526 SPECIAL PLANT PATHOLOGY 



complished by^biting insects. If a leaf is attacked, it dies back to the 

 stem. If the basal part of the stem is infected, the plant rapidly 

 succumbs. This rapid wilting is due to the fact that the organism 

 lives in masses in the vessels of the xylem by which the water taken 

 up by the roots is distributed throughout the plant, hence any occlusion 

 of these spiral and pitted vessels stops the water supply and the plant 

 suffers. Advanced stages of the disease may be characterized by the 

 disintegration of the vascular system and the formation of cavities in 

 the adjacent parenchymatous tissue. Smith sums up the cultural 

 characteristics of this organism, as follows: Stains readily; smooth; 

 white; viscid; glistening; slow grower on media; surface colonies small, 

 round, discrete; nogrowthat 37°C.orat 6°C. (i6days); aerobic; faculta- 

 tive anaerobic (with grape-sugar, cane-sugar or fruit-sugar); usually 

 it grays potato after a time; clouds peptone-bouillon and Dunham's 

 solution thinly; growth retarded in acid juice of cucumber-fruits; 

 also retarded or inhibited by juice of many vegetables, e.g. table-beet, 

 sugar-beet, turnip, etc.; grows on many media at 25°C., carrot, coco- 

 nut, etc.; thermal death point 43 °C.; optimum for growth 25° to 3o°C., 

 maximum, 34° to 35°C.; easily killed by dry-air, sunlight, freezing; 

 ammonia production moderate, in litmus milk persistent growth without 

 reduction or distinct change in color of litmus; killed readily by acids. 

 Group No. 222, 232, 2023. As the disease is distributed by insects, 

 the grower of cucurbits should endeavor to reduce the number of 

 these pests by the use of kerosene, or arsenate spray, and trap plants 

 should be grown to attract the insects away from the more valuable 

 plants. 



Oak {Quercus spp.) 



Decay {Polyporus sulphureus (Bull.) Fr. Figs. 189 and 190).- — The 

 decay induced by Polyporus sulphureus is often called the red heart-rot. 

 It attacks not only oaks, but also the chestnut, maples, black walnut, 

 butternut, alder, locust, etc. It is widely distributed in North America 

 and Europe. The sporophores of this fungus form a series of superim- 

 posed, fleshy brackets of a sulphur-yellow color, weighing in the aggre- 

 gate at times almost one hundred pounds (Fig. 189). The color some- 

 times may vary to an orange-red. The under surface is usually a light 

 yellow color and beset with numerous minute pores. At maturity, the 

 fruit bodies lose their soft character and become harder and more brittle. 



