Answer: Some begonias have variegated leaves, but if yours aren't of that variety, there could be a problem. Many things can lead to yellowing leaves: Are your begonias in a shady place? They are best adapted to shade, and excess sun can cause shade-lovers to form pale leaves. Inadquate moisture is another possibility - begoinas love a humusy, moist, yet well-drained soil. Avoid sprinkling water on the leavs, since this can invite fungal and bacterial diseases. Viral infections somtimes show up as streaks in leaves, as do some nutrient deficiencies.
Begonia leaves will yellow in fall as the tubers go dormant, but that's a ways away! I hope this information gives you some clues to the cause and possible solutions. Let us know if you need anything else!
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