This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
I first announced the occurrence of this dreadful parasite in Europe in the Gardeners' Chronicle, 25 August 1900 (fig. 374). The specimens were sent by Mr. (now Sir) F. Moore, F.L.S., and were obtained from County Antrim. Soon after this announcement was made the hue and cry commenced, and no political partisan ever succeeded in making more capital out of a point than was attempted to be made out of the advent on European shores of the American Gooseberry Mildew. According to various authorities, and for divers reasons, the fate of the British Empire has frequently hung in the balance, and only required the final tip, which happily never was administered, to send it headlong to destruction, but never before was the Empire so near the verge of destruction, as when this parasite made its home amongst us. The mildew is admittedly a very serious scourge to Gooseberries, and it was extremely impolitic, to say the least, of a person supplicating for the presence of the said disease only a few years after its advent, in order that it might reduce the quantity of gooseberries, which on account of an exceptionally good crop throughout the country, could not be sold at a profit. Soon after its occurrence in Ireland it appeared in considerable abundance in Worcestershire in the Evesham district, and a small amount was met with on one standard Gooseberry on a Ribes aureum stock in Kent. At the present day the mildew occurs in various districts in England, being especially abundant in the neighbourhood of the Wash. American Gooseberry Mildew has also appeared on the Continent, in Norway, Sweden, Denmark, Germany, Hungary, and Russia. It has been stated that the disease was introduced into Ireland and the Continent from America. As the mildew is a native of the United States, this statement may be quite true, but it lacks evidence. It has also been stated that the mildew came to England from the Continent, but out of a batch of standard Gooseberry bushes imported at the same time, only one showed a little mildew, and that only after it had been in this country for some time, so that the evidence is not convincing. The batch of Gooseberries in the Evesham district, where the mildew was first observed in England, were not imported, but raised on the spot. In the United States the mildew is not uncommon on the native Gooseberries, but does not assume the proportions of an epidemic; but it attacks European species so virulently that they cannot be cultivated at a profit. The mildew has also been met with sparingly on Black and Red Currants in this country. There is nothing unusual in the behaviour of American Gooseberry Mildew. Epidemics, that is severe outbreaks of a disease, are practically unknown, except under very exceptional circumstances, among the indigenous plants of any country. Such have grown along with the parasitic fungi, also indigenous, for such a long time, that they have become accustomed to each other, and neither can take any great advantage of the other. On the other hand, when you introduce a plant into a new district, it often suffers severely for a time. The same is true when a parasitic fungus invades a new country, as in the present instance; at first it simply runs riot with those plants it can attack, but gradually loses its virulence. When the Hollyhock Rust first invaded this country, it rendered the cultivation of Hollyhocks practically impossible for a time. The rust is yet with us, and will probably remain, but it has found its level, and Hollyhocks are grown in abundance. (See Vol. II, p. 54).
Fig. 374. - American Gooseberry Mildew.
A, Leaves and fruit attacked (nat. size), b, Ascus from winter fruit containing 8 spores, x 400. C, Perithecia or winter form of fruit, x 150. D, Conidial or summer form of reproduction, x 200.
The American Gooseberry Mildew first attacks the tips of the young shoots, rarely extending backwards for more than a few inches; it soon afterwards appears on the fruit. It first appears as a pure-white, thin mildew, much resembling in general appearance the European Gooseberry Mildew. The mildew soon becomes powdery, due to the formation of summer spores, which are scattered by wind, birds, insects, etc, and infect adjoining plants, on which the mildew soon appears and produces summer spores, which infect other plants, and thus the disease spreads with great rapidity unless promptly checked. Soon after the summer spores have been scattered, the mildew changes to a dull-brown colour, and becomes thicker in texture, more especially on the twigs and berries; in fact, the patches of mildew become so thick and felty that they can be readily scraped off. Later in the season the felt-like patches of mould become studded with small black fruits resembling in size the winter fruits of the European mildew. Some of these remain attached to the mildew on the twigs until the following season, others fall to the ground, but in both instances the spores are liberated in the spring, and are capable of infecting the young shoots of Gooseberries.
Fig. 375. - American Gooseberry Mildew. Appearance of afflicted shoots in September.
The preventive measures that have proved most satisfactory in combating this disease are as follows. Commence spraying with a solution of potassium sulphide ( = liver of sulphur) 1 oz. in 3 gall, of water, just when the leaf buds are expanding, and repeat about every ten days, or more frequently if heavy rains occur. This method is adopted as a means of preventing the spread of the disease by means of summer spores. With the object of eradicating the disease, a different line of treatment should be pursued. Somewhat early in the autumn every shoot should be pruned back for at least 6 in. This practically removes all the shoots bearing mildew and winter fruit of the fungus. In some places the prunings are dumped in heaps to be disposed of at some convenient time. In other places the prunings are not collected at all, but left lying on the ground around the bushes. The prunings should be collected and burned at once. The time to prune is somewhat difficult to determine. If it is done too early in the season, and is followed by mild weather, new shoots are sometimes made which may become infected, and so carry over the disease to the next season. If pruning, on the other hand, is unduly delayed, many of the winter spores drop to the ground, and, unless due precautions are taken, endanger the bushes for the following season. This can be guarded against by digging over the ground during the winter, by which means all winter fruit of the fungus, whether lying on the ground or on fallen diseased leaves, are buried and rendered harmless. In places where the methods outlined above have been honestly carried out, the disease has been reduced to a minimum, or altogether banished.