This disease, which is called Brown Rot of stone-fruits, mold, blossom blight, twig blight, peach Rot, Brown Rot canker and other names, was not given serious consideration in America prior to 1881. It is now a well-known fungous trouble wherever the peach is grown, both in Europe and in the United States. The pathogene causing Brown Rot probably came from some foreign country. History shows that the disease has been more serious in some years than others in America. In 1887 it attracted no little attention in Maryland and Delaware. In 1891,1893, and during subsequent years Brown Rot has been of considerable importance in the Delaware and Chesapeake peninsula. Alabama growers experienced a severe epiphytotic in 1897, while in 1900 the disease was the most conspicuous and the most destructive in Georgia since the beginning of stone - fruit culture in that state.
Brown Rot is most prevalent and most destructive in the warmer peach-growing states, such as have already been enumerated. In warm, wet seasons the trouble is severe in the northern states. The light - colored varieties are generally regarded as the most susceptible. Those showing least rot in Georgia are the Carman, Early Crawford and others. Observations on the question of varietal resistance need to be extended.
The average annual loss to the peach-growers of this country because of Brown Rot is placed at $5,000,000. For the year 1900 Georgia growers are said to have lost from $500,000 to $700,000, and the average annual loss is estimated to be not less than 40 per cent. Again, in 1909 the loss of peaches due to Brown Rot in Georgia, with only one-third of an average crop, is said to have reached $1,000,-000. Almost a total loss of the peach-crop is reported from Alabama in 1897, and similar losses occur in other southern states. While in the northern states and in Canada peach-growers do not experience any such calamities, yet these regions are by no means exempt from the disease, and in some years the outbreak is severe. The loss from this disease is not limited to the fruit-growers, but is felt by the transportation companies, the commission men and the consumers. A shipment of fruit which shows great promise as it leaves the orchard may reach the market in a worthless condition. The disease is destructive in one or more of the following ways: (1) blossoms may be blighted, destroying the set of fruit; (2) twigs may be blighted, thus inflicting serious injury on the tree; (3) large limbs may be cankered, which form of the trouble is of great importance in parts of New York State; (4) the crop of fruit itself may be partially or wholly destroyed as a result of attacks by the Brown Rot pathogene; green fruits, if injured by insects or hail, may be rotted extensively; and likewise ripe fruits on the tree, in transit, or in market may be wholly ruined as a result of rotting. Symptoms. The fruits are most commonly affected (Fig. 71). However, other organs of the host are also susceptible to the disease; these include the blossoms, twigs, limbs (Figs. 72 and 73) and occasionally the leaves.
Fig. 71. - Brown Rot on peach.
In America the flowers are commonly affected, resulting in a serious blossom blight. When the blossoms are opened, the petals turn brown and shrivel, but do not fall. Grayish tufts composed of the fruiting structures of the pathogene show on the affected blossoms. Twig blight follows blossom blight very closely, the former being a direct result of the latter. The leaves on such twigs wither and die and cling to the twig, as in the case of Fire Blight of pears. Twig blight may also result from the spread of the pathogene from affected fruit into the twig by way of the fruit-pedicel. This happens very commonly in New York State orchards. From the twig the causal pathogene passes into the larger limb where it spreads out, forming a canker (Figs. 72 and 73). The Brown Rot canker is a definite dead area in the bark, the surface is sunken, and the lesion is accompanied by a flow of gum (Fig. 73). The disease is said to affect peach leaves, showing itself as a shot - hole.
Fig. 72. - Brown Rot canker of peach, general appearance in the orchard.
Fruits generally show signs of Brown Rot after they are half grown, the susceptibility of individuals increasing as they approach maturity. The lesion on the peach is at first evident as a small, more or less circular, dark-brown, decayed area with a rather indefinite line of demarcation between the healthy and diseased portions. This rapidly enlarges, and soon the fruiting structures of the pathogene appear on the surface as grayish tufts (Fig. 71). At first these tufts occur sparingly, but in a day or so the original rotted spot may become densely dotted with the characteristic ashen mold (Fig. 71). With the enlargement of the lesion the whole fruit becomes involved, is brownish, shrunken, and eventually shrivels into a dark mummy. The mummy may cling to the tree or it may fall to the ground. While on the tree mummies cling together in groups of two or more. Cause of Brown Rot.
Fig. 73. - Brown Rot canker on peach limbs.
