Answer: Dahlias are usually not too difficult to grow if given plenty of sun, and a good rich soil well supplied with organic matter. They also require adequate moisture throughout the season, so they need a moist yet well drained (not soggy) soil. The mulch should not be a problem as it helps maintain both constant soil temperature and moisture.
There are two common reasons for yellowing as you described. One is overwatering and the other is a lack of nitrogen. If you used a wood based mulch it is possible that the plants are being temporarily deprived of nitrogen due to the natural decomposition processes of the mulch. Since dahlias are heavy feeders in any case, you might wish to side dress the plants with compost or a balanced granular fertilizer according to the label instructions. Alternatively you could apply a water soluble fertilizer such as MiracleGro on a routine basis according to the label instructions in an effort to supply the plants with nitrogen.
Slugs normally chew holes in the leaves and would not cause yellowing per se. To control the slugs you can hand pick them, catch them in traps baited with beer, or try a commercial slug bait according to the label directions.
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