First Report of Root Rot Caused by Pythium myriotylum of Onion in Idaho

Abstract

In August 2017, onion plants (Allium cepa cv. Joaquin) were observed in one field in southwest Idaho with a high incidence of reduced bulb size. One area, approximately 3 ha in size, comprising approximately 20% of the total field, appeared to be affected. In this area, over half of the bulbs appeared smaller by at least 50% compared with nonaffected areas of the field. Closer inspection showed that the roots of the stunted bulbs were discolored and volume reduced. To determine the causal agent, root pieces, each 3 to 5 mm in length, sampled from symptomatic plants were surface sterilized (2% sodium hypochlorite) for 30 s and rinsed twice in sterile water prior to plating onto potato dextrose agar (PDA) amended with penicillin G (0.2 g/liter) and streptomycin sulfate (0.8 g/liter). After 5 days at 21°C in the dark, colonies were observed, and hyphal tips were transferred to fresh PDA plates to obtain pure cultures. After 7 days, suspect Pythium colonies were present that had abundant, white, aseptate hyphae up to 9 µm in width and sporangia. Species identity was confirmed through ribosomal DNA (rDNA) ITS sequencing from a representative isolate (designated OP1), as previously described (Woodhall et al. 2013). The resulting sequence (GenBank MT256180) was 99% identical to the sequence from Pythium myriotylum strain CBS 254.70 (GenBank AY598678). To confirm pathogenicity, onion seeds (cv. Vaquero) were planted in individual pots containing premium potting soil (Scotts, Marysville, OH). The plants were grown in a controlled environment chamber set at 21°C for 1 month until they were approximately 10 cm in height. Plants were watered as required. Plants were then inoculated with a plug of PDA (10 mm in diameter) that was either fully colonized with isolate OP1 or sterile in the case of the control treatment. Six plants were inoculated for each treatment. The plugs were placed adjacent to the main root mass at approximately 5 cm depth. Plants were grown for a further 18 days under the same conditions and then were destructively harvested and visually assessed for root symptoms. On all plants inoculated with isolate OP1, the roots had brown discoloration. The roots of the control plants appeared healthy with no discoloration. Root length was also significantly less (t test, P < 0.05) for the inoculated plants (mean = 18 mm) than for control plants (49 mm). Pythium spp. was reisolated from all inoculated plants, but no Pythium spp. was isolated from control plants. Several Pythium species can infect onion roots (Vincelli 2008). To our knowledge this is the first report of P. myriotylum affecting onions and in Idaho. P. myriotylum mainly occurs in warmer regions and has a wide host range including wheat, beans, and corn (Serrano et al. 2008; Wang et al. 2003). These crops are often grown in rotation with onion crops in southwest Idaho, and therefore growers may need to consider the potential presence of P. myriotylum in their fields.