Account

ACCOUNT In the present section a brief account will be given of some of the more important plant diseases and under the following headings— (a) Physiological Plant Diseases.

(b) Plant Diseases due to Bacteria.

(c) Plant Diseases due to Viruses.

(d) Plant Diseases due to Fungi and Slime-Fungi.

(e) Plant Diseases due to Higher Plants.

Physiological Plant Diseases.—Plants, like animals, may show deficiency diseases due to the lack of some important chemi cal constituent in their food supply. Thus a lack of iron salts in the soil produces the condition called "chlorosis," in which the green pigment of the plant, chlorophyll, fails to develop. Lack of potash, such as often occurs in poorly manured soils or in light soils leached by heavy rainfall, produces very characteristic symptoms of disease in certain plants, e.g., potato, tobacco and cereals. Similarly for other essential food elements such as mag nesium, phosphate and nitrogen. Conversely disease conditions may result from the presence of an excess of certain chemicals in the soil. To this class belong various kinds of chlorosis, due to excess of lime or manganese in the soil. The most important diseases of this type are those associated with the accumulation of soil alkali. These are met with in irrigated regions where they often constitute the chief agricultural problem.

Among diseases due to unsuitable environmental conditions may be cited those caused by too high temperature, such as sun scorch of leaves, heat canker, etc. The effects of too low temper ature are familiar as frost injury to leaves, young shoots, fruit, etc., but a more important illustration is seen in the winter killing of trees in those countries which experience severe winter freez ing. The development of "winter-hardy" varieties of cultivated plants is of great importance in countries possessing the conti nental type of climate.

Injuries due to noxious chemicals in the air are well shown by plants growing in industrial areas. The more important chemicals in this connection are gases, such as coal gas and sul phur dioxide, fumes such as sulphuric acid, and dusts such as soot and cement dust. The latter act by choking up the breathing pores of the plants, but this action is often accompanied by the poisonous effects of gases associated with the dust. In this con nection should also be mentioned the injuries which arise from the injudicious use of chemical protective agents such as sprays.

Plant Diseases Due to Bacteria.

The most important dis eases of this type are the fire blight (Bacillus amylovorus) of apples, pears and similar fruits; the angular leaf spot or black arm disease of cotton (Pseudomonas malvacearum) ; the olive tubercle (P. Savastanoi) ; citrus canker (P. citri) ; various wilt diseases (Bacillus tracheiphilus in cucurbits, B. solanacearum in tomato, potato. etc.) ; soft rots of a number of vegetables (B. carotovorus and allied species) ; two leaf spot diseases of tobacco (Bacterium angulatum and B. tabaci) ; and crown gall (P. tume faciens) on a large variety of herbaceous and woody plants.

Plant Diseases Due to Viruses.

This important group was formerly placed among the physiological diseases, but it is now known that they are of infectious or contagious nature. Detailed investigation has failed to show the presence of a visible parasite, but in other respects the resemblance to the parasitic type of disease is very close. The disease can in many cases be trans mitted by the inoculation of the juice of a diseased plant into a healthy one. Where this simple method of transmission fails, infection of a healthy plant can be obtained by grafting on it a shoot of a diseased plant. In certain cases it has been proved that the juice of a diseased plant is able to convey infection after it has been filtered through a porcelain cup, whence the current view with regard to these diseases is that they are caused by filter-passing organisms. The symptoms of virus disease are con fined to the shoot portions of the plant, and take the form typically of mottling ("mosaic") of the leaves, with as a rule a certain amount of puckering, distortion or inrolling. The habit of growth may also be affected, and often there is dwarfing, pre mature death, and a marked diminution in the yield. The chief agents of transmission are various leaf-biting or leaf-sucking insects, particularly aphides. The number of crop plants which are known to show virus diseases is very large and includes potato, tomato, tobacco, cucumber, clovers, spinach, hop, sugar cane, sugar beet, peach and many others. Certain kinds of or namental variegation are also infectious and are thus not dis tinguishable from virus disease.