Patología Vegetal, Departamento de Producción Vegetal, Universidad Politécnica de Valencia, Camino de Vera 14, 46020 Valencia, Spain
Tiger nut (Cyperus esculentus L.) is cultivated in Spain for the production of tiger nut milk. Over the past 5 years, important economic losses resulting from a new tuber rot have been observed near Valencia in eastern Spain. Affected tubers were covered by a white mycelium that turned black as the disease advanced, leading to a general rotting of tubers. Aboveground plant parts showed some early decay and under high-moisture environmental conditions the white mycelium was present on the soil surface. This mycelium showed pyriform swellings characteristic of Rosellinia necatrix Prill., and coremia were occasionally observed on external surfaces of tubers after incubation in a moist chamber for 1 to 2 months. Coremia produced small ellipsoid or obovoid single-celled conidia 3.7 to 5.0 × 2.0 to 2.2 μm. The teleomorph has not been observed. Pathogenicity tests were conducted with inoculum produced on wheat (Triticum aestivum L.) seeds that were soaked for 12 h in flasks filled with distilled water. Each flask contained 300 ml of seeds that were subsequently autoclaved after excess water was drained. Two fungal disks of a 2-week-old culture of R. necatrix grown on potato dextrose agar were placed aseptically in each flask. The flasks were incubated at 25°C for 4 weeks, and shaken once a week to avoid clustering of inoculum. Two plastic pots (35 cm in diameter) per isolate were filled with a sterilized mixture of equal portions (vol/vol) of soil, sand, and peat moss, and inoculum was added at a concentration of 30 g of infected wheat seeds per 1,200 g of substrate (1). Healthy tubers were surface disinfested in 1.5% (vol/vol) sodium hypochlorite for 1 min, washed twice in sterile water, sown, and subsequently thinned to one per pot after emergence. Plants were grown under field conditions. Six months after inoculation, symptomatic tubers appeared similar to those originally observed in the field. The fungus was reisolated from affected tubers, confirming Koch's postulates. Pathogenicity tests were also conducted on avocado (cv. Reed) and almond (cv. Garriges) seedlings, and apple rootstock (MM-106) as very susceptible host plants for R. necatrix (1). Inoculated plants showed symptoms of wilting and death 4 weeks after inoculation. The fungus was reisolated from affected plants. This is the first report of C. esculentus as a host of R. necatrix.
Reference: (1) A. Sztejnberg and Z. Madar. Plant Dis. 64: 662, 1980.