Several pests and diseases cause dark venation in tomato plants, including the potato virus, broad mite and bacterial cankers. Curly top virus, psyllid infestation and phosphorus deficiency all cause purple-colored veins.
About this Author
Jessica Alzarana has a Bachelor of Music in music composition from the University of North Texas, and is currently pursuing a graduate degree in music therapy from Texas Woman's University. Alzarana's essays have been published by UNICEF State of the World Children's Report, BootsNAll, Alarana Atelier, eHow, and Travels.