# A practical guide to two-stage sporulation of Pyricularia oryzae: introducing a filter paper method and comparison with existing methods using strains from diverse grass hosts

## TL;DR

This paper introduces a new filter paper method for spore production in Pyricularia oryzae, a rice pathogen, and compares it with existing methods across strains from various grass hosts.

## Contribution

The study introduces a new filter paper method for spore production and evaluates it alongside existing methods across diverse grass host strains of Pyricularia oryzae.

## Key findings

- The filter paper method achieved inoculum-competent spore concentrations in 78% of tested strains without concentration steps.
- Spores dried on filter paper retained germination and virulence after six months of frozen storage at -40°C.
- The filter paper method showed broader applicability across strains compared to other methods.

## Abstract

Pyricularia oryzae is a major fungal pathogen responsible for significant yield losses in rice. In recent years, diverse pathotypes have emerged as threats to other economically important grasses, including ryegrass, oats, wheat and foxtail millet. Research on host–pathogen interactions involving this species requires reliable spore production for inoculation. However, as a hemibiotrophic pathogen, P. oryzae often sporulates poorly on artificial media and typically requires specialized two-stage protocols for consistent spore production. Although several such methods have been developed, all were optimized for rice-derived strains and have not been systematically evaluated across strains from other hosts. There is also a practical need for a simple setup that allows advance preparation and frozen storage of spore stocks. Therefore, we developed a new two-stage filter paper method and compared it with four published protocols across 23 strains from 13 grass hosts.

Comparative analysis showed strain specific differences in sporulation across methods, with no consistent link to phylogenetic lineage. The filter paper method reached an inoculum-competent concentration (defined here as \documentclass[12pt]{minimal}
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\begin{document}$$\ge 5\times 10^{4}$$\end{document} spores/mL, suitable for routine spray inoculation) without any concentration step in 18 of 23 strains (78%), compared with TARI 16/23 (70%), IRRI 15/23 (65%), corn grain 14/23 (61%), and mycelial mat 3/23 (13%). Spores dried on filter paper were ready to use upon thawing and retained germination with no change in virulence after six months of storage at -40 \documentclass[12pt]{minimal}
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\begin{document}$$^\circ $$\end{document}C. Step by step protocols with illustrations are provided for all five methods, together with practical guidance for choosing a method based on laboratory conditions, available resources, and research objectives.

This study provides a comparative evaluation of two-stage sporulation methods for Pyricularia strains across diverse grass hosts. Among the five methods, the newly developed filter paper method shows the broadest applicability across strains while maintaining yields comparable to established protocols. It can be prepared for frozen storage and used directly after thawing, enabling advance preparation and bulk stocking of inoculum for virulence profiling, resistance breeding, and disease management. These findings are particularly relevant for laboratories in regions that are affected by, or at risk of, outbreaks caused by this pathogen.