April 2016. An unusual symptom on sweet potatoes in Marshall Islands. It was thought to be virus. It does not look like little leaf (phytoplasma), and mites could not be found. There is a drought and it was wondered if this unusual symptom was to do with an extremely dry dry season. The soil is very sandy and it could be caused by nutrient deficiency? (Later, checks were made in the field for mites and thrips, but none were found).
There were a number of solutions offered:
(i) Boron deficiency which could be related to the dry conditions. It was asked if there were any corky patches in the roots? Corky veins, splitting and corkinness in other parts are common symptoms of boron deficiency.
(ii) Thrips – often they show symptoms along the veins; they feed on the underside of the leaves. There is an entry on Dendrothripoides ipomoeae, the sweetpotato thrips on PaDIL (http://www.padil.gov.au/pests-and-diseases/pest/main/136410).
(iii) Broad mites. Must look at the youngest leaves to see the mites. If it was broad mite, there should be more curling and distortion of the leaves, rather than damage on the leaves.
(iv) Scab, caused by the fungus, Elsinoe batatas. This is unlikely in the circumstances as Marshall Islands are going through a drought, but the symptoms are similar. The photo (top, left) was taken in Fiji. It was suggested that undersides of the young leaves were examined for brown patches along the major veins and also for torn and distorted leaves. The fungus attacks the leaves early, and prevents the veins from expanding and as a result the leaf blades tear. Often there are minute brown spots between the veins.
(v) Hormonal weed killer.