Workers in the different cherry districts disagree as to the chemicals to be used for control of leaf spot: The severity of leaf spot varies widely among areas, the length of the season over which protection must be provided is different, and the severity of the injury to the tree by the fungicide varies between districts. The effects of the fungicide on the tree, however, usually are different in degree only. Some general statements therefore apply in most cases.

The early work on fungicides for cherry leaf spot was primarily concerned with bordeaux mixture. Many tests of lime-sulfur solution, elemental sulfur preparations, copper compounds, and organic fungicides followed. Most commercial growers now use sulfur on sweet cherries and one of the proprietary copper compounds on sour cherries, although a considerable amount of bordeaux mixture is still used and the organic fungicides are gaining in usage each year.

The choice of a fungicide for use on sour cherries is partly determined by the effect of the fungicide on the tree and fruit. Fungicidal sprays cause various types of leaf and fruit injury such as leaf scorching, leaf spotting, leaf yellowing and dropping, and fruit scald. They also affect leaf size and photosynthetic activity, the size of the fruit, the solids and acid content of the juice, the color of the fruit, the yield, canning quality, and so on. The aim in any control program is to balance the various factors as precisely as possible in order to control the disease with the least injury to the tree and fruit.

Sour cherries of acceptable size commonly run about 100 to 125 fruits a pound. Variations in size of 10 or 15 cherries a pound have been general in Pennsylvania with different fungicides. The solids content of the juice is normally about 14 percent, with 1.5 to 4.0 percent variation common between fungicides. The acid content of the fruit has varied from 0.8 to 1.5 percent with different fungicides. The weight of pits has varied from 6.5 to 8.3 percent of the total fruit weight. The color of the fruit has varied from a very light to a very dark red.

Those characteristics are related in that any fungicide that reduces size of the fruit usually increases the percentage of solids and acid in the fruit and total weight of pits in a ton of fruit. The color of the fruit does not seem to be closely related to the other effects. All the variations are important because one or more of them affects in turn the yield and grade of raw fruit, the amount of waste and yield of cans of fruit the ton of raw fruit at the canning factory, and the attractiveness of the product to the consumer.

The effects of the fungicide on fruit quality and yield are of major importance in commercial cherry growing and canning where differences of to or 15 percent may mean the difference between profit and loss. It should be kept in mind, however, that failure to control leaf spot is usually much more serious than the injurious effects of the fungicide applied for its control.

Bordeaux mixture, at a concentration of 2 pounds of copper sulfate and 6 pounds of hydrated spray lime to 100 gallons of water, has been one of the most effective fungicides. Concentrations varying from about 1.5-3-100 to 6-8-100 are now used. It has caused severe leaf injury when used during wet or abnormally dry weather or on foliage on which aphid honeydew was present. It has dwarfed the fruit more than any other treatment. The fruit has been dark red in color with a high content of solids and acid.

The low cost and high degree of effectiveness of bordeaux mixture has made it one of the best materials for control of cherry leaf spot on sour cherries in the nursery, on nonbearing orchard trees where no more than four or five sprays are required, and in the sprays before bloom, at petal fall, and after harvest on bearing trees. It has given satisfactory results when used all through the season on bearing trees in northeastern Wisconsin and comparable conditions. In other areas the use of bordeaux mixture during the period of rapid fruit growth has caused excessive dwarfing of fruit.

The proprietary copper compounds, of which Copoloid, Copper Hydro, Copper A, Cupro-K, and Bordow are examples, have been used at rates of 8 to 12 ounces of actual copper plus 3 pounds of hydrated spray lime in 100 gallons of water. Properly used, any one of them has given fair to good control. As a group, they have been less effective than bordeaux mixture, but they have caused less leaf injury, and the number of leaves remaining on the tree has often been as high with one of them as with bordeaux mixture. They have sometimes been associated with an injury on the fruit that has occurred as a black line around the stem of the fruit and is very objectionable in canned cherries.

The copper compounds have been the most frequently used materials for leaf spot control on sour cherries since about 1940. They have represented a compromise between the older fungicides, bordeaux mixture and lime-sulfur solution, in that they have given less fruit dwarfing than bordeaux mixture and better leaf spot control than lime-sulfur. They still have considerable merit for use in some of the Great Lakes districts where injury by them is at a minimum, and in other areas where small orchards or garden trees do not justify extra labor and expense with a more complicated spray schedule in an effort to obtain maximum crops of perfect fruit. They have not been satisfactory in south central Pennsylvania because of excessive leaf and fruit injury. With the relatively large number of sprays required there for leaf spot control, crop reductions of 10 to 20 percent by a copper fungicide have been frequent in hot, dry harvest seasons.

None of the copper materials may be used on sweet cherries without danger of injury.

FERBAM preparations sold under such trade names as Fermate, Ferradow, and Karbam Black, have been used both alone and with elemental sulfur, usually one of the sulfur pastes. The usual concentration has been 1.5 pounds to 100 gallons of water of a product containing about 75 percent active ingredient. Two pounds has been the minimum effective concentration in Pennsylvania with the sprays started before bloom and continued at 7- to 14-day intervals until harvest. One and one-half pounds has been adequate when used with one of the elemental sulfurs.

Ferbam has not usually caused any visible injury to the tree or reduction in fruit size. The large fruits have been comparatively low in solids content, largely or entirely because of their size, and have been unsatisfactory to some canners because of this.

F. H. LEWIS is a professor of plant pathology at Pennsylvania State College and pathologist at the Pennsylvania State College Fruit Research Laboratory, Arendtsville, Pa. He is a graduate of Clemson Agricultural and Mechanical College and obtained his doctor's degree in plant pathology from Cornell University in 1943. He has worked on cherry leaf spot and the effects of fungicides on cherry fruit quality in New York and Pennsylvania since 1940.