The pathogene, the fungus Sclerotinia cinerea, hibernates in both the fallen and hanging mummies, and in the cankers. From sclerotial crusts in the fallen mummies arise apothecia (Fig. 74) in the spring of the year. The hanging mummies furnish a habitation for co-nidia through the winter; in the spring these spores are carried to the susceptible parts. These old clinging mummies as well as the cankers also contain mycelium of the fungus, and in the spring conidia are developed from these sources. The source of the primary inoculum is found, then, in both the fallen and hanging mummies, and in the cankers. The inoculum itself consists of both ascospores and conidia, of which the latter are the more important. In fact, it sometimes appears that ascospores are not developed every year, and that they play a very minor role in the life-history of the Brown Rot fungus. Spores of either kind are carried to the blossoms, where blossom blight is induced. The mycelium grows throughout the blossoms, enters the twigs, and finally passes into the large limbs. Conidial tufts develop on the petals, and even on the cankers and blighted twigs, if the atmosphere has a high relative humidity. The conidia from these sources start the disease in other parts of the peach. From one or more of the several possible sources conidia are carried to the half-grown fruits. These spores germinate, and the tubes enter through an injury; the curculio and the peach scab fungus are the chief agents in making such wounds. The mycelium develops profusely within the tissues of the fruit, the result snowing externally as a Brown Rot. Conidial tufts soon appear and conidia are liberated to continue the destructive action of the fungus. Where two or more peaches touch each other, the mycelium grows from the infected peach through the point of contact to the other. In this way the fruits are made to cling to each other, several in a group.
Fig. 74. - Apothecia of the Brown Rot fungus; attached to fallen mummies.
The fungus is highly favored by a series of cloudy days accompanied by frequent showers, especially at picking time; consequently a great amount of fruit may be destroyed under such weather conditions. Prolonged drizzly weather is far more dangerous than a heavy rain followed by clearing. Hot weather favors the rapid growth of the fungus and increases the danger of its destroying the crop. On the other hand, in a dry, cool season the crop may be expected to remain relatively free from Brown Rot.
In applying control measures it should be borne in mind that: (1) all parts of the peach above ground are liable to attack; (2) the disease is most serious in warmer regions and in warm, wet seasons; (3) total destruction of the crop may come suddenly and unexpectedly; (4) the fruits and blossoms are affected seriously, which fact is of vital importance when measured in dollars; (5) the fungus overwinters in the mummies and in the cankers, and the mummies are found both on the ground and hanging to the tree; (6) the fungus enters fruits chiefly through wounds, and the curculio is a serious offender in making such wounds. Therefore, remove all cankers from the larger limbs, and prune out affected limbs of smaller size. The methods followed are described under another heading (see page 52). In pruning, knock off the hanging mummies. Bury the fallen mummies by plowing to a depth of several inches; this should be done before the blossoms open. The regular pruning operations assist to no little degree in the control of the Brown Rot. The removal of limbs admits sunlight and air, thus allowing the susceptible parts to dry off more quickly after rains.
The fruit should be sprayed. This can be done without fear of injury to the foliage by the use of self-boiled lime sulfur of the 8-8-50 formula (see page 438). For curculio, add 2 pounds of lead arsenate to 50 gallons of the fungicide. Never use lime sulfur solution as a summer spray, even at dilute strengths, since a 1 to 300 dilution may seriously burn peach - foliage. Spray: (1) about the time the blossoms fall; (2) two or three weeks later or about one month after the petals drop; (3) about one month before the fruit ripens.
Limited experiments indicate that lime sulfur solution 1-40 may be used safely and effectively before the blossom-buds open. This application purposes to prevent bud - infection, and is worthy of trial.
Quaintance, A. L. The brown rot of peaches, plums and other fruits.
Georgia Agr. Exp. Sta. Bul. 50: 237 - 269. 1900. Jehle, R. A. Peach cankers and their treatment. Cornell Univ. Agr.
Exp. Sta. Circ. 26: 53-64. 1914. Jehle, R. A. The brown rot canker of the peach. Phytopath. 3:105110. 1913. Smith, E. F. Peach rot and peach blight. Journ. Myc. 5:123 - 134.
1889. Smith, E. F. Peach blight, Monilia fructigena. Journ. Myc. 7:3638. 1891. Norton, J. B. S. Sclerotinia fructigena. St. Louis Acad. Sci. Trans.
12: 91-97. 1902. Scott, W. M., and Ayres, T. W. The control of peach Brown Rot and scab. U. S. Agr. Dept. Plant Indus. Bur. Bul. 174: 7 - 26. 1910.
Humphries, J. E. On Monilia fructigena. Bot. Gaz. 18: 85-93. 1893. Cooley, J. S. A study of the physiological relations of Sclerotinia cinerea (Bon.) Schroter. Missouri Bot. Gard. Annals, 1: 291 - 323.
1914.' Scott, W. M., and Quaintance, A. L. Spraying peaches for the control of Brown Rot, scab, and curculio. U. S. Agr. Dept. Farmers' Bul.
440: 5-40. 1911. Scott, W. M., and Quaintance, A. L. Control of the Brown Rot and plum curculio on peaches. U. S. Agr. Dept. Ent. Bur. Circ. 120:
5-7. 1910. Scott, W. M., and Ayers, W. T. The control of the peach Brown Rot.
Georgia State Bd. Ent. Bul. 30: 96 - 104. (Georgia State Hort.
Soc. Proc. 33:96 - 104.) 1909.