60 DISEASES OF GLASSHOUSE PLANTS 
the sporangiophore does not do this, but the new 
sporangium is produced on a side branch of the old 
sporangiophore some distance below the old sporangium. 
Sources of Infecttion.—The main sources of infection 
are the soil and water supply, but experiment has shown 
that the disease organisms are also carried over from 
one season to the next by seed-boxes and pots. Cracked 
pots are especially dangerous, and the discoloration and 
destruction of plant roots have frequently been traced to 
some crack or crevice which has harboured the resting 
spores of the fungus. These 
organisms, like many others, 
spend part of their existence 
in the living plant and the rest 
in hibernation over the winter 
in the decompésing soil humus. 
Composts rich in humus, such 
as old mushroom beds, are 
relatively more suited to per- 
meation by infection than poor 
soils. The water supply and 
drainage has proved to be a 
potent source of infection, and 
: preliminary experiments upon 
Fig. 6. Fiytopithora eryptogea: this disease led to the examina- 
(1) ye bodies—(a) con- ~~ 
gree) ZOURR Gree. tion of many nursery water 
(2) Ty pical aetedeet ment of conidia, 
showing how one conidium after 
another. is produced on the SUpplies and the consequent 
main jy youse conidia, “” proof of their powers of infection. 
Surface drainage is also dangerous, and has been shown in 
many cases to be the indirect cause of many epidemics 
of this disease. 
Control.—Sterilization of infected soil by steaming, 
baking, or with a 2 per cent solution of formaldehyde 
(1 gallon of commercial 40 per cent pure formaldehyde in 
49 gallons of water) will completely rid it of the disease 
organisms. Pots may be sterilized by boiling in water 
or by treatment with formaldehyde, while boxes are best 
sterilized with formaldehyde, as steam and hot water 
