11] Potato Disease and Allied Diseases 33 
In seed boxes the soil may be freed from this fungus 
by heating it to 150° F. Mixing charcoal with the 
upper layer of the seed bed reduces the disease. Take 
two boxes of soil in which plants have damped off. 
Heat one of them to a temperature of at least 150° F. 
Sow each of the boxes very thickly with cress seed. 
Keep the soil very wet and use water which has been 
previously heated to 150° F. Keep the boxes in a 
shady place. When the plants grow many of those in 
the unheated box will damp off, whereas those in the 
heated box will not. Now introduce some of the soil 
and dying plants from the unheated box into the 
heated box; the plants begin to damp off. 
Onion mildew (Peronospora schleident). 
This disease is usually present wherever onions are 
grown. It is easily recognized by the yellowing of the 
leaves and also by the ‘“‘necking” of the plant, i.e. 
elongation of the stem between the leaves and the 
bulbs. The leaves which are attacked soon collapse. 
Like the potato disease it does most damage in dull 
warm weather. On the leaves of a diseased plant a 
dull violet-coloured growth forms. This is the fruiting 
stage of the fungus and consists of tree-like conidiophores 
bearing conidia on the tips of their branches. These 
somewhat resemble those of the potato but each 
conidiophore bears a larger number (see Fig. 11). In 
water these conidia give rise to zoospores which are 
capable of further infection. If damp weather con- 
tinues the disease spreads rapidly. In dry weather, 
however, it soon disappears. 
This fungus also forms winter spores like those 
of Pythium de Baryanum which serve to carry the 
P. F. 3 
