DISEASES AND INSECTS 



DISEASES AND INSECTS 1023 



leaf-curl, heart-rot and canker of trees, mildew of 

 many plants, rusts and smuts of cereals (Figs. 1287, 

 1288, Kansas Experiment Station) ; in fact the mere 

 enumeration of the more common fungous diseases of 

 plants would fill many columns in this volume. 



Algse, low forms of green plants, most of them living 

 in water or very damp places. Few are known to pro- 



1281. Currant foliage attacked by the leaf-spot fungus. ( X H) 



duce disease in plants. The red rust of tea is one of 

 the best known algal diseases. 



Parasitic angiosperms, flowering plants, of which 

 there is no inconsiderable number, causing more or 

 less injury to the plants upon which they live. These 

 parasites are usually markedly degenerate in one or 

 more respects, as a result of their parasitism, being 

 often without true roots, or without leaves and fre- 

 quently without chlorophyl green. As examples we 

 may mention the mistletoes, dodders and broom 

 rapes. 



Insects (see page 1034) cause such diseases as 

 galls and similar malformations. 



Nematode worms, minute all but microscopic 

 in size and multiplying rapidly, they constitute 

 one of the greatest crop pests, especially in warm 

 or tropical countries. They usually infest the roots, 

 causing galls or swellings. Some species injure the 

 plants by destroying the fine feeding roots as in the 

 case of the nematode parasites of oats so destruc- 

 tive in certain countries of northern Europe. Over 

 400 different plants are known to be subject to 

 the nematode root-gall disease. (See pp. 1041-2.) 



Physiological disease is a term under which is 

 included all those diseases the cause of which 

 cannot be attributed to some parasitic organism. 

 Their origin is variously attributed to abnormal 

 enzymic activity, disturbed nutrition, and the like. 

 The best-known of these are peach-yellows, chloro- 

 sis of the vine, tip-burn (Fig. 1291), mosaic disease 

 of tobacco and leaf-roll disease of potatoes. 



The various parasitic organisms cause disease in 

 one of two ways, either by the secretion of toxines 

 and enzymes which at once kill the plant tissues and 

 change them into forms readily available as food for 

 the invader; or the toxins and enzymes secreted merely 

 stimulate or irritate the plant tissues in such a way as 

 to result in abnormal tissue growth or diversion of the 

 food substances of the host to the advantage of the 

 parasite making its home between or in the cells of the 

 host. Both types of disease-production have the same 

 ultimate result, the serious injury or destruction of 

 the infested plant, although the former is usually the 

 more rapid and destructive. Of the first type, rots, 

 blights and leaf-spots are the best examples, and are 



characterized by the rapid death and destruction of 

 the affected tissues; of the second type, galls, leaf- 

 curls, rusts and smuts are good examples and are char- 

 acterized by a rather long period of association of the 

 parasite with the living tissues of its host before 

 marked injury or death of the plant results. 



The causal agent is usually associated with the tissues 

 of the host, either the dead or living, during its entire 

 cycle of development. The apple-scab pathogen, 

 Venturia in&qualis, will serve admirably to illustrate. 

 It passes the summer on the surface of leaf and fruit. 

 In the autumn when the infested leaves fall to the 

 ground, the fungus, which as a parasite has invaded only 

 the cuticle of the leaf or fruit, now penetrates the 

 dead tissues and develops there during the autumn the 

 winter form of fruit bodies, the minute globose black 

 perithecia, in which during the warm days of early 

 spring the ascospores are rapidly developed. These 

 ascospores (Fig. 1292), eight in a sac, ripen and are 

 discharged by the spring rains that come during the 

 blossoming period. The old leaves on the ground are 

 filled with millions of these minute perithecia with 

 many sacs of ascospores in each perithecium. The 

 spores are shot into the air during the rain and being 

 exceedingly light are carried to the opening leaves and 

 forming fruits, where they germinate, sending out 

 mycelial threads into the cuticle of leaf or fruit form- 

 ing the characteristic dense dark green or black mats 

 or crusts, the scab-spots. The leaves become crumpled 

 and injured, the young fruits grow one-sided, or if the 

 stem be attacked, soon drop from the tree, thus giving 

 no set of fruit. On the scab-spots the conidia or sum- 

 mer spores cut off from the tips of upright branches 

 in great numbers, are carried by the wind to other 

 leaves and fruits where, with the next rain, they germ- 

 inate and give rise to new scab-spots and more conidia. 



The life-cycle as given for the apple-scab fungus is 

 typical of many of the fungous pathogens of our crops. 

 It must be remembered, however, that each pathogen 

 has habits peculiar to itself; hence the necessity for the 

 most careful study of each that we may know its 

 habits and peculiarities and thus be able successfully to 



1282. "Tip growth" of yellows. 



Left-hand specimen shows two small-leaved tips appearing in 

 October, two or three of the normal leaves still remaining near 

 the top. The middle specimen shows numerous tips appearing in 

 August. Right-hand specimen is a healthy twig, for comparison. 



