Copper (silicates, basic sulfates, chlorides, oxides, phosphate, and certain copper organic combinations).

Mercury (phenyl derivatives lactates, acetates, formamides). Dithiocarbamates (thiuram disulfides tetramethyl and morpholine; metallic methyl derivatives; iron, zinc, and lead; metallic ethylene derivatives; sodium, zinc, manganese). Chlorinated quinones (tetrachloroquinone).

Chlorinated naphthoquinones (dichloronaphthoquinone). Quinolinolates (copper and zinc compounds).

Quaternary compounds (quinoline, isoquinoline).

Glyoxalidines.

Nitrated phenols (dinitro compounds).

Chlorinated phenols (pentachlorophenates).

Thalamides (N-trichloromethylthio tetrahydrophthalimide).

Chromium compounds (complex double salts with other metals).

The control obtained with some of these new compounds is outstanding. The effect of ferric dimethyl dithiocarbamate (ferbam) on cedar-apple rust is an example. Before it was introduced, sulfur gave only indifferent control of this disease, but this iron dithiocarbamate has practically solved the problem of cedar-apple rust for the commercial apple grower. The same compound has eliminated the spray-injury problem in the production of high-quality pears in the Pacific Northwest.

Many of the new compounds have proved satisfactory for the control of plant diseases in the Tropics. An immense potential demand exists for the products in the Tropics, but their widespread use is limited by the relatively low purchasing power of many of the tropical farmers. Strangely enough, no organic compound has been found to replace bordeaux mixture for the control of the very destructive Sigatoka disease of bananas. So intensive is the attack of the fungus causing this leaf disease, banana plantations must be sprayed 15 to 17 times a year. Approximately 45 million pounds of copper sulfate are required each year to protect the 130 thousand acres of bananas growing in tropical America.

While these new compounds have given better control of the fungi, they have been less satisfactory against plant diseases caused by bacteria. Those diseases present a particularly difficult problem, for the bacteria are present in such enormous numbers that it is difficult to kill all of them by ordinary procedures.

Current research for the control of bacterial diseases of plants is centered on the possible use of antibiotics. These materials, derived from the fungi themselves, have given brilliant results in the field of human pathology, and research workers in various parts of the world are trying to determine if they are just as effective in their action against the bacteria that cause plant disease. The investigations are still in the preliminary stages, but even so it is evident that some antibiotics are absorbed by the plant and transported into the newly developing leaves and stems. This absorption suggests the possibility of protecting the entire plant against invasion by disease-producing bacteria.

Those tests are not limited to the use of antibiotics. The action of chemicals injected into the stem of living plants or applied as solutions to the soil around the roots also is being investigated as a possible mode of attack against virus diseases of plants; a group of important and destructive maladies that are difficult to control.

The improvements in fungicide materials would be of little value had they not been accompanied by an analogous development in the machines used for applying them. Bordeaux mixture was first applied by sprinkling it on the plants with brooms or brushes. This tedious procedure soon led to the development of simple, hand-operated hydraulic pumps. The spray material was forced, under pressure from the pump, through a jet-tipped rod and deposited on the various plant parts as the operator moved the spray rod among the plants. The bucket pump, the knapsack sprayer, the barrel-type sprayer, and the one- or two-cylinder hydraulic pumps (either hand-operated or driven by a chain-drive sprocket connected to one wheel of the wagon carrying the pump and spray supply in a barrel) are representative of the pioneer types of spraying equipment.

The gallons-per-minute output of all these rigs was low, operating them was hard work, and the acreage they could cover was limited. Nevertheless the benefit obtained, particularly when diseases required only one or two applications, was striking, indeed. Sprayed plants remained healthy and produced good crops, while the unsprayed plantings had to be abandoned because of disease.

Just as the development of some of the new organic fungicides evolved from compounds used in the manufacture of automobile tires, so did the development of efficient spray machinery evolved with the for automobiles. Indeed, modern control of plant diseases on a large scale became feasible just as soon as the gasoline engine was substituted for manpower. Pumps were redesigned to operate at higher speeds, develop more pressure, and have a higher gallons-per-minute output than was possible with manually operated machines. The gasoline engine also permitted the introduction of a power-driven agitator to keep the spray materials thoroughly in suspension, a feature that was inadequately provided for in the manually operated machines.

