Once infection has taken place, conidia begin to develop upon the surface of the peach in a few hours. The appearance of those spores completes the cycle which started the preceding year on a maturing peach that eventually mummified and dropped to the ground: Apothecia produced by the mummy ejected ascospores into the orchard air; the unfolding blossoms were infected; and conidia formed on the blighted parts finally infected the maturing fruit of the new crop.

Several variants in this cycle may occur. In mummied peaches that remained on the tree instead of falling to the ground, apothecia would not be formed. But often conidia are produced on the mummies in the trees the following spring; those spores, like the ascospores, can produce blossom blight. Occasionally the twig cankers formed one season can also produce spore tufts on their surfaces the following spring and may serve as a source of blossom infections. Spore production the second season on twig cankers, however, is rather uncommon in the case of the common American brown rot fungus, M. fructicola. It is common in the far West on twigs infected by the European fungus, M. laxa.

The American brown rot fungus grows best at temperatures between 70 and 80 F. and is killed by a temperature of 127 . It is not killed by exposure to freezing temperatures;it has been known to produce symptoms of rot in 12 days on peaches held at 36 . Periods of rainy weather encourage infections of blossoms and fruit and the production of spores on the infected parts.

THE FUNGUS would be serious enough if its effects were confined to the orchard. Unfortunately, though, it continues to destroy the fruit after it has been harvested. Conditions that favor the development of brown rot on the maturing fruit also scatter conidia throughout the tree. Some of those spores germinate immediately. Others may not start to grow until after the fruit is picked. In favorable seasons, a grower may deliver what he considers disease-free fruit to shippers, only to learn later that his fruit developed rot in transit and was quite rotten when it reached the market. Moreover, even apparently sound fruit may show brown rot infections and spoil after the housewife has bought it from the grocer. The destructive effects of the fungus are indeed far-reaching in some seasons it destroys more than 2 million bushels of peaches.

Considerable work has been expended on ways to combat the disease.

Unfortunately the removal of rotted fruit, cankers, and infected twigs cannot be depended upon to control brown rot. These sanitary measures do reduce materially the sources of infection and should be practiced along with pruning and removal (thinning) of fruit to make it feasible to protect the remaining fruit with fungicidal sprays or dusts the main control procedure.

Before 1907, peach growers used bordeaux mixture, potassium sulfide, and occasionally flowers of sulfur with Only indifferent success. W. M. Scott discovered in 1907 that a mixture of sulfur and stone-lime (called self-boiled lime-sulfur) gave effective control of the brown rot fungus.

The discovery was of great importance. The use of sulfur ended the possibility of injury that accompanied the use of bordeaux mixture for brown rot control during the growing season. Although self-boiled lime-sulfur has been replaced by mixtures of sulfur, lime, and a wetting agent, or by finely divided sulfur pastes, elemental sulfur in some form is still the most effective fungicide we know of to combat brown rot.

To control blossom blight, one has to apply sprays every 3 or 4 days during the blossom period. The development of high-capacity, rapid-delivery types of spray machinery has made it feasible to control blossom blight.

Wettable sulfur (6 pounds to 100 gallons), lime-sulfur (1 gallon to 100), ferbam (1 1/2 pounds to 100), and Phygon (2 pounds to 100), are all effective materials to use for blossom blight control if a sufficient number of closely spaced applications are made during the bloom period.

If all the blossoms on a peach tree opened in a day, they could all be protected by a single spray. As the blossom period generally extends 7 to 15 days, however, one has to spray at regular intervals to protect the blossoms as they open.

The control of blossom blight is of such fundamental importance that every peach grower should aim to do so. It is true that in some seasons the lack of rain at harvesttime greatly retards the development of fruit rot and a good crop may be harvested despite the blighting of many blossoms scattered through the trees. But the weather at harvest cannot be foreseen at the beginning of the season hence the need for the protection given by control of blossom blight.

Experiments over many years have standardized the control procedures after the blossom season. There may be local variations, but the general schedule calls for spraying at petal fall, when most of the shucks have dropped, 2 weeks later, and approximately 1 month before harvest.

Sulfur, 6 to 12 pounds, depending on the composition of the sulfur preparation and its degree of fineness, is added to 100 gallons of water for the sprays. Appropriate amounts of insecticides are added for the combined control of disease and insect pests.

Some growers apply sulfur, lime, and insecticides as dusts instead of as sprays. Properly applied, dusts give adequate control of the fungus and can be applied rapidly to large acreages during critical periods. Frequently the light dusting machines can be used when waterlogged soils make it impossible to move the heavy spray machines through the orchards.

In orchards where blossom blight has not been controlled (particularly in humid sections) additional protection against brown rot results from the application of preharvest dusts or sprays. One may use wettable sulfur, 6 pounds to 100 gallons of water; dilute lime-sulfur, 1 gallon to 100 gallons; or sulfur-lime dusts. The application of the fungicide should start about 3 weeks before harvest and should be repeated at intervals of not more than 7 days until the fruit is picked. Dusting is a particularly advantageous procedure for these pre-harvest applications, as the entire acreage can be treated rapidly and the dust does not leave an objectional deposit on the fruit.

JOHN C. DUNEGAN is a principal pathologist in the division of fruit and nut crops and diseases of the Bureau of Plant Industry, Soils, and Agricultural Engineering. He joined the staff in 1921 and until 1945 was engaged in field investigations of fruit diseases in Georgia and Arkansas. In 1945 he assumed leadership of the deciduous tree fruit disease project at the Plant Industry Station, Beltsville, Md.

The attention of the reader is directed to the section of color photographs, in which appear pictures of peaches infected with brown rot, anthracnose, and bacterial spot. Diseases of apple, pear, citrus fruit, strawberry, grape, and cherry are also illustrated.