Amaranthus hybridus (common name: amaranth) is a fast-growing crop with nutritious leaves and seeds that is cultivated in semi-arid regions throughout the world. In South Africa, cultivation of this crop as a leafy vegetable is increasing. In autumn 1997, extensive tissue discoloration and decay were observed in branches, stems, and root collars of mature A. hybridus in Bloemfontein, Free State Province. Symptoms included discolored phloem, xylem, and pith, black cankers, and weakened stems prone to wind breakage. Examination of these tissues revealed larval galleries of the pigweed weevil (Hypolixus haerens), the main insect pest of A. hybridus in South Africa (1). Six-month-old A. hybridus stems were split and small samples of discolored tissue adjacent to the larval galleries of each stem and the associated larvae were placed aseptically on corn-meal agar containing streptomycin and incubated for 4 to 7 days. The seven fungi most frequently isolated from discolored stem tissues (n = 166) were Fusarium subglutinans (46%), a Phomopsis sp. (11%), Alternaria alternata (10%), F. oxysporum (9%), F. solani (5%), a Phoma sp. (5%), and F. sambucinum (4%). The nine fungi most frequently isolated from larvae (n = 90) were F. subglutinans (46%), F. solani (8%), F. equiseti (8%), F. oxysporum (7%), A. alternata (6%), a Phomopsis sp. (4%), F. proliferatum (3%), F. sambucinum (2%), and a Phoma sp. (2%). Stems of greenhouse-grown A. hybridus were inoculated with the seven most common species isolated from the discolored stem tissues. One isolate of each species was used. Inoculations involved wounding stems by removing approximately 36 mm2 of the epidermis 5 cm above the soil, placing a colonized water agar plug on the wound, and wrapping Parafilm around the stems at the wound site. Wounded and nonwounded (untreated) controls were also included. A noncolonized water agar plug was applied to wounded controls but not to nonwounded controls. Ten plants per isolate and 10 wounded and nonwounded control plants were used in each of two separate trials (180 total plants). Treatments were assigned randomly. Four weeks after inoculation, canker lengths were measured and stem sections were surface disinfected and transferred to water agar plates. The presence of the fungi was confirmed after 20 days. Only F. sambucinum, F. oxysporum, and F. subglutinans caused cankers with frequencies of 100, 100, and 65% (n = 20), and mean lesion lengths of 30, 26, and 10 mm, respectively. Lesions were never observed on either of the controls. Discoloration and cankers were similar to that observed in the field. F. sambucinum, F. oxysporum, and F. subglutinans were recovered from 65, 50, and 60% of the tissues, respectively, and none of the Fusarium spp. were recovered from the control treatments (n = 20 for all). In artificial inoculations, these species can act as pathogens independent of the pigweed weevil and are likely the cause of the discoloration, decay, and cankers observed in branches, stems, and root collars of mature A. hybridus. However, there are no prior reports of a Fusarium sp. causing disease on A. hybridus, and H. haerens larvae were observed in all symptomatic stems in the field. Further studies are needed to determine the potential for significant disease loss associated with this insect-fungal association and the potential role of these fungi in further weakening Amaranthus stems that are colonized by H. haerens.
Reference: (1) S. vdM. Louw et al. Afr. Crop Sci. J. 3:93, 1995.