310 GARDEN GUIDE 



from the normal, produced by some agent which interferes with the 

 usual growth or functions of the plant. For example, a certain 

 Cabbage plant wilts while another nearby does not. On pulling up 

 the wilted plant we find that the roots are swollen to many times 

 their normal size. The wilting and the abnormal size of the roots 

 are symptoms of a disease known as clubroot. 



The following are some of the symptoms exhibited by diseased 

 plants: Rot is the effect produced when a considerable portion, usually 

 of a fleshy plant part, is rapidly killed. We may have soft rots, hard 

 rots, dry rots and wet rots. Rots of stems are called stem rots, rots 

 of roots, root rots, etc. Blight is the effect caused by a rapid kiUing 

 of tissue and usually refers to leaves and shoots. Spot is a small area 

 of dead tissue. There are leaf spots, pod spots, fruit spots, etc. Wilt 

 results from injury to the water conducting vessels or from some injury 

 to the water absorbing organs of the root. Scab is superficial injury 

 resulting in corky growth and craclang. Wart is an irregular oyt- 

 growth from a plant organ, leaf, stem or root. Canker is a definite 

 dead area in the bark usually on stems or on h'mbs of trees. Mildew 

 is a white fungous growth upon the sm-face of a plant; there are powdery 

 and downy mildews. Mosaic is a peculiar Ught and dark green 

 mottling of leaves produced in certain diseases. Knots are swellings 

 produced by an overgrowth of tissue. Burn is a killing of leaves 

 especially at the edges or tips. Smut is a black powdery mass, while 

 a rust usually consists of reddish to brown powdery pustules. Leaf 

 curl is a distortion of leaves due to the presence of a parasite. Galls 

 are abnormal swellings. Many other similar descriptive terms are 

 used to designate symptoms, t)ut the above illustrations will suffice to 

 indicate what constitutes symptoms. 



The Cause of Disease 



We must distinguish between injuries caused by insects and those 

 caused by diseases, although the two may have some points in com- 

 mon. Diseases are usually caused by (1) parasitic slime moulds, (2) 

 bacteria, or, (3) fungi, all of which are low forms of plant fife. Rut 

 there are some diseases which are not due to the presence of a parasite. 

 There are many lower organisms which are not harmful; in fact, many 

 are beneficial; but certain ones have so modified their habits that they 

 are able to live upon the higher plants and use the food prepared by the 

 latter. They are then called parasites and by their manner of Hving 

 they injure the plants upon which they exist and produce in them a 

 diseased condition. 



Slime moulds, though very common, are not well known to most 

 people because of their small size. In their active life phase, they 



