The disease here discussed is known commonly as California peach blight, blight, shot-hole, Fruit Spot, and sometimes as brown spot, pustular spot and winter blight. The earliest record of it comes from France in 1843. In recent years, particularly from 1905 to 1907, the disease was the subject of considerable comment in California. The first report of it from that State dates back only to 1900, although doubtless the parasite had been there for many years prior to that time. It was first observed in Michigan peach orchards in 1893, and subsequently has been seen in Indiana, Oregon, Ohio and New York. The distribution in America would appear rather limited. It occurs widely in Algeria on apricots, peaches and cherries; in France on almonds; in Australia and New Zealand on plums and other stone - fruits; and in Europe commonly on cherries.
This blight disease constitutes a well - known and important factor in peach growing throughout California. Most damage, however, is wrought in the more humid localities, where no variety seems to be immune. Trees are more or less weakened, depending on the severity of the attack, and frequently crops are destroyed. Buds are killed, green fruiting twigs are spotted, and leaves are dwarfed. Premature defoliation is not at all uncommon.
The effects of the Coryneum Blight are shown early in the spring. This is particularly true under California conditions. The most evident symptoms of blight consist in a gumming and death of the buds on the fruiting wood, accompanied by a splitting of the bark on branches of the current year's growth. Affected buds may die before spring and thus fail to start, or they may start and die later after the young fruit sets. Blighted twigs become spotted, and an exudation of gum ensues; finally death occurs .The spots may show as early as the first of February, at least in California. These are depressed areas in the bark, and are in reality cankers. The leaves may be affected when young. The lesions produced are circular, brownish spots, with dark-red margins; finally the affected tissue falls away, leaving a hole in the leaf where each dead spot first occurred. Where several such holes occur there results a shot-hole appearance. This shot-holing is more common as a result of the first lesions produced; lesions developed later in the season do not exhibit this peculiarity. On the latter spots small grayish tufts of the pathogene may be seen. Lesions produced on peach-fruits resemble somewhat the effect induced by the San Jose scale (Fig. 82) At first the spots are small and purplish red. As the diseased portion enlarges, a light - colored area develops in the center (Fig. 82). Later the spots are brown and confluent. In extreme cases the fruit becomes cracked and a flow of gum follows.
The lesions described above are induced by the fungus Coryneum Beijerinckii. The chief period of infection appears to be from December to February; most inoculations occur about January first. In California no infections occur after the end of April; in Oregon these take place from May 10 to June 15. Rainy, weather, favors infection The conidia of the fungus .are carried to the buds, leaves and fruits, germination occurring in a short time in the presence of moisture. After infection, spore-cushions break through the skin of the various parts affected. These are visible to the naked eye as dark spots, and are most abundant about leaf scars and roughened portions of the bark on defoliated twigs in the spring and summer. They also develop on Leaf Spots. The conidia appear to live through the summer lodged in the bud - scales, being very resistant to desiccation.
It has been found in the Pacific Coast regions that Coryneum Blight can be controlled by fall and spring spraying. Peaches should be sprayed about November first, just as soon as possible after the fruit is harvested. Bordeaux mixture 6-6-50 is regarded as the best fungicide available at present. This autumn application alone has been effective under California conditions, and it may also prove entirely satisfactory for Oregon orchards. The present recommendations, however, for Oregon conditions include supplementary sprayings in the spring, using self-boiled lime sulfur 8-8 - 50. Two applications are made as follows: (1) May tenth; (2) June first; if the disease is unusually prevalent and the weather rainy, the second spring application may be made about May 20 or 25, and a third spraying about June 5 or 10.
Fig. 82. - Coryneum Blight lesions on peaches.
Smith, R. E. California peach blight. California Agr. Exp. Sta. Bul. 191:73 - 98. 1907.
Jackson, H. S. Peach diseases. California peach blight and fruit spot. Oregon Crop Pest and Hort. Bienn. rept. 1911-1912: 255 - 257. 1913.
Cordley, A. B., and Cate, C. C. Spraying for peach fruit spot. Oregon Agr. Exp. Sta. Bul. 106:4 - 15. 1909.