Answer:
Mold needs a few factors to grow
1) high humidity
2) warm temperatures
3) food source
If you were to keep humidity and temperature constant, I would say that wheat bread would grow mold faster. Rye bread is often more acidic because Lactobacillus maybe used in the dough raising process. The anaerobic respiration of lactobacillus can mean higher acidity of the dough and therefore a lower pH value.
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Mold will grow quicker on white bread than it will grow on rye or plain brown wheat bread.
It also depends on the formulation of the bread, whether or not it contains artificial or natural preservatives, the quality of the ingredients - and their original state of freshness - and then, of course, the conditions under which the bread was produced and the conditions in which it is being stored.
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It depends on what moldy environment you'd prefer to keep your bread in my friends...
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Put five to 10 drops of water on the piece of bread and store in a warm, dark place.
After 5 to 10 days, you should notice some fuzzy white, black, orange or green spots on the bread. Once visible, the mold will grow noticeably each day.