7S FUNGOID DISEASES 



Once inside the plant, the germ tube branches 

 and ramifies in all directions, piercing the cell wall 

 and causing- death of the infected tissues. The 

 hyphcC in the leaf send out aerial branches through 

 the stomata bearing conidia (Fig. 21, B), which 

 become detached, carried in the air to healthy plants, 

 and hence the rapid spread of the disease, for all 

 this goes on in the course of a few days. Ulti- 

 mately the whole haulm succumbs, Phytophthora 

 being assisted in the later stages of decomposition 

 by other fungi. Infection of the tubers by the 

 spread of the fungus down the stem to the tubers 

 is said not to occur, but takes place, due to the 

 conidia, or zoospores produced in them, being 

 washed down through the soil to the young tubers. 



The infected tubers may decay at once, or if 

 the infection is but slight and the soil not too wet, 

 they may not decay until storage, especially if the 

 temperature be high and the circulation of air 

 limited. Such decaying tubers serve as centres of 

 infection, and the whole of the tubers in the imme- 

 diate area may become rotten. If stored in a cool 

 dry place with free circulation of air the slightly 

 diseased tubers survive, but when planted they 

 either decay outright, or give rise to short com- 

 pletely diseased shoots a few inches high, which 

 bear huge crops of conidia of the fungus in their 

 short life, and so infect the healthy plants. The 

 diseased tubers show slightly sunken patches (some- 



