32 

 THE POWDERY^MILDEWS. 



By. J. S. Dash, Macdonald College. 



The powdery mildews belong to the family Erysiphaceae of the 

 large class of fungi known as the Ascomycetes, being placed in the order 

 Perisporiales of that group. Tney are often known as "mildews," 

 "blights" or "white" mildews. 



During tlie summer they are very evident on a large variety of plants, 

 covering the leaves and often the stems and fruits w^ith a white cobweb- 

 by mass of mycelium, on which large numbers of conidia or summier 

 spores are produced. These give the host an appearance as though it w^ere 

 covered with a white pow^der. 



The mildews are found practically all over the world, but are most 

 abundant in north temperate regions, and their distrioution is apparently 

 not closely restricted by slight differerces in climate. As a group, mois- 

 ture is very essential for most vigorous development. 



This family has long been subject to an undue multiplication of spe- 

 cies, but Salmon, w^hose monograph is perhaps the most recent and 

 most reliable w^ork, includes in the familv forty-nine species and eleven 

 varieties. 



The following genera are included in the Erysiphaceae: — Podosphae- 

 ra, Sphaerotheca, Uncinula, Microsphaera, Erysiphe and Phyllactinia. 



CHARACTERISTICS. 



The characteristic feature of the Erysiphaceae is their true parasitism. 

 Other characters are the abundant formation of conidia, capable of 

 causing quick infection of the host, and the dehiscent perithecia, pos- 

 sessing outgro^vths or appendages of a definite form and containing the 

 non-septate ascospores. 



HOST PIvANTS. 



According to Duggar, the various species and varieties have been re- 

 ported upon about 1500 species of phanerogams. The plants that seem 

 to be least affected are some belonging to the orders Liliaceae, Iridaceae 

 and som.e other m.onocotyledons. Herbs, shrubs and trees are equally af- 

 fected, and sometimes a single species may be found upon plants of all 

 three sorts. 



The mildews attack and injure many species of cultivated and na- 

 tive plants. As examples we cite the mildews affecting the grape, goose- 

 berry, rose, cherry, plum, apple and various forest and ornamental trees. 

 The leaves are usually the chief parts affected, thereby lowering the vi- 

 tality of the plant; stems, twigs and fruit are frequently attacked, as in 

 the case of the gooseberry mildew. 



