To imbibe and germinate urediniospores require free water or near
saturated atmospheres in which both adsorption of water is
maximal and capillary condensation pads form. The rate of spore germination
is
influenced by the amount of free water available.
During imbibition, spores swell and capillary pads form beneath spores
in
contact with the substratum. These pads greatly increase
the area of contact with the substratum. Pad area fluctuates and
may depend on the balance between the rate of capillary
condensation and the rate of uptake of fluid by spores. When spores, on
leaves or cellulose acetate, are misted with water they
imbibe rapidly and germinate. Adhesion pads which form under fully imbibed,
germinating spores are morphologically similar to
capillary pads but they differ in composition. On synthetic surfaces, during
imbibition of spores and under microscopic glass beads,
pads are composed principally of volatile capillary condensation and are
removed when samples are freeze-dried. On host surfaces,
when spores are fully imbibed and germinating, pads contain non-volatile
soluble materials derived from both pathogen and host.
These substances are not removed by extended freeze-drying.