LAUSR

Botryosphaeria dothidea causes severe disease of apple trees in China. The process of conidium germination, colonization, and infection of apple fruit and branches was examined on 'Fuji' apple and the effect of temperature, surface wetness and relative humidity (RH), and host surface washates on these processes was studied in controlled environments. Initial germ tube development and hyphal growth resulted in the colonization of the host surface without forming an infection structure. Hyphae expanded radially across the host surface and, after entering lenticels, developed into a dense mycelium mass and/or differentiated pseudo-parenchyma. Hyphae from the bottom of the pseudo-parenchyma either directly penetrated the lenticel surface intercellularly through the cell layer, or formed an undifferentiated hypha that invaded the lenticel through cracks formed during the lenticel development. Conidial germination and hyphal colonization occurred at 10 to 40oC, with an optimum of approx. 28oC. Conidial germination required an RH > 95% or surface wetness, but for hyphal colonization an RH > 90% was sufficient. Conidia germinated and formed germ tubes within 1 hour under optimum conditions. However, the pathogen required a longer period at RH > 90% or surface wetness for hyphae to colonize and form pseudo-parenchyma or dense mycelia on the host surface. Hyphal colonization is a crucial stage for infection of apple tissues by B. dothidea.