I haven't been successful with growing daikon. No roots develop, only green tops. Also too heavy application of manures and high nitrogen fertilizer favors top growth only. The black rot bacterial disease can cause blackish streaks in the daikon roots.
Boron deficiency can cause a grayish discoloration of the central portion of the roots.
I've noticed lots of aphids on the daikon plant. Daikon matures in about 50-60 days after planting the seeds. Varieties such as Cherry Belle, Early Scarlet Globe, Comet, Sora, Sparkler, Red King, Fancy Red and others are ideal for home planting. The radishes that I'm growing splits. Too much water after the roots reach harvestable size causes root splitting.