Genetic diversity and pathogenicity of pea downy mildew (Peronospora viciae f. sp. pisi) in New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Applied Science at Lincoln University

Abstract

Downy mildew of peas is caused by the biotrophic pathogen Peronospora viciae f. sp pisi (Pvp) which occurs sporadically throughout temperate pea growing regions across the world. Severe infections can completely disrupt commercial production by reducing crop quality and yield; early season systemic infections can ultimately prevent seed from being produced. Control strategies are largely preventative, with the use of tolerant cultivars being the most simple and cost effective, however, the genetic diversity within Pvp often correlates with varying tolerances towards cultivars. To date, there are 14 known pathotypes of Pvp throughout global pea growing regions but the New Zealand Pvp population has not been examined. This is the first study in New Zealand to assess the genetic diversity of Pvp. The results has indicated that the causal organism of pea downy mildew is Peronospora viciae f. sp. pisi with genetically different isolates being present in pea populations in New Zealand. Preliminary bioassays have been developed that has enabled the potential for future rigorous screenings of pea cultivars. To characterise the genetic diversity of Pvp in New Zealand’s pea growing regions pods were collected from infected plants from 7 sites across the North and South Islands of New Zealand in the 2018-2019 growing season. An examination of the partial ITS1, complete 5.8S and partial ITS2 region via sequencing and RFLP analysis did not indicate any genetic variation between the representative samples. In contrast, a RAPD analysis of selected representative samples examined a larger portion of the genome and indicated genetic dissimilarities within and between sites. Despite the limited number of analysed samples the results indicated that the surveyed pea growing regions in the North Island have a more variable Pvp population that the surveyed South Island sites. Overall, genetic variation within Pvp in New Zealand is minimal, however, it is expected that further studies with a more representative sample size would identify greater variation. A range of bioassays tested various methods of inoculation and growth chamber conditions, whilst using different sources of inoculum to develop a method to screen pea plants against Pvp in a controlled environment. Infection was achieved using fresh sporangia and soil collected from sites with a known history of hosting Pvp infected plants. No infection was achieved with dehydrated, infected field pods. Only two plants expressed signs of Pvp infection, thus no discernable laboratory conditions were identified to facilitate disease expression. The successful inoculation methods identified in this study could be used in future studies to investigate the most conducive conditions for disease expression in a controlled environment.