Bacterial Spot of Peach

John C. Dunegan.

Under the unwieldy title, "Observations on a Hitherto Unreported Bacterial Disease the Cause of which enters the Plant through Ordinary Stomata," Erwin F. Smith, one of the pioneer plant bacteriologists, described in 1902 a yellow, motile bacterial organism. The bacterium, now called Xanthomonas pruni, was found to be the cause of a destructive disease of the Japanese plums in Michigan. Later research in other sections proved that it also attacked the leaves, branches, and the fruit of the peach, almond, apricot, nectarine, and one variety, English Morello, of cultivated cherry.

On all of them, instead of rotting the fruit or wilting the leaves and branches, Xanthomonas pruni merely kills small spots of tissue. Bacterial spot is, therefore, an apt name for the disease.

Years ago the organism destroyed the extensive plum industry in Georgia. It is commercially important now on plums in Japan and New Zealand, but in eastern United States it is the peach industry that suffers most from attacks of X. pruni. It is not known to occur in the fruit sections west of the Rocky Mountains. Because of its prevalence on peach, I limit this discussion to a description of the disease on that host.

The organism can enter uninjured peach tissues through the normal openings, the stomata, on the under side of the leaves and on the fruit and twigs whenever high humidity or rains occur during the growing season. Entrance through the stomata has been demonstrated repeatedly, in experiments, in the leaves and twigs.

It has not been demonstrated experimentally in the fruit, yet it occurs under natural conditions in untold numbers of instances on peaches every season. We do not know whether infection results from the entrance of a single bacterium or whether a number of individual organisms must enter through one opening to set up the cycle of infection within the host tissue.

Once inside the peach leaf, fruit, or twig tissue, the bacteria multiply and fill the spaces normally existing between various cells. The individual cells are cut off from the normal interchange of oxygen, nutrients, and water vapor and are exposed to toxins produced by the bacteria surrounding them.

Likewise it is evident from the microscopic study of young spots on the leaves and fruit that the organism can dissolve the material that forms the boundary zone between the walls of two adjacent cells. As the number of bacteria increases, the individual cells die and are split apart, so that the normal cellular structure is completely disorganized. Eventually the mass of bacteria becomes so great that the outer host tissues are ruptured, and the bacteria ooze to the surface of the leaf, fruit, or twig, spreading over the surface and drying into a thin film. Once this oozing has occurred, there is no further destruction of tissue in this individual spot.

Actual tissue destroyed by the bacterial mass involves only a tiny area. In a typical fruit spot, for example, the area is but a few cells wide and not more than 8 or 10 cells deep. It is, indeed, a microscopic pocket filled with bacteria.

THE DISEASE would be of little commercial importance if the killing of small pockets of cells were the only effect of the bacterial invasion. That destruction of the cells is followed by renewed growth of the surrounding tissues in the fruit. It forms callus tissue, which seals the area destroyed by the bacteria from the surrounding healthy tissue. The subsequent growth of the peach, particularly its rapid increase in diameter as it approaches maturity, sets up a series of strains that result in the development of extensive cracks and roughened areas that make the peach unmerchantable. Actually the injury results from the renewed growth of the tissue, for the callus tissue cannot expand as the peach increases in diameter.

As INFECTIONS of fruit may occur from the beginning of the season until the fruit is harvested, the severity of the injury depends on how early the infection took place. Infections that occur just before harvest, after the fruit has nearly reached its maximum diameter, have little effect on the appearance of the fruit because there are no growth tensions to cause Cracking at that late date.

The cycle of disease development on the leaves follows much the same pattern. The leaf spots are generally angular, one-sixteenth to one-eighth inch long, and usually confined to areas between the small veins. Pale green at first, they change to purple and finally to dark brown. As with the fruit, the oozing of the bacteria to the surface of the leaf ends the cycle as far as the individual spot is concerned.

The development of bacterial spots on the leaf is followed not by renewed tissue growth, as on the fruit, but by the dropping out of the disease area. That causes a shot-hole effect. Affected leaves, even though there are only a few spots on them, soon become yellow and fall to the ground. In favorable seasons the trees may be defoliated by midseason. The reason for that is not clearly understood, but presumably it is the result of the toxins the bacteria produce.

