Incidence and control of black spot syndrome of tiger nut

ABSTRACT Tiger nut (Cyperus esculentum) is a very profitable crop in Valencia, Spain, but in the last years, part of the harvested tubers presents black spots in the skin making them unmarketable. Surveys performed in two consecutive years showed that about 10% of the tubers were severely affected by the black spot syndrome whose aetiology is unknown. Disease control procedures based on selection of tubers used as seed (seed tubers) or treatment with hot‐water and/or chemicals were assayed in greenhouse. These assays showed that that this syndrome had a negative impact on the germination rate, tuber size and yield. Selection of asymptomatic seed tubers reduced drastically the incidence of the black spot syndrome with respect to using seed tubers with severe symptoms (selection of healthy seed tubers was not possible because the causal agent is undetermined). Thermal treatment of seed tubers with severe symptoms reduced the number of unmarketable harvested tubers by half but was detrimental for the germination. Chemical treatments of seed tubers with severe symptoms decreased the incidence of the black spot syndrome about 40% for sodium hypochlorite and about 10% for hydrochloric acid, trisodium phosphate and the fungicide trioxystrobin. The black spot syndrome, of unknown etiology, causes considerable economical losses in tiger nut crops in Eastern Spain. Our results showed that the best method to control this syndrome was the selection of asymptomatic tubers used as seed, which showed lower incidence of the black spot syndrome and an improvement in the germination rate and the yield of marketable tubers. Thermal and chemical treatments also reduced the incidence of this syndrome. The figure shows the comparison of harvested tubers of tiger nut in pots in greenhouse from seed tubers with severe symptoms of the black spot syndrome (S) and from asymptomatic seed tubers (A). Time‐course incidence of the black spot syndrome (proportion of unmarketable tubers, showing severe symptoms), germination rate (proportion of germinated seed tubers), yield of marketable tubers (weight per pot). Dots represent posterior medians and vertical segments credible intervals at 95%.