10 Bulletin 313 



Phytophthora infestans (Mont.) deBary. — On leaves and 

 stems and in tubers, of the potato. Causes large, dark spots on the 

 leaves, appearing somewhat water-soaked and with abundant white 

 growth on lower surface. Spreads rapidly and causes death of 

 plants. Tuber rots, usually beginning at surface ; the rot is brown 

 or chocolate-colored, usually irregularly streaked giving a marbled 

 effect. Conidiophores emerging from stomata, single or in clusters 

 of 2 to 4, branching scorpiose-cymosely ; conidia ovoid with slight 

 wall near the apex, 27-30 x 15-20 /x. 



Common in the mountainous districts of the state and very rare 

 in other parts. 



30. Conidiophores and conidia, 1/12. 



Phytophthora phaseoli Thaxter. — On pods, young shoots, 

 flowers and buds of lima beans. Stunts or completely checks the 

 growths which become covered with a dense white mycelium. The 

 conidiophores emerge through the stomata ; are single or clustered, 

 simple or branched from below and cymosely branched above swell- 

 ings near the tips ; conidia oval or elliptic, papillate, 35-50 x 24 //,. 



Common throughout the state. 



31. Conidiophores and conidia, 1/12. 32. Immature conidiimi, 1/12. 



Rhysotheca australis (Speg.) Wilson. — On leaves of Sicyos 

 angiilatus L. Causes spots of various sizes and shapes but boimded 

 by leaf veins. Conidiophores arising in clusters from the stomata, 

 .500-650 X 9-11 fi, with 5-7 main branches ; the last branches, 10-14 /x. 

 Conidia ellipsoid, 14-17 x 10-13 /*. 



33. Infected leaf of Sicyos angulatus. 34. Conidiophores and conidia, 1/12. 



Rhysotheca geranii Peck. — On the leaves Geranium carolinia- 

 num. On the under side of the leaf ; conspicuous downy white, defi- 

 nite areas ; conidiophores fasciculate, monopodially 2-3 branched, 

 branches short, 90-350 x 9-12 ;«,; conidia obovoid, hyaline 18-25 x 15 



fi. 



35. Conidiophores and conidia, 1/12. 



Rhysotheca halstedii (Farlow) Wilson. — On the lower leaves 

 of Ambrosia artemisiifolia. Originates as small spots finally cover- 

 ing the entire leaf which turns yellow. Conidiophores fasciculate, 

 slender, 300-750 /a, alternately branched 3-5 times, final branches 

 8-15 long, conidia oval or elliptical, 18-30x14-25/*. 



36. Conidiophores and conidia, 1/12. 



