BUREAU OF PLANT INDUSTRY. 205 



THE PHILIPPINE DOWNY MILDEW OF MAIZE. 



Good proo-ress was made in the investi<!;ation of the extremely de- 

 structive downy mildew of maize in the Philippine Islands. There 

 are several downy mildews in the Orient which have attracted in- 

 creasing attention recently because of their widespread destruction 

 of maize and related crops in the East and their threatened invasion 

 of our own country. The Philippine species is the most immediately 

 dangerous of the group. It was found that this maize mildew is 

 generally distributed throughout the thousand-mile extent of the 

 scattered Philippine Islands, thriving even in districts of higher 

 altitude where the climate closely approximates that of our South- 

 ern States. 



This downy mildcAv is the most destructive of all known corn 

 diseases. In the Philippine Islands it commonly causes losses of 

 from 80 to TO per cent and at times destroys all but a very few plants 

 in extensive fields. In some districts it is so severe that it has caused 

 the abandoning of corn culture for other more fortunate crops. 



Detailed information was obtained on many baffling points in the 

 hitherto obscure life history of the causal fungus. At least two 

 strains were found to be involved. Conidium production and the 

 consequent spreading of infection were discovered to take place only 

 at night, and even then only Avhen the plants Avere covered with a 

 thin layer of moisture. 



Under favorable conditions the downy mildew was found to spread 

 with very great rapidit3\ The spores are distributed by wind over 

 considerable distances; and, once started, the plant-to-plant infection 

 is progressively destructive, because the diseased individuals begin 

 producing large numbers of conidia within one or tAvo weeks. To 

 make matters worse, this conidium production may continue for 

 more than two months, and corn in the field is susceptible for six or 

 even eight weeks. When attacked, the plants are almost invariably 

 rendered quite barren, and as a rule they also are so stunted, weak- 

 ened, and deformed as to be useless for fodder even if they sur\ive. 



By extensive inoculation experiments it was learned tliat the dis- 

 ease apparently attacks all varieties of maize with equal virulence. 

 It also attacks teosinte, some sorghums, sugar cane, and Saccharum 

 spontaneum and Miscanthus japonicus, two widely distributed Avild 

 grasses of the Orient, Avhich serve to maintain the disease from crop 

 to crop even in the absence of maize. 



Two types of spores were encountered, the fungus forming, on 

 sugar cane and these wild grasses, not only conidia but also heavy- 

 walled sjiores which are extreni(>ly resistant. The latter could easily 

 withstand sufflciently long transportation to enable them to rea( h 

 the T'nited States on introduced plants, seeds, or packing. The 

 connection between the two types of spores has not yet been definitely 

 established, but in the Philippines the resistant spores are not neces- 

 sary to the continuance of the disease. 



Because of the extensive distril)ution of these tw'o wild grasses, 

 it is practically impossible to eradicate the downy mildew from the 

 Philippines or from the Orient as a whole, but in the United States 

 where the grasses do not occur this probably could be accomplished.^ 

 However, means of control were devised which, even in the Philip- 



