106 FARMERS' INSTITUTES. 



upon it, causing decay. This fungus is said to attack not only the potato, 

 but also the smaller bittersweet {Sola')ui7n Dulcamara) tomato, petunia and 

 other plants of the potato family. The fact that this fungus is the direct 

 ■cause of the rot was established about 1860 by Dr. A. DeBarry, now 

 Professor of Botany at Strassburg. Previous to that time this fungus was 

 known and occasionally assigned as the cause of the rot, but the best author- 

 ities then believed that all fungi were a consequence, and never a cause, of 

 disease; and even when it became demonstrated that certain fungi could 

 attack living plants, it was long contended that they only attacked feeble 

 plants, or those in a morbid condition. 



This fungus was long'known under the name of Bofrytis infestans, but at 

 the time when it was discovered to be the cause of the rot it was known as 

 Peronospora infestans, and since that time the genus Peronospora has been 

 divided and it is now classified as Pliytophtliora infestans. It was found on 

 the blighted leaves of the potato, and was carefully studied by Dr. De Barry, 

 who found its branching vegetable threads extending through the living 

 tissue of the leaf surrounding the decayed spots. By numerous experiments 

 he proved that it was the living and not the dead tissue which supported the 

 fungus. Usually nearly all the cells of the potato are filled with starch 

 grains, but where the fungus threads are present there are fewer, and where 

 the tuber is completely blackened by the disease only the empty cells remain 

 and the threads of the fungus, which are very soft and delicate, have nearly 

 or quite dissolved. These thread-like branches of the fungus pass all through 

 the tubers, stems, and leaves, extracting nourishment and causing decay. 

 Before the potato plant dies, however, and consequently the fungus also, 

 branches of the fungus may extend into the air and bear fruit. These 

 fruiting branches are most abundant on the lower surface of the leaves, and 

 form the white frosty patches already mentioned. The branches generally 

 emerge from minute openings or breathing pores (stomata) which are 

 abundant on the lower surface of most leaves. The seed-like bodies or spores 

 which grow singly on the ends of the branched fruiting stems, and appear 

 to the unaided eye like a fine white powder, are produced in large numbers, 

 and as they fall succeeding crops are produced from the same branches. 

 These ^pores germinate in three ways. When placed in water, as on the moist 

 surface of a potato leaf or tuber, some of them will in a few days swell up 

 and expel their contents in a mass which emits a germinating tube that 

 penetrates the plant, either through one of the minute openings mentioned, 

 or, more commonly, directly through the surface. Usually, however, pre- 

 paratory to germination, and especially in darkness, the contents of a spore 

 breaks up into from 6 to 16 bodies called zoospores, or living spores, which 

 escape and move about for a time in the moisture, propelled by two whip-like 

 cilia or tails attached to a joint on this lower surface. These lively bodies, 

 appearing like little animals, finally settle down upon the plant, lose their 

 cilia, take on a cell wall, and soon germinate and penetrate the plant as in 

 the former case. Sometimes, instead of the contents of a spore dividing 

 directly into zoospores, a short germinating tube is produced, at the end of 

 which a single spore is formed, the contents of which divides into zoospores. 



The spores of the fungus (conidia) in hot dry weather do not live more 

 than a day or two, and never live over winter, but in suitable weather they 

 may live for several weeks and be blown through the air to great distances, 

 carrying the disease to other fields. When the spores fall upon the ground. 



