Perhaps the most common of strawberry diseases is the Leaf Spot. It is also called spot disease, sun-burn, sun-scald, Leaf Blight, and, erroneously, strawberry rust. These names will recall to the mind of the reader the disease under consideration.
This Leaf Spot is found in Europe and America. First studies were made in France in 1865. About 1880 it began to attract more than usual attention in the United States, and for five or six years thereafter complaints of its prevalence and destruc-tiveness came from many parts of this country. While the trouble is now known in practically all parts of the United States, the strawberry - growers of the northeastern quarter of the country are the heaviest losers on account of this disease.
The disease is usually less severe in Florida, Louisiana and other southern states.
In many cases the amount of injury done by the Leaf Spot is doubtless over-estimated. Yet it must be remembered that the disease does some damage nearly every year, so that after all the pest is not only one of the most common but also one of the most destructive to strawberries. In certain localities it is of comparatively slight import, while in other places whole plantations have been completely destroyed. In southern Illinois, a leading strawberry section, Leaf Spot is regarded as one of the worst enemies of this fruit. In Connecticut, one grower is said to have experienced a loss of $250 an acre, while the damage done in 1904 to Maryland strawberries is estimated at 21 per cent of the total acreage. This approximates a loss of $50,000.
In the ordinary cases of attack the foliage is impaired; in severe cases the leaves are affected to the extent that the plants die. In any case, the prospect for the next year's crop is usually threatened. If the fruit - pedicels are attacked, the berries never mature but remain small, become shriveled and worthless.
The most serious injury is said to occur on such varieties as the Wilson, Hunn, Beeder Wood, Warfield, Sanafee, Photo, Monmouth and Ohio. The varieties which resist the disease to a noticeable degree are the Marshall, Brandywine, Lady Thompson, Michel's Early, Daisy, Eureka and Jewell. It is not guaranteed, however, that the resistance and susceptibility herein indicated will be found applicable or reliable in all strawberry sections. It is well to bear in mind that ecologic factors may have no little influence in this respect. Accordingly, then, observations as to varietal resistance should be carefully considered. It is generally held that vigorous plants are no more resistant than the more feeble individuals. It has also been observed that plants growing on heavy, wet, undrained land are more severely diseased than those on light, well-drained soil. Plants exposed to the sun are said to be more affected than those which are growing under shade; it appears that shade has a preventive effect, a phenomenon not easily explained. Moisture alone increases the amount of Leaf Spot, yet during an excessively rainy season plants remain quite free from the disease. This freedom from Leaf Spot during a very wet season has been explained on the grounds that cloudy weather offers much the same conditions as those of shade. In a year of continued drought the disease is likewise less common. In general, conditions most favorable to the disease are hot, bright days with occasional showers or heavy dews.
The disease makes its appearance on the leaves (Fig. 123), calyx and fruit - pedicels. In the beginning, the spots are very small, deep purple or red, and are usually first evident on the upper surface of the leaves. These areas rapidly increase in size, and at the same time the color of the central portion changes from a purple to a reddish brown; eventually the lesions become grayish or whitish in the center (Fig. 123). The border remains purplish, shading off to a reddish brown coloration towards the healthy. The single lesions vary from i to i of an inch in diameter, but several spots may coalesce to form a large irregular blotch. Severely affected leaves turn brown, this discoloration beginning at their tips; finally such leaves shrivel and die. The lesions on the other susceptible organs are similar to those on the leaves.
The Leaf Spot disease of strawberries is produced by the fungus Mycosphcerella Fragarice. The mycelium of the fungus grows in and between the cells, and through the action of these threads upon the host-cell contents there is a response expressed in the spots as described. After a short time the hyphae amass themselves beneath the cuticle of either surface of the leaf, and from these masses conidiophores bearing conidia arise. These spores develop, are scattered and produce infections throughout the summer; with the advent of winter conidial formation ceases, but the mycelium remains alive in the leaves until the following spring. At this time a new crop of conidia develop, which begin the cycle anew by infecting again the strawberry plants. Should any of these spores perchance fall on the common cinquefoil, the disease would be produced. It should also be mentioned that this plant acts as a weed - host, carrying the fungus through the winter and furnishing a source of the inoculum in the spring, as does the strawberry plant itself.
It has been noted that conidial production ceases in the fall and that the winter is spent as mycelium in the old leaves. Toward winter the fungus makes preparations for other means of hibernation. Masses of mycelium appear on the surface of the leaf; these are sclerotial bodies which are capable of producing conidia in the spring following. A third means of over-wintering is by the perithecia; these arise in the fall, but remain immature until March or April, when mature ascospores may be found therein. Unlike the ascospores of most ascomycetous fungi, they are not ejected from the perithecia nor are they blown to the new leaves. They germinate within the perithecia, even while yet in the asci, their germtubes emerging both through the ostiola and through breaks in the walls of the perithecia. At these points the germtubes cease growth and form conidia which are cut off and carried to the strawberry and cinquefoil. The first infections of the year occur most commonly about the time the fruit sets; later ones develop throughout the growing - season. The conidia germinate in the presence of moisture, and the tubes penetrate the plant more commonly through the cuticle, rarely through the stomata.
Fig. 123. - Strawberry Leaf Spot.
It has been shown that the mycelium does not descend to the roots or stems to hibernate. But, recapitulating, the winter is spent as follows: (1) as mycelium in the leaves; (2) as sclero - tial bodies on the surfaces of the leaves; (3) as immature perithecia in the leaves.
In selecting varieties for planting, avoid the more susceptible sorts. Set in well-drained soil, and use only healthy plants. Remove diseased leaves before setting. Spray with bordeaux mixture 4-4-50 before the blossoms open to protect the plants against first infections. Repeat the application two or three times as the occasion demands. In ordinary cases, the first spraying is done just after the fruit is harvested. If there is an unusual prevalence of Leaf Spot, renew the settings annually. Mowing and burning after harvest will often given a new set of leaves which go into the winter quite free from the fungus.
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