I grew my first tomato plant in a raised cedar planter. I used the best soil I could buy and the plant seemed to flourish and yield a crop of 3 doz. fruit. I also used 2 bamboo stakes to support the plant. About three weeks before the crop was finished the lower leaves turned yellow and seemed to spread up the stalks of the plant. I pruned it back and pulled up the bamboo supports only to discover the parts in the soil were covered with a heavy black powdery substance that looked like black mold. My question: if this is mold, does this mean the soil is ruined and should not be used for any other plantings? Thank you.
There are several tomato diseases which could cause the symptoms you describe, but fusarium wilt and tomato black root rot seem like top possibilities. Fusarium is a fungus that damages the roots and stem and causes yellowing of the leaves:
Black root rot caused by Thielaviopsis fungi is less common but has been seen in California. Check this fact sheet for more information:
And here are a few more tomato diseases in case those don't match:
In any case it is best to avoid growing tomatoes, potatoes, eggplant, and other relatives in the Solanaceae family in that pot for the next few years. You may want to get rid of the soil in the planter entirely just to be safe and then use 10% bleach to clean the planter.