An understanding of the cyclic chain of events necessary to produce a severe outbreak of disease often enables us to select its weakest link and suggests ways of breaking the infection cycle and thus of controlling a disease. Because infected seed carry the organisms causing bean blight and cabbage black rot from one crop year to the next, we can use disease-free seed to break the chain and avoid heavy losses from those diseases. The bacteria that cause cucumber wilt are carried from plant to plant by cucumber beetles, and so control measures for the wilt include spraying to control the beetles. Losses from other diseases can be reduced through treatment of seed, selection of disease-resistant stock, and crop rotation. More effective, less expensive, and more easily applied control measures are needed for many diseases, however.

A later article explains how the variability of fungi affects the control of Plant diseases. We know much less about variability in the plant disease bacteria. The vigorous pathogens apparently survive in largest numbers. This continual selection makes pathogenicity the ability to cause disease; possibly the most stable characteristic in nature.

BACTERIAL DISEASES of plants occur in almost every place that is reasonably moist or warm. Their destructiveness varies from year to year and place to place. A part of this variation can be attributed to presence or absence of a critical environmental condition under which the bacteria operate on the host plant.

Along with temperature, moisture is extremely important. Abundant water in the soil and high relative humidity in the air encourage the plants to take in as much liquid as possible. The leaves are usually covered easily with water and any chemicals that might resist bacterial action are diluted. This often predisposes the cells to pathogenic invasion. Likewise, under those conditions, the stomata of the leaves may be wide open. Many of the pathogenic bacteria are spread from one plant to another by the distribution of splashing rain and running water. In fact, under the influence of beating rain, leaves may become partly water-soaked. Then most any soil bacterium particularly a pathogenic form is able to grow and to produce a necrotic lesion. Conversely, carrots and potatoes, for example, that have dried properly after digging are quite resistant to soft rot.

Temperature has an especially important part in the development of most bacterial diseases. This is seen in the tomato wilt disease, which may be present in various plantings but may not become epidemic until the warm weather raises the temperature of the soil. Bean blight is also a high-temperature disease. Infection, however, may actually take place over a wide range of temperatures, but the high temperature is needed for typical symptoms. Temperature is likewise a critical item in the development or lack of development of crown gall. Tomato plants grown at 89 F. failed to develop galls while those at 82 produced typically large cells. Air and soil temperatures are important for the tomato canker disease which develops best at a soil temperature of 82 . Cold storage effectively inhibits the bacterial rots that develop so rapidly with suitable warmth.

Mineral nutrition may influence disease development. Stewart's wilt disease of corn increases as the nitrogen level increases, within limits. At high levels of nitrogen, infected plants die in a few weeks. Virulent strains of this bacterium grow well on inorganic nitrogen, while avirulent strains require organic nitrogen. Weakly and highly virulent strains are equally destructive in nitrogen-deficient plants, but highly virulent strains are much more so in plants that receive liberal amounts of nitrogen. The tomato wilt disease is affected strikingly by the nutrient level of the plants.

The number of hours of sunlight during any season of the year also influences disease development. In the greenhouse less tomato wilt develops during an 18-hour day (simulating summer conditions) than under a 12-hour day (winter or spring conditions).

Nutrition, length of daylight, and temperature are all closely associated in the development of disease conditions. It is hard to assess the intensity of the effects of each taken alone. The three conditions influence the balance of inorganic and organic nitrogen and the carbohydrate supply in the plant sap and thereby favor or inhibit the pathogenic bacteria, depending on their requirements.

IN STUDIES of bacterial plant diseases, the scientist must identify the organism causing the disease and be certain that his cultures have not become contaminated with an unwanted species. Studies under the microscope of properly prepared samples of the growth developing in cultures and of bacterial ooze or tissue from diseased plants furnish some evidence.

Because the simple cells of the pathogenic bacteria are so similar in appearance, however, additional tests of the effects of their action on various substances must be used to identify them. Their type of growth and colony formation on various semisolid or liquid culture media in petri plates or test tubes, their ability to ferment selected nutrient substances or to produce acids or gas on them, and their ability to withstand more or less unfavorable physical conditions (such as high and low temperatures or certain chemicals) are among the reactions that are commonly used in identification. Their ability to cause plant diseases is the factor chiefly responsible for our present interest in them. That factor also is an aid in identifying an unknown organism.

TO DETERMINE the identity of bacteria, one has to have laboratory equipment for making culture media and maintaining cultures, for microscopic examinations, and for various physiological determinations.

Suitable precautions and special techniques also are necessary to get and maintain cultures and media that are free from contaminations.

The Committee on Bacteriological Technique of the Society of American Bacteriologists, Biotech Publications, Geneva, N. Y., publishes a series of frequently revised leaflets about the latest techniques. Most up-to-date bacteriologists follow Bergey's Manual of Determinative Bacteriology, published by Williams and Wilkins, Baltimore, Md., as the standard textbook on identification. The sixth edition of this book was published in 1948. In that manual the plant pathogenic bacteria are classified into seven genera Agrobacterium, Bacterium, Corynebacterium, Erwinia, Pseudomonas, Streptomyces, and Xanthomonas.

Charlotte Elliott, a former associate of Dr. Erwin F. Smith in the Department of Agriculture, wrote a book, Manual of Bacterial Plant Pathogens. The second edition, published in 1951 by the Chronica Botanica Company, of Waltham, Mass., gives the names and descriptions of plant pathogenic bacteria, their reactions on culture media, symptoms of the diseases they produce, their host plants, geographical distribution, control methods if known, and citations to research literature.

HOW DOES a plant pathologist make sure that a plant disease is caused by a given species of bacterium? Robert Koch in 1882 worked out the rules of proof to follow for animal diseases. The rigid logic of his requirements applies equally well to bacteria causing plant disease.

Briefly stated, the postulates of Koch require that: (1) The bacterium must be associated in every case with the disease, and conversely the disease must not appear without it. (2) The micro-organism must be- isolated in pure culture and its specific morphological and physiological characteristics determined. (3) When the host is inoculated with the bacterium under favorable conditions, the characteristic symptoms of the disease must develop. (4) The micro-organism must be re-isolated from the inoculated plant and identified as that first isolated from the diseased host.

In this brief- review we have not attempted to present detailed information on individual diseases. Losses caused by various important bacterial diseases, symptoms, means of spread and control measures when known are given in other articles on specific crops.

A. J. RIKER is a member of the department of plant pathology, University of Wisconsin. He has made many contributions to our knowledge of bacterial diseases Of plants and has conducted research on the basic reasons for diseased growth. Dr. Riker is also an authority on the diseases of forest trees.

A. C. HILDEBRANDT, also of the department of Plant pathology, University of Wisconsin, conducts research on fundamental phases of bacterial diseases of plants with special emphasis on factors affecting the growth of organisms and plant tissue in culture media.