22 - 24 September 2009

Plant disease surveillance is important for early detection of incursions, monitoring disease status during eradication programs, and to demonstrate that an area is free of a particular pathogen, for trade purposes. Surveillance for plant pathogens currently relies on detection of symptoms by suitably skilled personnel. The difficulties of distinguishing diseases based on symptoms, and of timing surveillance to coincide with symptom expression, can result in new pathogens not being detected until they are already widespread. In epidemiological studies, monitoring has relied on symptom expression in crops or trap plants. However, there may be logistical challenges in assuring a timely supply of trap plants, and the amount of inoculum may be underestimated if conditions are suboptimal for disease development. More efficient surveillance techniques for plant pathogens are therefore needed. The combination of air sampling and PCR-based diagnostics allows fast, reliable, accurate, sensitive and specific detection of air-borne fungal pathogens. This project aims to apply quantitative PCR techniques, in combination with air sampling, for detection of air-borne fungal plant pathogens, and to determine the strengths and constraints of these tools in relation to plant health surveillance.

Methodology has been developed for PCR-based detection and quantification of fungal pathogens from air samples, using three model pathogens which occur on pulse and oilseed crops grown in rotation with cereals in South Australia. The model pathogens are Leptosphaeria maculans, causal agent of blackleg of canola, Mycosphaerella pinodes, one of a complex of pathogens causing Ascochyta blight of peas, and Ascochyta rabiei, which causes Ascochyta blight of chickpeas. These organisms were chosen because their ascospores are reasonably distinctive morphologically, real-time PCR tests are available for them, and airborne ascospores of the three organisms are expected to be at different concentrations in the field.

The technique for extracting DNA prior to PCR has been optimised and the PCR-based methodology has been proved to be both sensitive and specific to the three model pathogens. Results of monitoring for the three model pathogens in 2008 indicated that the PCR-based tests were in good agreement with microscopy, and there was broad agreement with trap plant data. The methodology enabled truthing of epidemiological models for blackleg and blackspot. Valuable epidemiological information was obtained as it was possible to use this methodology to gather data during months when trap plants cannot be grown. For example, numerous ascospores of M. pinodes were released on rain days in December and January, indicating that fruiting bodies of the fungus do not require further maturation before releasing spores, even after a period of over a month with no rain. As expected, spore trapping and trap plants both failed to detect air-borne ascospores of A. rabiei, which are normally absent, or present at only very low levels in South Australia.