. Fungous les maladies des plantes : avec des chapitres sur la physiologie, les méthodes de culture et de technique . Les champignons dans l'agriculture. Phycomycètes 1694. La sélection n'a pas eu augmentation visible de la résistance. 5. L'hybridation et la culture des semis, suivi d'une sélection rigoureuse, semblent offrir une méthode plus logique de se guérir, de variétés résistantes aux maladies que la sélection. La tomate est parfois sujet à cette maladie, mais jusqu'à présent, comme c'est maintenant connu, il n'est pas gravement touché dans n'importe quelle partie du monde. Le champignon. Le mycélium du Phytophthora, comme celui des autres membres de cette famille,

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. Fungous diseases of plants : with chapters on physiology, culture methods and technique . Fungi in agriculture. PHYCOMYCETES 169 4. Selection has not given visible increase of resistance. 5. Hybridization and the growing of seedling plants, followed by careful selection, seem to offer a more logical method of se- curing disease-resistant varieties than does selection. The tomato is occasionally subject to this disease, but so far as is now known it is not seriously affected in any part of the world. The fungus. The mycelium of the Phytophthora, like that of the other members of this family, is unicellular, and the haustoria are filamentous. The conidiophores arise singly or in groups of from two to four from the stomates. The conidio- phore is branched, and at the tip of each branch a conidium is produced. The conidium is pushed to one side and the branch continues. The continua- tion is, however, larger than the tip which produced the conid- ium, so that this further growth is marked by an enlargement of the branch, making a very characteristic form of conidio- phore (Fig. 62). The conidia are ovate and usually measure 27- 30 x 15-20 fi. The conidia ger- minate readily when fresh, by the production of about eight zoo- spores. Germination may be secured in water but apparently not in nutrient solutions. The zoospores are motile for a brief time, perhaps seldom longer than a hour. They then come to rest and appear spherical and invested with a wall. Germination readily follows, and the germ tube penetrates the leaf either by stomates or by boring through the cuticle. The conidia serve not only to spread the disease rapidly from leaf to leaf, but they also fall upon the soil and may. Fig. 62. Section of Potato Leaf and Conidiophores of the Phy- tophthora. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original

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