218 



FUNGOUS DISEASES OF PLANTS 



insufficient data. The evidence is more conclusive with reference to the mil- 

 dew on species of Hordeum and also the one on the Brome grasses. Salmon 

 . . . has investigated both of these. The mildew on barley (Hordeum vul- 

 gare] will infect this species and also Hordeum distichum, H. decipiens, 

 H. Hexastichum, H. intermedium, and H. Zeocriton, but will not pass 

 over to Hordeum jubatum, H. bulbosum, H. murinum, H. secalinum, 

 H. sylvaticum. In some of these cases, however, the number of trials is 

 very small. 



Morphological. With only one or two exceptions (notably 

 Sph&rotheca Mors-uvce) the superficial mycelium of these plants 



consists of colorless hyphae, 

 considerably septate, each 

 cell being ordinarily uninu- 

 cleate. In all species except 

 two, so far as is known, the 

 haustoria penetrate the epi- 

 dermal cells in the form 

 of short, swollen branches. 

 However, in one common 

 mildew of shrubs and trees 

 (Phyllactinia Coryled) hy- 

 phal branches grow through 

 the stomata and into the 

 intercellular spaces. These branches may in turn develop haustoria, 

 which enter the cells in contact with this intercellular hypha. As a 

 rule conidial production in all forms begins whenever a con- 

 siderable mycelium has been developed. These conidia consist, 

 quite generally, of a single chain of cylindrical or more or less 

 barrel-shaped unicellular portions produced in basipetal order on 

 short, erect conidiophores, developed directly from a hyphal cell. 

 The conidia are capable of immediate germination, and since 

 they are produced in quantity, they frequently give the mealy 

 or powdery appearance to the parts affected. They serve for the 

 rapid propagation of these fungi. The conidial stage was for a 

 long time unconnected with the perithecium form and was then 

 known under the form-generic name Oidium. The minute char- 

 acteristics of the oidial stages have not been sufficiently studied. 

 It is proper to use the name Oidium for any conidial form the 

 perfect stage of which is unknown or indetermined. 



FIG. 84. HABIT OF A POWDERY MILDEW 



