Xanthomonas leaf spot symptoms are mostly seen on pumpkin, winter squash, gourds and cucumber and in some rare cases on watermelon. he leaf spots on watermelon start as small spots with a large yellow halo around it. The same symptom of leaf spots with a yellow halo is also visible from underside of the watermelon leaves. Spots may be much larger with a yellow halo around it. High temperatures and wet conditions favor symptom development. You may start to see large yellow discolored section on the leaves with large necrotic sections. The spots may enlarge and merge with other spots to form sections of blighted tissue on pumpkin leaves. The pathogen is seed-borne.