Brown Rot of Peach

John C. Dunegan.

Brown rot has been the scourge of peach growing in the more humid sections of the United States since Colonial times. Sometimes it is called simply the rot because it occurs so commonly and affects so adversely the fortunes of all who handle and eat peaches.

Its first symptom on the fruit is a small, brown spot, which rapidly enlarges and soon destroys an entire peach and all peaches near it. Masses of gray spores form on the surface. As the fungus develops best in warm, humid weather, brown rot long was considered merely the aftermath of such conditions. By 1880, however, people realized that rot did not develop solely as the result of the "delicate" nature of the peach but that a fungus caused it.

Now we know also that a common insect, the plum curculio, is implicated in the spread of the brown rot disease. The punctures the insect makes when it feeds and lays eggs furnish ideal points of entry for the spores of brown rot. Although brown rot can be very serious even if the plum curculio is absent, it cannot possibly be controlled during harvest periods of warm, rainy weather if the insect is prevalent in an orchard.

For many years the fungus, Monolinia fructicola, was considered to be identical with a brown rot fungus that attacks plums, apples, and pears in Europe. The two are distinct, however. The fungus on peaches, plums, cherries, and (only rarely) apples and pears in the eastern part of the United States is a native of the region. Probably it occurred as a disease of wild plums long before the settlement of the Western Hemisphere.

The European brown rot fungus is now known to be present in California, Washington, and Oregon and in a few scattered localities east of the Rocky Mountains. Its activities are described by E. E. Wilson in another article in this volume (p. 886). I shall discuss only the common brown rot fungus so prevalent in the eastern part of the United States.

The life cycle of the brown rot fungus is a vicious circle: The fungus constantly multiplies its sources of infection so that the disease can become more destructive, given proper weather conditions, each succeeding year. To control the fungus, by interrupting its development at some point, one has to know its life cycle.

As I indicated, brown rot starts as a speck that rapidly enlarges and may involve the whole peach in a few days with a brown, rather firm rot. Masses of spores conidia are soon formed on the rotten surface. The spores may be blown by wind, washed or splashed by raindrops, or carried by insects or man to other peaches, which in turn develop the characteristic brown spots and eventually are destroyed. Sometimes the destruction of the fruit can involve one-half or more of the crop in less than a week. Complete destruction of the crop occurs less often now than 50 years ago, but it may still occur during periods of favorable weather in orchards that are poorly cared for.

The fungus completely penetrates the tissues of the peach. The rotten peach left hanging on the tree loses moisture and shrivels. By the end of the growing season it becomes a dry, distorted object that is aptly called a mummy. The peach stone is covered by the tough, leathery remains of the Peach cells held in place by the fungus threads (mycelia), which have spread all through the flesh. The skin remains as a covering. Just beneath the skin the fungus threads are closely interwoven and form a hard rind. The mummy can withstand very adverse conditions.

If the infected peach had dropped to the ground immediately instead of drying up in the tree, very likely it would have disintegrated under the action of various molds and bacteria. Only rarely do rotting peaches on the ground form mummies. Once the peach has dried into a mummy, however, it may fall to the ground, but in that condition it is not affected by soil organisms, and many persist for several years.

If the mummy is partly or completely covered by soil, the fungus produces another stage in its life cycle the following spring when the peach blossoms begin to open. Small, brown, goblet-shaped structures the apothecia develop from the part of the mummy embedded in the soil and unfold on the surface of the ground.

The apothecia are never formed on mummies lying only on the surface of the ground. Some part of it must be buried in the soil for apothecia to form. The mummy, however, does not have to be intact, as numerous apothecia will develop from buried fragments. That phenomenon will be repeated for several years. In a series of experiments I once made, I observed apothecia produced from the same group of peach mummies for six successive seasons. J. B. Pollock reported finding apothecia in Michigan from plum mummies 10 years old.

The upper surface of the apothecium is the spore-bearing layer. It consists of closely packed vertical sacs the asci each containing eight spores (ascospores). The asci are separated from each other by sterile, threadlike structures (paraphyses), so that each sac or ascus stands as a separate entity, with its apex pointing upward.

When the individual spores are mature, the base of the ascus absorbs moisture and the spores are forcibly ejected into the air above the apothecium. There they are swept away by air currents, and some may be carried upward into the peach trees. As the formation of spores takes several days, the discharge of ascospores into the air is more or less continuous. Often a blow on a group of apothecia will bring about the sudden release of a whole group of ascospores, which will be visible for an instant as a smokelike cloud above the apothecia. The number of ascospores discharged into the air in an orchard soon reaches astronomical proportions, but, fortunately, only a few land on the unfolding peach blossoms. The ascospore that happens to lodge on the sticky surface of the pistil of the blossom germinates and sends a mycelial thread down the style into the very small peach, causing the phase of the disease called blossom blight. Having destroyed the young peach, the fungus grows down the stem and into the twig, killing the tissues and forming a stem canker.

Unless the conditions are extremely favorable for the growth of the fungus, the number of blossoms blighted or destroyed by the fungus is not enough to effect materially the size of the peach crop. Blossom blight is an important and serious phase of the brown rot disease, however.

Its importance lies in the fact that the blighted blossoms and the cankers on the twigs soon are covered with masses of spores conidia which can infect and blight other blossoms. Moreover, the twig tissue killed by the fungus decomposes into a gummy substance, which oozes to the surface, surrounds the blighted blossom, and prevents it from dropping.

Every time it rains during the rest of the spring, a fresh crop of conidia is produced on the blighted parts. The green peaches are not readily infected by the fungus, but as the peaches start to mature they become extremely susceptible. conidia formed on nearby blighted blossoms are scattered by the wind and raindrops to the maturing peaches, which soon develop the telltale spots.