112 THE PHYCOMYCETES 



Opinions differ regarding the time of occurrence of reduc- 

 tional division. Wager (1900) maintains that it takes place just 

 before fertilization, and Kriiger (1910) and Arens (1929, 1929a) 

 at the time of the first division of the zygote nucleus. It is prob- 

 able that Kriiger and Arens are correct, but this point requires 

 elucidation. 



In most downy mildews the oospores constitute the hibernat- 

 ing stage. They usually form within necrotic tissues near the 

 end of the pathogenic cycle and are liberated by decay of the 

 host tissues. InPla^jiopara vkicola [Arens (1929, 1929a)], how- 

 ever, they form all summer long. In Peronoplasmopara ciibensis 

 they form most abundantly in the youngest host tissues. In 

 Sclerospora graminicola they continue to fall to the ground as 

 the leaf parenchyma disintegrates [Weston and Weber (1928)]. 

 In Peronospora tabachia they form only in tissues that have been 

 dead a week or more. Essentially nothinsr is known about the 

 influence of environmental factors on oospore formation among 

 any of the downy mildews. 



The essentiality of oospores in the hibernation of P. tabacina 

 was established by Dixon et al. (1935). By epidemiological 

 methods they determined that this pathogen overwinters as 

 oospores in North Carolina. Primary infections in all areas ap- 

 peared in seed beds sown on the sites of old beds. These in- 

 fections antedated by 7 to 19 days those in near-by seed beds 

 located on new sites. Furthermore sporulation occurred in beds 

 located on old sites before the time when sporangia could be 

 entrapped from the air in the same locality. 



Oospore germination has been observed in a few species. Greg- 

 ory (1912) and Arens (1929, 1929a) noted that a short germ 

 tube arises from the oospore of Plasmopara viticola and that a 

 sporangium forms at the end of this tube. Swarm spores emitted 

 from this germ sporangium are capable of inducing primary in- 

 fection. In Sclerospora graininicola Hiura (1935) observed germ- 

 tube formation, which may be assumed to cause infection after 

 penetration of the grass host. In respect to the factors which 

 favor oospore germination, temperatures of 20 to 25° C have been 

 found to be optimum for both P. viticola and S. graminicola. 

 Manifestly because of the paucity of observational data the 

 germination of oospores of downy mildews is \\'orthy of ex- 

 tensive investigation. 



