DISEASES OF CULTIVATED PLANTS 431 



memory of many close observers. Like most leaf -spot troubles which 

 are strictly due to parasitic fungi, this tomato disease has been worst 

 in seasons of abundant rainfall. The same applies to the shot-hole 

 disease of the plum and the allied leaf -spot of cherry. The explana- 

 tion appears to lie in more favorable conditions for spore germina- 

 tion and for the growth or spread of the parasitic organisms which 

 produce the diseased conditions. Biting or sucking insects also open 

 the way for the entrance of parasitic diseases. 



LEAP IMPAIRMENT THROUGH FUNGUS COVERINGS. 



In addition to the leaf-spots or dead areas in leaves to which 

 reference has just been made, we have most noticeable examples of 

 the spread of the mycelium of certain powdery mildews over the leaf 

 surfaces. Casual observers note that these spread over the leaves and 

 stems of roses, over the leaves of lilac, of oak, of peach, of grape, of 

 forcinghouse cucumbers, of bean and pea and upon other plants. 

 While the development of these fungi or powdery mildews occurs 

 often rather late in the season, they are nevertheless damaging to 

 the host plant over which they spread. Above and beyond the inter- 

 ference with the leaf action the impairment of the photosynthetic or 

 sunlight processes of the leaves of the plants by which all real in- 

 crease in substance is made to the plants, these mildews develop suck- 

 ing or penetrating organs of the threads of the mycelium. These or- 

 gans called haustoria penetrate the leaf epidermis and must do this 

 for the purpose of food extraction it is needless to add that all food 

 extraction from the plant acts as robbery. 



Furthermore, the mildew-covered leaves drop to the ground in 

 fall and there afford the fungus the needed conditions for the devel- 

 opment of the resting or winter stages of its course by which it is 

 again ready to attack the plants the following season. Because so 

 largely external in development these powdery mildews are usually 

 comparatively easy of control. 



WILT DISEASES SEEDLING COLLAPSE. 



The stems or branches of plants may suffer from localized at- 

 tack by parasitic fungi as well as from hail, insect attack and me- 

 chanical agencies. The symptoms which follow will be found char- 

 acteristic. In certain ones as in the clover anthracnose and in the 

 fusarium of clover stems, we have the lesions accompanied by discol- 

 orations in which the fungus occupies a subordinate place outwardly. 

 On the other hand the spots or sori of the rusts upon grains and 

 grasses and the spots caused by the anthracnose of wheat, oats, rye, 

 etc., show commonly a crowded occupation of the area by the para- 

 sitic fungus. 



There are many examples of the effects of such lesions. Fuller 

 discussions will be found under the description of the particular dis- 

 eases. The anthracnose of the bean as well as that of the pea arc 

 good illustrations where these attack seedlings. Even clearer symp- 

 toms come out in potato rosette where the fungus parasite at early 

 stages of growth may kill off the stem attacked, while in later attack 

 will cause such impaired development of the plant that stem or axial 

 lengthening is arrested and a rosette appearance results. A still more 



