Spruce broom rust of the broom. The main damage from the rust is simply overall volume and growth loss. The number of brooms as well as their proximity to the main stem can also play a role in determining the extent of the damage to the tree. "Chrysomyxa arctostaphyli" is an obligate parasite, and as with most other rust fungi diseases, spruce broom rust requires two different host plants to carry out its life cycle and is therefore referred to as heteroecious. The primary or aecial host of the rust is spruce ("Picea" spp.), and the secondary or telia host of the rust
Four causes after the ethologist Nikolaas Tinbergen and based on Aristotle's four causes, are complementary categories of explanations for animal behaviour. They are also commonly referred to as levels of analysis. The four questions are on function, what an adaptation does that is selected for in evolution; phylogeny, the evolutionary history of an organism, revealing its relationships to other species; mechanism, namely the proximate cause of a behaviour, such as the role of testosterone in aggression; and ontogeny, the development of an organism from egg to embryo to adult. Four causes The "four causes" are elements of an influential principle in Aristotelian