop, but shrivel up or remain sour, and in some cases dry up entirely. In others, again, they acquire a mawkish taste, lose flavor and sweetness, and make only inferior or bad raisins. These many different characteristics of the plague depend evidently on the stage of infection. They do not follow each other in any certain succession, nor do they all appear on the same vine. Some vines show one face of the disease, other vines show another, and the observer must have been previously acquainted with the disease before he can readily recognize it.
Nature and Cause.
—The cause of the vine plague is not known. No deadly fungus has so far been found on the vine, nor has any other deadly parasite been found on the diseased vines. In California the vine plague has been studied by N. B. Pierce, of the Agricultural Department at Washington. He suggested once that the disease was of bacterial nature, but has not proved his theory, his investigations not yet being finished. Mr. E. Dowlen has also been investigating this disease, and at one time thought it caused by a fungus, which, however, was proved later by Dr. H. W. Harkness to belong to the non-injurious kind. No insects of any kind prey on the vines in sufficient numbers to cause the serious symptoms of the vine plague.[7] Whatever may be the true cause of the vine plague, certain it is that it resembles in its advent and spreading such diseases in men as cholera, yellow fever or the Oriental plague. The vine plague appears to be especially promoted by warm, moist air and rain, but it is not confined to damp places, nor has it as yet been ascertained in what relation it stands to locality and climate.
[7] The most interesting and correct account of the vine plague yet published is found in an essay on “The Mysterious Vine Disease,” by Newton B. Pierce, read before the State Horticultural Convention, at Los Angeles, March, 1890, and published in California—A Journal of Rural Industry, May 10, 1890; Vol. 3, No. 18.
In California it first made its general appearance in Anaheim in Orange county, in the month of August, 1884, when vineyards of old Mission vines suddenly stopped growing, and the grapes failed to color and ripen, while many of the vines died the same year. The plague attacks in preference vines growing on poor, sandy or alkaline soil, or in vineyards underlaid with hardpan. The weak vines succumb the first of any. This is the reason why so many vineyardists doubt the existence of any particular disease, contributing the poor condition of the vineyard to anything else than the true cause.
N. B. Pierce, who has now spent a year in studying the vine plague, has found many similarities between it and the mal nero of Italy; but the descriptions of the foreign investigators are both contradictory and insufficient, and, without a personal investigation of the Italian or French vines, the identity of our vine plague with any foreign disease cannot be established. It is to be hoped that the United States Congress will make such investigations possible. At present we do not even know whether the vine plague is original in this country or whether it was imported from foreign countries. The general opinion in the first attacked district is that the disease was imported there with grapevines brought from Europe. So far I have not been able to ascertain when and by whom such vines were imported, but I am satisfied that in the course of time it will be found that foreign grapevines were imported to the vineyards where shortly afterwards this disease first appeared.
Damages.
—The damages caused by the vine plague may be summed up as follows: The leaves turn spotted and yellow, finally dry up and fall off. The canes fail to mature, or mature only in spots. Later in the fall, they die from the tips, which turn black and become brittle. The berries either dry up or shrivel up, and fail to mature, or at any rate become mawkish or bitter. The yield becomes less and less every year, although, the first year that the vines are touched by the disease, the yield is often unusually large. In severe cases the vine dies in from one to three years, but a few may linger longer.
Remedies.
—The vine plague has existed in this State for six or eight years, but as yet few, if any, efforts have been made to extinguish it, and only during the last year have any experiments been carried on. The solutions of bluestone and lime which many expected would prove beneficial to the vines attacked by the plague have, in my opinion, done little or no good. Spraying the vines when in full foliage with the I X L compound greatly benefits the vines, and proves a powerful stimulant and the best remedy yet employed.
LEAF-HOPPER (Erythroneura comes).
Characteristics.
—This pernicious little pest is a bug which multiplies in enormous quantities and sucks the sap out of the vine leaves. Many use the name of thrips to denote this insect, but this is incorrect, as the thrips is an entirely different, much smaller, insect, which so far has never been injurious to the vines of this coast. In size the leaf-hopper is, at maturity, about one-tenth of an inch. In color it is yellowish white, with a few red spots. When the insect approaches maturity, it jumps, but the undeveloped insect or larva only crawls, principally on the underside of the vine leaves, where their cast-off skins can be seen in all stages and sizes. The eggs are laid in the veins of the leaves. The glossy globules which are always seen on leaves where the leaf-hopper is found are not the eggs, as has been supposed by many, but is only the vomit which, when irritated, the hopper throws out either as a defense, or because it desires to rid itself of an unnecessary burden. The leaf-hopper hatches at least two times, or possibly three times, during the summer. Many of the insects remain over during winter time. They feed on almost anything, such as alfilerilla (Erodium), etc., but are especially fond of the grapevines, and even in the early spring flock onto the young vine shoots, leaving the less desirable weeds. In some localities this insect is known variously as the white fly, the vine-hopper, or incorrectly as the thrips.
Damages.
—The hopper punctures the leaves and causes them to dry up and fall, thus exposing the grapes to the hot sun. The excrement of the hoppers also covers the grapes largely, and spoils their appearance and keeping quality, at least as table grapes. It is principally the table grapes and wine grapes which are injured by this insect; the former are made unfit for shipment, and the latter do not color well when deprived of their leaves. If the grapevines are kept growing, the grapes are less injured, and some growers even contend that the hopper is advantageous, as it causes the leaves to fall and the grapes to mature.
Distribution.
—It is not known whether the leaf-hopper is a native of California, and I hardly believe it is. It does not exist in Southern California, but in Northern California and in the San Joaquin valley it is common. In the grape districts of Southern California there is found another variety of leaf-hopper almost twice the size and of a brilliant green color, which only once appeared in such quantity as to do any damage at all. Generally it is quite rare. The Erythroneura comes, however, occurs in countless numbers, and often rises in clouds when the vines are approached. In some years it is less common than in others, and after having been plentiful for several years gradually diminishes in quantity, but never disappears entirely.
Remedies.
—Pasturing the vineyards with sheep as soon as the grapes are picked is very beneficial. The sheep destroy both hoppers and leaves, and the following season always finds the hoppers greatly diminished in numbers. The sheep do no injury to low-pruned vines, and in Fresno many vineyardists pasture their vines regularly every year in October and November, or as soon as the grapes are picked and the vineyards are made accessible.
The gauze bell consists of a bell-shaped cover made of wire netting, large enough to cover the vine. The inside of the bell is sprayed with petroleum, and then turned over the vine. A shake is then given the vine, when many leaf-hoppers will fly up and stick in the petroleum. It will only pay to use this remedy on table grapes; for raisin grapes it is too expensive.