I have some napa cabbage plants that are trying to finish heading. I live in zone 8 Southeast NC near the coast so we coldest temps here at night are usually above 25 degrees F. The plants are freezing when it gets below 32 but they bounce back by about 10am and look okay. The problem that I've been having is the oldest/biggest leaves start to yellow at the tips and then eventually yellow all the way down. Is this from the frost damage or fungus problem? I recently mulched with wheat straw about a week ago to help protect the roots and hold moisture. I'm on an organic program. I will get some pictures and post soon.
thx Joe

working on getting pix to show. click on the picture below for the slideshow of my garden. these were taken 12-20-08
cheers!
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to me it looks like frost damage...similar looking to sun scald. Hopefully someone else will know for sure. Thanks for posting!