Suppose a dominant allele causes a plant disease that usually kills the plant before it can reproduce. over time, what would probably happen to the frequency of that allele in the population?

Over time, the frequency of this allele would decrease in the population. Since the specific allele is dominant and kills the plant before even reproducing, the number of plants having this allele in their genetic material will be difficult to increase. Therefore, the presence of this allele in the population will probably decrease.