Early adhesion of Blumeria graminis to plant and artificial surfaces demonstrated by centrifugation

Abstract

The relative strength of adhesion by Blumeria graminis conidia and germlings to barley leaves and to clean (hydrophilic) or silanized (hydrophobic) glass, was assessed using centrifugation. Ungerminated conidia were incubated for 10 or 30 min before centrifugation, while extended incubation, for up to 12 h, allowed germlings to reach different developmental stages before treatment. After incubation, mounts were subjected to relative centrifugal force (RCF) ranging from 0.13 × 10 3 to 26.00 × 10 3. Two key features indicating adhesion strength were the minimum RCF required for displacement of any fungal units, and the percentage of these displaced by the maximum RCF. By these criteria, adhesion was far stronger after germination, presumably because germ tubes adhered to substrata. Ultimately, germlings adhered more strongly to leaves, where appressoria differentiated, than to clean glass where only multiple short germ tubes were formed. Ungerminated conidia adhered relatively weakly to host leaves and most strongly to silanized glass. Increasing incubation time did not increase their adhesion to leaves but conidia incubated for 30 min adhered more strongly to clean and silanized glass than those incubated for only 10 min. Thus, in general, the strength of adhesion by ungerminated conidia correlated to known differences in the speed of conidial extracellular material (ECM) release and to the quantity of ECM released on the different substrata. However, even on leaves where ungerminated conidia showed the weakest adhesion, about 80% of conidia were not displaced by RCF of 0.13 × 10 3. Under these circumstances conidia had been subject to about 4.2 × 10 −9 N force acting to displace them. This is far greater than the forces likely to be generated by wind acting on conidia deposited on the leaves of a barley crop. Thus, adhesion by ungerminated conidia is sufficient to hold the majority of spores in place even under very windy field conditions.