Bordeaux mixture, the first important spray used against this disease, is one of the most efficient protectant fungicides known, but it often causes severe russeting of fruit and injury to the leaves. It was therefore largely replaced by liquid lime-sulfur.

Lime-sulfur is efficient against the fungus, but may cause serious spray injury under some conditions. While it is still used in substantial amounts, especially in the pre-blossom applications, it is being increasingly replaced by less injurious materials. The wettable sulfurs have been extensively substituted for lime-sulfur, especially in the after-bloom treatments. However, even the wettable sulfurs may cause objectionable spray injury in some situations.

Extensive research is in progress to develop sprays that will control scab efficiently with still less danger of spray injury. Much attention is being given to organic chemical compounds, which seem to offer much promise. It should be recognized that many years of extensive orchard-spraying experiments are necessary in order to assess the practical value of a fungicide for apple scab control. It must stand the test under the range of conditions found from year to year and place to place, and it must be compatible with the insecticides with which it must be mixed.

One of the great deficiencies of any spray program based wholly on protection of the susceptible parts by repeated applications is that, under favorable conditions for scab infections, no economically practical number of treatments would fully protect all the young developing parts. Under favorable conditions for spraying, the bud and fruit parts can be kept protected with a workable degree of efficiency because all the blossom buds are exposed to infection in a short time. New leaves, however, are put forth from day to day for many weeks. Under severe conditions for scab infection, the best practical spray program based wholly on protection will not prevent the fungus from establishing itself in the unprotected leaves in sufficient amount to develop an abundant supply of ascospores the next spring. Furthermore, the infected leaves furnish a source of conidia that constantly threatens further fruit and leaf infection throughout the season. That problem is being attacked from two main approaches.

One approach is through the use of spray materials, especially in early season, that have some value in eradicating the fungus after it infects or in checking the production of conidia if the fungus survives. The lime-sulfur-lead arsenate spray has a considerable value in these respects, and on that account it is extensively used in the pre-bloom sprays in areas where scab is severe and experience has shown that the spray injury is not too great. Various spray materials, especially organic mercury or other organic compounds, are being developed with the aim of eradicating the scab fungus from early infections, as well as protecting against infection. This approach through use of sprays that have both protectant and eradicant properties is very logical and promising. Substantial progress has been made. The degree of success with which such materials and methods can be developed remains to be seen.

Another approach to this problem is through supplementing the best available spray programs by a direct attack on the scab fungus with an eradicant fungicide. This method is adapted for situations in which scab occurrence is severe and the best available tree spray programs are uncertain of control or cause too much spray injury. This method has been used extensively by Wisconsin apple growers for nearly 10 years. By means of a special boom a single application of spray is applied to the orchard floor in the spring after the ground is free of surface water and before apple buds break enough to expose susceptible tissue. Commercial preparations containing the sodium salt of dinitro-o-cresol or the triethanolamine salt of dinitro-o-sec.-butylphenol, one-half gallon in 100 gallons of water, are applied under 400 to 600 pounds pressure at the rate of 600 gallons an acre. Such treatments commonly eliminate 95 to 99 percent of the ascospores, with corresponding reduction in ascospore infections. Where this method is used, orchards are not cultivated during the period of ascospore discharge, because cultivation may lessen the effectiveness of the treatment. In Wisconsin this ground spraying has practically eliminated the danger of failure of well-executed spray programs on account of very severe occurrence of scab or because of unavoidable delay of critical applications. It has also made possible the successful use of the milder fungicides, thereby avoiding some serious problems of spray injury.

Perhaps an ideal fungicide for control of apple scab and many other diseases would be one that would penetrate into the susceptible plant tissues, kill or inactivate the fungus or other causal agent in infections that had already occurred, and render the plant immune to further infection for a substantial period of time. A search for such internally active materials has been started. Work in this field is encouraged by the striking successes with internally active substances for combating infectious diseases of man and animals and insect pests of plants.

Much progress has been made in improving spray machinery and methods. The tendency is towards increased mechanization and the elimination of hand labor. Fixed or automatic spray heads or booms are rapidly replacing hand-operated spray outlets, and extensive research is in progress on spraying with concentrated materials instead of highly diluted fungicides. Dusting continues to be preferred to spraying in some situations, especially where very quick coverage is necessary or ground or weather conditions are more favorable for dusting than spraying. In most cases dusting against apple scab is used as a supplement to spraying, rather than as a complete substitute.

In conclusion, a control program designed for use under very severe conditions of scab occurrence is outlined. It is offered as an illustrative program that has been very successfully used in Wisconsin. rather than as a recommendation for adoption in other States.

The ground spray as outlined in the foregoing.

Prebloom sprays designed primarily to keep the young bud and fruit parts protected from the time they are sufficiently exposed to warrant spraying. Begin after the cluster buds have broken open and the tips of the young blossom buds and leaves have grown out about one-fourth inch and spray at intervals of not more than 7 days. About three prebloom sprays are usually required in Wisconsin, the last of which should come at the open-cluster stage. Liquid lime-sulfur, 1-50, is a standard material. Other fungicides may be substituted.

A spray in bloom 10 days after the last pre-bloom treatment, applied only if the calyx spray is not due by that time. A mild spray, such as a wettable sulfur or a dithiocarbamate, without an insecticide, is used.

After-bloom sprays designed to keep the fruit protected until harvest. The time and number of treatments vary somewhat with the seasonal conditions. The milder fungicides, such as wettable sulfurs or dithiocarbamates, are used. Ordinarily five or six treatments are given after bloom at about 2-week intervals, beginning with the calyx spray. Without the ground spray, the after-bloom treatments with mild fungicides would usually come at intervals of about 10 days or less, and lime-sulfur might be desirable in the final treatment on the more susceptible varieties to guard against late-season infection and the development of scab in storage.

Insecticides are used in this program as recommended by entomologists and the spray dates are set to meet the needs for control of insects as well as scab.

Excellent spray materials, equipment, and methods are now available, and apple scab control can be assured if they are correctly used. However, there is no substitute for competent planning of each detailed spray program or for timeliness and thoroughness in carrying it out.

G. W. KEITT is a professor of plant pathology and chairman of the department of plant pathology of the University of Wisconsin.

Crystals of tomato bushy stunt virus.