Pyrenophora tritici-repentis the causal agent of tan spot: characterisation of New Zealand populations

Abstract

Tan spot, caused by the ascomycete fungus Pyrenophora tritici-repentis, is one of the most economically significant and devastating foliar diseases of wheat. In this study, the distribution, genetic diversity and race structure, sensitivity to commonly used fungicides and virulence on different wheat cultivars of P. tritici-repentis was investigated.

A total of 15 different populations of P. tritici-repentis were collected during the 2013-2014 survey period. The survey showed that P. tritici-repentis was widely distributed throughout the South Island of New Zealand. A follow-up farmer questionnaire revealed that the re-use and sowing of untreated wheat seed and continuous sowing of wheat over 10-20 years may have exacerbated tan spot infestations.

A total of 12 single spore P. tritici-repentis isolates were chosen to represent the geographic distribution and diversity of P. tritici-repentis in New Zealand. The identities of all isolates were confirmed through sequencing of the internal transcribed spacer (ITS), β-tubulin regions and species specific (PtrUnique) PCR primer products. All New Zealand P. tritici-repentis isolates had the Ptr ToxA gene ToxA, but lacked the Ptr ToxB gene ToxB. This result indicated that the New Zealand isolates most likely belonged to virulence races 1 or 2. Results were confirmed by comparing New Zealand isolates against a range of fully characterised P. tritici-repentis international isolates.

Three different molecular methods (UP-PCR, RAPD and microsatellite analysis) were utilised in this study to assess genetic diversity of a range of New Zealand and international P. tritici-repentis isolates. A total of 13 different genotypes were detected with all three methods using 12 different primers from all of the New Zealand and international P. tritici-repentis isolates. Although all three methods provided different levels of information, they indicated that the genetic structure of the South Island populations of P. tritici-repentis was clonal. The New Zealand isolates were genetically distinct when compared to a range of international P. tritici-repentis isolates. These results suggested that New Zealand populations of P. tritici-repentis either do not undergo frequent genetic recombination or were part of a recent incursion. However, the exact explanation could not be identified.

The in vitro fungicide bioassay showed that all ten of the assessed fungicides effectively reduced the in vitro mycelial growth of P. tritici-repentis to <50%. The fungicides isopyrazam, propiconazole and prothioconazole had the lowest EC50 values which indicated they were the most effective fungicides against P. tritici-repentis.

The inoculation of ten wheat cultivars in a glasshouse environment with P. tritici-repentis revealed that at least eight of the ten cultivars were susceptible to tan spot. Of the ten wheat cultivars assessed, Empress and Duchess had the lowest disease severity and displayed symptoms characteristic of resistance towards P. tritici-repentis.

Overall, this study has provided a new insight into the current distribution, genetic diversity and race structure, sensitivity to a broad range of fungicides and virulence on a range of commonly cultivated wheat cultivars of P. tritici-repentis in New Zealand. The findings of this study will also provide wheat growers with valuable knowledge that will facilitate the development and implementation of future tan spot control strategies in New Zealand.