Symptoms and Signs
"Chocolate blotch" occurred in several lima bean fields in 2012. Symptoms appeared late in the season and were observed in fields as far south as southern Merced County and as far north as Yolo County. Symptoms appeared during the pod filling stage or later. Chocolate blotch is characterized by dark necrotic blotches, which occur on leaves in various shapes and sizes and are randomly distributed over the leaf surface. The necrotic blotches resemble leafminer damage, only chocolate blotches lack the leafminer galleries (mines inside the leaf surface). The severity can range from light to heavy.
Symptoms have been observed in the varieties UC Beija-Flor, UC-92 and UC Luna.
Comments on the Disorder
The cause of this disorder is currently unknown, but it is thought to be of abiotic origin. No detectable pattern of symptoms was noticed. However, in one field symptoms were more pronounced where irrigation water had stood and the soil had been well saturated. Plant tissue analysis indicated boron may be involved, but symptoms are not typical for boron toxicity. No bacterial or fungal agents were isolated from leaf tissue. Blotches should be checked for galleries with a hand lens to ensure they are surface blotches rather than tunnel-like galleries caused by leafminers.