By Donald Reddick

Some knowledge of the habits of the organism causing a disease is usually necessary in order successfully to combat it and prevent its ravages. Those diseases caused by powdery mildew fungi (which are surface infestations) can be cured. Practically all others must be prevented.

Fungi attacking parts of plants above ground are usually disseminated by means of spores. Water is often necessary to liberate the spores from the fungus proper, and is nearly always necessary to permit spore germination and infection of other plants. Heavy dew sometimes furnishes sufficient moisture, but prolonged drizzling rains are more favorable. For this reason a fungicide, in order to be effective against such parasites, must be applied before the rain. If it is going to rain to-morrow, spray to-day. But how know whether it is going to rain? This can best be told from a study of the United States weather maps, which are printed and distributed from the many weather stations, or else appear in the daily papers. Storm periods, indicated by a " low " barometer, travel quite regularly from west to east, and are usually accompanied or followed by rain. This can be determined by noting the amount of precipitation, if any, in the wake of the storm. Local conditions are often a factor to be considered. A few minutes' study of the weather map each day will soon make one reasonably efficient in predicting the weather. See Chap. I.

It is unfortunate that a definite system of naming plant diseases has not been formulated. Diseases of plants of a similar nature should bear the same common name. The term "blight" is commonly used for many kinds or forms of diseases. It might well be restricted to bacterial diseases like fire-blight of pear or bean blight. When some definite system of naming diseases is adopted, it is likely that a tabulation of methods of control will be somewhat simplified, for if the term " blight" is restricted to bacterial diseases of the nature of pear blight, it will be understood that certain control measures, such as spraying, will not be effective. At present, each case must be considered separately, and in the following pages the popular names are used. These names are followed by the technical botanical name of the organism causing the disease, in italics, and this by a brief description of the disease, the most prominent symptom being mentioned first.

Certain General or Unclassified Diseases

Damping-off. - A term applied to the decay of young seedlings or cuttings at or near the surface of the ground. The trouble is due to the action of various organisms, especially Pythium deBaryanum, Phytophthora cactorum, Rhizoctonia sp., etc. Wet soil, confined atmosphere, and crowded plants are conducive to damping-off.

Control. - Steam-sterilize seed or cutting beds. Sterilize nursery seed beds with formalin, using 1 gallon of 1 per cent solution to the square foot, i.e. 1 pint of formalin in 12-15 gallons of water.

Cedema or Dropsy. - A disorder of various plants under glass, as tomatoes, violets, geraniums, which have insufficient sunlight, stimulating temperature and soil, and too much moisture. It has also been observed on twigs of the apple. It is usually indicated by elevated corky or spongy points or masses, much resembling fungous injury. The leaves curl. The only remedy is to improve conditions under which the plants are grown.

Smuts of cereals. - Practically every cereal is attacked by a specific smut fungus, and most of them by two perfectly distinct species. These smuts are confined to a single species of cereal, and never cross from one to another. Some of the smuts produce a loose black spore-mass (loose smuts), while in others (covered smuts) the seed coat of the grain is not affected, so that the smut is not detected until the grain is broken open. The most important difference to be noted, however, is the method of wintering. In some the spores adhere to the surface of the seed and infect the young seedling plant at the time of germination, while in the other case the spores fall upon the blossoms and grow down into the seed directly, there lying dormant until the seed is planted.