By Donald Reddick
Plant diseases are caused by parasitic fungi or by bacteria, or other vegetable parasites; or by forms of physiological disturbance. Each disease calls for special treatment. Most plant diseases must be prevented, not cured.
It should be understood that spraying is only one of the control measures effective against plant diseases. Many diseases are not affected by spraying, though perhaps more are susceptible to this treatment than to any other.
A satisfactory fungicide must be one that does not injure the plants and at the same time is effective against the parasite. For spraying, additional requirements are imposed; it should not dissolve readily in rain water; it should adhere to foliage and fruit; in some cases it should be colorless in order not to make ornamentals more unsightly than when diseased. The fungicide which has been used most for general purposes is bordeaux mixture. Lately some other preparations, particularly lime-sulfur combinations, have come into use, and in many cases are supplanting bordeaux. There are in addition a large number of other substances which have fungicidal value and are in more limited use for specific cases.
Destroying affected parts. - It is important that all affected parts should be removed and burned, if possible. In the fall all leaves and fruit that have been attacked by fungi should be raked up and burned. Diseased branches should be severed at some distance below the lowest visible point of attack. Fungous diseases often spread rapidly, and prompt action is usually necessary. Practice clean and tidy culture.
Rotation of crops. - One of the most effective and practical means of heading off fungous diseases. Especially applicable to diseases of roots or root-crops, but also to many other diseases of annual plants.
Sterilizing by steam. - An effective fungicidal practice for several soil-inhabiting organisms which attack roots and stems. This includes nematode worms. It is especially applicable in the greenhouse, where it may be applied (a) through sub-irrigation tile or through specially laid perforated steam pipes in the bottom of the bed. Cover the beds with blankets, introduce steam under pressure of 40 to 80 pounds for two hours. Insert thermometers at various places to see that the soil is being uniformly heated. (6) A large galvanized iron tight box may be constructed with finely perforated trays 4 to 6 inches in depth. Soil placed in these trays and steamed for two hours as above will be freed from parasitic organisms. In this case the frames should be sprayed with a solution of formalin, 1 pint in 10 gallons of water.
Steam sterilization of soil may be used on intensively cultivated areas or extensive seed-beds. A portable boiler is necessary. The beds are sterilized after they have been prepared for seed, and just before the seed is sown. A galvanized pan 10 by 6 feet and 6 inches deep is inverted, and the edges are pushed down into the soil one or two inches. The pan is connected with the steam boiler by means of a steam hose and live steam is run into the pan from about forty minutes under a pressure of 100 pounds and up. The higher the pressure the more thoroughly the soil will be sterilized.
The cost of sterilizing is approximately three-fourths of a cent the square foot. It should be noted that soil sterilization has an invigorating effect on the plants, and it will be necessary to run greenhouses at a lower temperature (5°-10°) both night and day. Field sterilization also kills weed seeds, and with the reduction of the cost of weeding makes the process practicable.
Bordeaux mixture. - A bluish-green copper compound that settles out when freshly slaked lime and a solution of copper sulfate (blue254 FUNGICIDES AND GERMICIDES FOR PLANT DISEASES stone) are mixed. Many formulas have been recommended and used. The 5-5-50 formula may be regarded as standard. In such a formula the first figure refers to the number of pounds of copper sulfate, the second to the stone or hydrated lime, and the third to the number of gallons of water. Bordeaux must often be used as weak as 2-2-50, on account of injury to some plants.