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. Since there is no pathogen, abiotic diseases are not infectious. Cultural factors like light, temperature, soil chemistry/soil structure, media air:water ratios, 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 are sometimes players, too (even though there's a question re whether of not viral particles are living OR microbes – 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) which 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 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. 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. These would be plants not growing in the light, temperature, soil, fertility, ....., "sweet spot". Circling back to the disease tetrahedron, we can see compost that contains the virulent inoculum provides 2 of the 4 essential sides needed for a disease to develop – environment and inoculum. Time, the third essential side will not likely be on the side of the grower, so you can't use that to your advantage. We know there will be potential hosts (it'sa garden!), but not whether the hosts will be susceptible to the pathogen, or weak enough for the pathogen to infect the host. When you start to analyze, it doesn't make a lot of sense to take the position you'll build the size of your compost heap at any cost. Al

Appears you're correct. Treatments for powdery mildew must be applied at the first hint of disease. Harvest the pink tomato and let it ripen on the counter. Then open up that area as much as possible to increase air circulation. Water at ground level. Remove the brown portions of the leaves because thy won't ever re-green and, right now, they are limiting air circulation within the plant.

In a pot or ground? If in a pot, as an experiment, try fertilizing them with potassium silicate, 1/2 tsp./gal, with every watering. Otherwise, there are silicate fertilizers that can be spread on the ground, e.g., rice hull ashes. Potassium silicate can be bought online. See http://link.springer.com/article/10.1007/s11104-006-9085-4

I think powdery mildew, if eaten, is unlikely to hurt you, but the taste may be off, and there is a chance that you could have a serious allergic reaction. I should add that if you haven't gotten an allergic or immune system reaction after washing it off, because of course you never get it all off, you're probably good to go. Such allergic reactions, and reactions from compromised immune systems, are why vegetable vendors stay way clear of powdery mildew.