Most growers consider the fruit injury the most important phase of the bacterial spot disease. Really, though, it is the constant loss of the leaves (50 to 200 a day) that causes the most damage. The trees are weakened by the defoliation. The buds for the succeeding season also are adversely affected. If the defoliation occurs by midsummer, new growth is stimulated in the fall and the trees enter the winter season in a weakened condition only to succumb to low-temperature injury. So damaging is such premature defoliation that it means success or failure when peach trees have been established on light, sandy soils.

Bacterial lesions on the twigs and branches have little effect on the subsequent growth of the tree; they are on the surface and are soon separated from the surrounding healthy cells by a layer of callus tissue.

They are, however, of vital importance in the life cycle of the bacteria. They are means by which the bacteria survive the winter. In the less humid North Central States the bacteria infect the twigs late in the fall but apparently do not produce visible symptoms until the following spring. Along the Atlantic seaboard typical lesions can be detected on the twigs in the winter.

Regardless of the time the symptoms develop on the twigs, however, all investigators agree that the organism overwinters in the twigs and that bacteria oozing from the overwintered lesions start the cycle of leaf, fruit, and twig infections the succeeding year.

Wind and rain are the main factors in the spread of the organism. If the early spring weather is dry, therefore, the disease may appear merely as cone-shaped areas of infected leaves here and there on the tree. If the early spring is rainy, however, the disease may seem to develop throughout the tree at once.

ONCE primary infections develop in the spring, the subsequent spread of the bacterial organism depends upon an abundance of moisture. The disease thus is much more prevalent during rainy than dry seasons. So necessary is water in the spread of the organism that the disease is more severe on the west side of the trees because the leaves may remain damp with dew later in the morning than those on the east side. Many leaves show a series of spots in a straight line parallel to the long axis of the leaf blade an indication that a bacteria-laden drop of dew has rolled down the leaf.

Temperature influences the time of the first appearance of the disease in the spring and its subsequent development. The organism grows most luxuriantly in the laboratory at temperatures between 75 and 83 F. In the orchard the disease spreads most rapidly when the temperatures range between 70 and 85 . Within that range the individual spots develop profusely. Bacteria may ooze from them within 7 days, whereas at temperatures below 70 the cycle of development of the individual spots may be prolonged to 20 or 25 days. The growth rate of the bacterial organism is reduced by temperatures above 85 . It ceases to multiply at about 100 but remains alive and can renew its activities when the temperatures drop. In laboratory experiments it has required an exposure of 10 minutes at 124 to kill the organism.

BECAUSE THE DISEASE is more severe on weak trees that are poorly cared for, the first step in control is to induce vigorous growth. In some sections fairly satisfactory commercial control can be obtained merely by pruning, cultivation, and the use of nitrate of soda.

In many orchards on light, sandy soils, however, it is hard to keep the trees growing vigorously, and additional control measures are needed. Many types and combinations of spray materials have been tested. The susceptibility of peach foliage to chemical injuries is an important detail; many bactericides cannot be used because of the excessive injury they cause.

One mixture, a zinc-lime spray, prepared by dissolving 8 pounds of zinc sulfate in 100 gallons of water and then adding 8 pounds of hydrated lime, has been used extensively in many districts. The mixture does not injure peach leaves (except in a few localities along the Atlantic seaboard). Carefully applied, it has given a fair degree of control in most seasons. But this has to be applied at least six times, starting at petal fall and at intervals of 2 weeks thereafter. Moreover, while the zinc-lime spray is compatible with arsenate of lead, it is not compatible with the new organic insecticides that are widely used to control insects in peach orchards.

A SATISFACTORY control for the bacterial spot disease is one of the most urgent needs of the peach industry. Some progress has been made in the development of less susceptible varieties and this may be the final solution to the problem. Pending the development and the commercial adoption of resistant varieties, however, the search must continue for effective spray materials to protect the orchards now in bearing.

The possibility of killing the bacteria in the overwintering twig lesions during the dormant season, when more potent materials may be used on the peach without producing injury, is being investigated in several parts of the country. That is a logical approach to the problem, for the elimination (or at least marked reduction in the number) of overwintering sources of infection would necessarily affect the number of primary infections initiated the following spring.

JOHN C. DUNEGAN joined the staff of the Bureau of Plant Industry, Soils, and Agricultural Engineering in 1921. He studied the control of peach diseases in Georgia front 1921 to 1928, and from 1928 to 1945 investigated apple and peach diseases in Arkansas. Since 1945 he has been in charge of the investigation of deciduous fruit tree diseases in the United States.