Tomato plants Tops drying
What would cause tomato plants to turn yellow at the tops first? Water stress? Fungus?
Cowlitz County Washington
I must give credit to the Yakima County Master Gardeners for helping me with an answer to your question. I would suggest that you follow up with the Master Gardener program in your County.
To quote from Ortho Home Gardener’s Problem Solver, “The yellowing of foliage from a loss or breakdown of chlorophyll. Yellowing may result from disease or insect infestation, poor growing conditions, or lack of nutrients.” So both your concerns are possible causes of leaf yellowing. It may be helpful if you capture the symptoms with digital images and repost those images on this website.
More information is needed to diagnose this problem. FS145E Growing Tomatoes in Home Gardens- a Washington State University Extension Fact Sheet may provide some information. Also see: WSU Hortsense which is a good reference (good pictures) for the more common problems tomato plants can experience.
Hope this information helps!