Identification and pathogenicity of Pythium species associated with poor growth of tomato plants in the Australian processing tomato industry

Abstract

The Australian Processing Tomato Industry (APTI) has observed a decline in yield over recent years and speculates that soil-borne pathogens may be a contributing factor. The decline is characterised by poor stand establishment and the presence of stunted plants sporadically spread throughout the field. In preliminary surveys, Pythium spp. were commonly isolated from the roots of plants showing symptoms of poor growth. The aim of this study was to identify those Pythium species and assess their pathogenicity. Four field surveys were undertaken during the 2016/17 season and plants exhibiting symptoms of putative Pythium disease were collected. Pythium isolates were identified based on morphology and ITS sequences. Eight species of Pythium were identified including some common pathogens and some species which have not been previously reported on tomatoes. The in vitro pathogenicity test showed P. aphanidermatum, P. ultimum var. ultimum and P. irregulare were the most aggressive pathogens. In a glasshouse test P. ultimum var. ultimum and P. recalcitrans significantly reduced plant height 1 month after inoculation. Overall, the pathogenicity results confirmed that pathogens in the genus Pythium were most damaging in the pre-emergence and early seedling phases of tomato plant growth. However, some species also had the capacity to continue to reduce the growth of plants at later stages, potentially leading to yield reduction. Further studies are necessary to better understand the aetiology of these and additional Pythium species in the APTI context, and their ultimate effect on tomato yield.