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 1 5-20 fji. The conidia ger- 

 minate readily when fresh, by the FIG. 62. SECTION OF POTATO LEAF 

 production of about eight zoo- AND CONIDIOPHORES OF THE PHY- 

 spores. Germination may be TOPHTHORA 



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 



