I have never come across any hard information on how much
aspirin in how much water is enough or on how much would
be too much. My advice would be to crush two with a mortar
and pestle (or two spoons and a ball peen hammer), put them
in the blender, fill with water, blend, drench the soil around the
base of the plant with it. The roots will take it in, and it should
have the same effect as a foliar spray as the salycilic acid
makes its way up the through the plant, which is to stimulate
the plant's natural immune system.
Murphy's Oil Soap (original formula) and Castille Soap are both
organic, vegetable derived soaps. I do not know which cheap
dish soaps would lack artificial chemicals (that may or may not
be safe to spray on your plants).
Yucca extract:
http://www.groworganic.com/item_PSA0...act_Quart.html
(There are probably other brands of that, too, that was simply
one that I remembered seeing that probably has some useful
information in the description.)