At first there were rather cumbersome rigs powered by heavy, one-cylinder, two-cycle engines of low horsepower. The machines gradually were more refined. Wooden wheels on the truck were supplanted by steel wheels, which in turn were replaced by rubber-tired wheels. The heavy, one-cylinder engines were replaced by two-cylinder and four-cylinder engines, which developed increasing horsepower. Pump pressures increased from 100 to 600 Pounds pressure to the square inch, and the output in gallons a minute from 2 to 3 to as high as 40 or 50, With the present-day high-capacity machines.

The increase in pump capacity necessitated larger tanks, and machines with 400- and 600-gallon tanks became standard equipment. The limit on tank size appears to be 600 gallons because the weight of that much water, 4,800 pounds, is about the maximum that can be drawn over soft ground. Some large commercial orchards use pumps in a central station and a network of pipes with suitable outlets or risers spaced through the planting. It would seem to be an ideal solution, but large investment in pipes and machinery is required, pipes break in winter, and the spray chemicals corrode equipment all the time. Stationary spray plants are practical only in the very large commercial operations. They probably reached their maximum use in the banana plantings in Central America.

Power-driven spray machines also have been developed for use with vegetable crops. The problem there is to cover each plant in the rows adequately with fungicides. That is done by a series of nozzles on horizontal booms arranged so that the spray envelopes the plants. In some machines the booms simultaneously spray all plants in a swath 40 feet wide. Their development has made possible the protection of extensive acreages of row crops.

Likewise, for use in vineyards and plantings of brambles on trellises, spray machines have been devised with vertical instead of horizontal booms. Some machines are designed to cover both sides of an individual row. Others spray on both sides of the machine as it moves up the rows.

The motor-driven spray rigs required two or three men. Wartime manpower shortages and high wages meant that faster and cheaper ways of applying sprays had to be found. First in citrus orchards and then in peach, apple, and pear orchards a new machine was put into operation. It was called a speed sprayer. It had a 600-gallon tank, an engine of 80 horsepower or more, conventional pumps, and spray nozzles arranged as adjustable arcs on both sides of the machine. Men were not needed to direct the spray to various parts of the tree. The spray, as it was forced from the nozzles, was caught in the blast of air from a rapidly revolving propeller and blown into the trees. The machine was so heavy that it could be pulled only by a tractor, but the controls were so arranged that the driver could spray the trees.

Many years earlier, the use of fine powders or dusts had cut the cost of operation and reduced the amount of water needed. But the method has faults: The deposits of fungicidal dusts apparently do not adhere quite so well as spray deposits. Because of the lighter weight of the equipment and the speed of application, however, dusting is used often with success.

All in all, the trend in 1953 was to combine the advantages of several methods. One was to increase the adherence and effectiveness of dusts by injecting water and adhesives into the dust stream under relatively low pressure (70 pounds the square inch). Another was to use machines equipped with highly efficient fans or specially designed turbines, which develop high-velocity air blasts carrying either dust or liquid particles to all parts of the plants to be protected. Sometimes these modern efficient blowers were available as units for the modernization of older equipment at a minimum cost. Still another procedure was to use sprays in concentrated form. If, say, 20 gallons of dilute spray were needed to protect a given area, the same amount of fungicide was applied in 5 gallons of water a 4 times (or 4x) concentration. Such concentrated sprays reduce the amount of water needed, but to get proper protection the rate of delivery of the pumps and nozzles and the forward movement of the spray machine must be in careful adjustment.

JOHN C. DUNEGAN is a pathologist in charge of investigations of fungus and bacterial diseases of deciduous fruit trees in the division of fruit and nut crops and diseases, Bureau of Plant Industry, Soils, and Agricultural Engineering.

S. P. DOOLITTLE is a pathologist in charge of investigations of diseases of tomatoes, peppers, cucurbits, and certain other vegetable crops in the division of vegetable crops and diseases in the same bureau.