This is a copy/paste of something I wrote and left on another forum site not too long ago, and saved because questions about disease come up so often:
There are two categories of plant diseases that affect plants. "Abiotic" diseases have no pathogen, and are generally caused by any of a number of cultural factors. Excess water retention in container media is one of the most frequent cause of abiotic disease(s). Since there is no pathogen, abiotic diseases are not infectious. Cultural factors like light, temperature, soil chemistry/soil structure, air pollution, pesticide/herbicide residue, insect herbivory, ….. can individually or collectively cause abiotic diseases, or, even though not infectious, set the stage for "biotic" diseases, which involve living pathogens.
In this case, the OP asks about the infectious/fungal/microbial pathogen, powdery mildew. Fungi are the most common pathogen affecting plants in our gardens, but bacterial and viral pathogens (even though there's a question re whether of not viral particles are living OR microbes – are not really applicable to this topic).
Plant pathologists often describe the disease process using a 4-sided figure called the disease tetrahedron (think "pyramid – 3 sides + the bottom").
Each side of the tetrahedron represents an essential part of the infection process. There must be a
virulent pathogen (capable of infecting the plant) that is
genetically capable of recognizing its host. Either
the environment must be conducive to the development of the pathogen, or the pathogen must be able to stress/weaken the plant. The 4th requirement is the time duration of favorable conditions required for diseases to develop and spread. Whenever conditions are such that any one (or more) of the 4 requirements is missing from the equation, there is no immediate threat.
Since the best medicines are prophylactic, the grower who makes sure plants are healthy and not stressed is ahead of the game. Regular prophylactic applications of fungicide when environmental factors favor development of the diseases is better than remedial treatment. Biotic diseases are better able to overcome the natural defenses of plants weakened by stress that occurs when the plant is forced to deal with cultural conditions near or beyond the limits the plant is genetically programmed to deal with, so again, keeping plants healthy is your/their best defense against disease pathogens AND insect herbivory.
Regarding fungi, if they need warm temperatures does it mean that there is no risk for plants left outside during winter? It depends on the fungi. Circling back to the disease tetrahedron, since fungal spores are almost ubiquitous, we can be reasonably sure there will be spores available as an inoculum, but cold temps might not provide the environment the particular species of fungus needs to germinate; still, that doesn't mean the spores are not present waiting for when conditions ARE favorable, so the answer is
not necessarily. We also know there will be potential hosts, but not whether or not the hosts will be susceptible to the pathogen, or weak enough for the pathogen to infect the host.
Al
