I planted my tomatoes from seeds two months ago. They get enough sunlight and rain. I transplanted them in 5 gallon pots, and now they are about 6-8 inhes tall with healthy, dark green leaves. Lately, however, some leaves are turning black not only on the edges but also on the whole leaf, and curling downward. What could be causing these? Tomato mosaic virus, or too much nitrogen-rich fertilizer? Hope youcan help me. Thanks.