Family Araliaceae 109 



Symptoms. The disease first attacks the petioles, 

 resulting in the drooping of the leaflets. In severe 

 cases, the leaf stalks are killed at the base where they 

 join the stem. This causes the leaves to droop over 

 the stem. The diseased areas usually become soft 

 and slimy. On the leaves, the spots are dark green, 

 watersoaked, and bent, soon becoming dry white in 

 the center with a prominent dark green water-soaked 

 margin (fig. 19 b). In wet weather, the disease at- 

 tacks the stem, and from there works downward to 

 the root causing it to decay (fig. 19 c). 



The Organism. The mycelium (fig. 19 d) of Phy- 

 tophthora cactorum somewhat resembles P. infestans, 

 the cause of late blight of the Irish potato, but differs 

 from it in producing an abundance of sexual or oos- 

 pores (fig. 19 g) within the dead tissue. The oospores 

 pass over the winter unaffected by cold weather. In 

 the spring they germinate, each sending out a germ 

 tube (fig. 19 h) which later may bear from one to 

 two conidia. These as claimed by Rosenbaum 1 

 may germinate by means of a germ tube or by 

 swarm spores. The conidia germinate in the same 

 way (fig. 19 e and f). 



Control. Downy mildew may be controlled by 

 spraying with 3-3-50 Bordeaux. Diseased plants 

 should be pulled out and destroyed by fire. Plant- 

 ing the roots deep in the soil will also protect them 

 from rotting. This seems to prevent the working 

 downward of the disease from the stem to the roots. 



Rosenbaum, J., New York (Cornell) Agr. Expt. Sta. Bui. 363: 

 65-106, 1915. 



