The American gooseberry, mildew is indigenous to the United States, where it is has been known for at least three - quarters of a century. It probably originated in this country on wild gooseberries before this fruit was brought under cultivation. It is believed on good authority that the fungus was carried from the United States to southwestern Russia about 1890, from which point it spread into neighboring European countries. The disease made its first appearance in western Europe in 1900, when it was found in Ireland. Undoubtedly the mildew fungus was introduced from America about that time. In 1901 it broke out again and became increasingly prevalent during the four years succeeding. With Russia and Ireland as centers, the disease soon became prevalent throughout Europe wherever gooseberries were cultivated.
The disease is far more serious in Europe than in America. It is a well-recognized fact that when a fungus is introduced into a new and favorable climate and among new varieties of hosts, it is liable to become more destructive. In this respect the gooseberry mildew pathogene is no exception. It is now established in the Old World and has become a permanent factor in gooseberry culture. While the fungus is less virulent in America, yet it is one of the most serious enemies of this fruit. The berries are usually rendered unsalable, and sometimes are even destroyed. The vitality of affected bushes is reduced, and they are more susceptible to winter injury than those which remain free from mildew.
The lower parts are first affected. The disease appears in May or June. In severe cases not a single berry remains free from the attacks of the pathogene. The leaves and young canes are also affected. At first there is a superficial whitish growth having a moldy, powdery appearance. Several spots may coalesce to form large patches. The leaves and tips of the stems may become distorted in serious cases. Affected berries are sometimes dwarfed. Finally the fruit may crack and decay. In later stages the mildew patches become buff or rusty - brown in color. Scattered through the felty growth, small black specks may be observed.
The American gooseberry mildew fungus is known as Sphcerotheca mors-uvce. The small specks mentioned above are perithecia of the parasite. Within each of these bodies is found a single ascus containing eight ascospores. These spores remain in the perithecium during the winter, either on the fallen leaves or on the canes of the past season's growth. Sometimes, when the leaves disintegrate, the perithecia hibernate in or on the soil. In the spring when the leaves are half grown, or later, the ascospores are forcibly ejected from the perithecia. The spores come to rest on the young leaves or fruits. Following germination of the ascospores a weft of mycelium develops which grows superficially. At certain places, however, small sucker-like bodies, called haustoria, are sent into the host cells; these are the organs of feeding. Nourishment is derived from the host and the mycelium grows. In the early part of the growing-season summer spores are formed. They are produced in chains on erect branches sent up from the weft of surface mycelium. The conidia (spores) are produced in great numbers, and soon fall away, giving the lesion a dusty or powdery aspect, whence the name Powdery Mildew. The conidia are scattered by the wind, and they give rise to new infections similar to those just described. Toward autumn the perithecia are formed, and these serve to carry the fungus through the winter.
This is one of the most difficult of Powdery Mildews to control, particularly in wet seasons. However, if remedial measures are thoroughly applied, less difficulty will be experienced. The work may be done at a slight expense.
The standard remedy has been to spray with potassium-sul-fide (liver-of sulfur), one ounce to two or three gallons of water. In New York State and elsewhere lime sulfur solution, at the rate of one to forty, has proven effective. In Oregon it has been found that an application of lime sulfur at dormant" strength on dormant branches, followed by a solution diluted one to thirty, gives excellent satisfaction. It is suggested that it may be desirable to use potassium-sulfide in later applications, since lime sulfur leaves a deposit on the sprayed parts. The applications of a fungicide should be made as follows: (1) when buds break open; (2) at intervals of ten days until at least five applications have been made. The more susceptible varieties may demand more sprayings.
Pruning in the fall may reduce the amount of the inoculum, although there are few experimental data on which to base positive recommendations. The rapidity of the spread of the fungus in Europe has necessitated legislation concerning it. Considerable money has already been spent along this line in England, but the disease is still serious in spite of all efforts. It is advisable that weeds be kept from gooseberry plantations, especially beneath bushes in order to allow free circulation of air.
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