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Genetic structure, survival mechanisms and spread of downy mildew in Western Australian and Australian vineyards

Taylor, Andrew (2018) Genetic structure, survival mechanisms and spread of downy mildew in Western Australian and Australian vineyards. PhD thesis, Murdoch University.

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Western Australia (WA) was one of few grape growing regions of the world free from grapevine downy mildew, caused by the oomycete Plasmopara viticola, until two separate detections occurred in 1997 and 1998. The pathogen is now established in all growing regions of the state and growers are required to actively manage the disease due to potential economic loss. The introduction of P. viticola into WA raised a number of questions about the source of the incursion(s), its population structure, ability to survive in a climate not considered optimal for its development and the subsequent economic impact it is having on the WA viticulture industry.

Using a number of different detection methods 51 microsatellite markers have been identified for P. viticola. These markers were identified from isolates of European and American populations and had not been tested on Australian isolates. Using three isolates collected from geographically separated locations in Australia all 51 microsatellites were tested for their capacity to detect polymorphism within the Australian populations and the sequences lodged in GenBank. A total of 18 microsatellites were identified showing polymorphism.

A total of 413 samples from Australia (286), North America (69), France (32), Brazil (8) and Uruguay (18) were collected as part of a global survey. Both fresh leaf samples and DNA trapped on FTA cards were collected and the populations analysed using 16 of the 18 microsatellites previously identified. The populations were analysed for genetic diversity, mode of reproduction and cryptic P. viticola species present. It was determined the Australian and South American populations more closely relate to the French samples than those of North America, the origin of P. viticola. The only cryptic species discovered outside of North America was P. viticola clade aestivalis. The WA population had the lowest genetic diversity of all populations, likely a result of the more recent introduction of the pathogen and suggests the 1997 and 1998 incursions are linked. Genetic analysis of the WA populations indicate they have a clonal mode of reproduction whereas the American and French populations are sexually reproducing. This is the first record of clonal reproduction within P. viticola and highlights a greater plasticity within the mode of reproduction than previously identified.

With relatively mild winters in comparison to northern hemisphere grape production systems it was hypothesised P. viticola could overwinter as mycelium in dormant grape buds in WA. To test this hypothesis a number of field trials and vineyard observations were conducted over two seasons. It was discovered P. viticola can infect green buds, and has the capacity to produce sporangiophores from them, but there is no evidence of any ability to overwinter and produce shoots infected with P. viticola the following season. This finding contrasts long held assumptions about the overwintering ability of P. viticola in dormant grape material.

Oospores are considered important for both P. viticola survival and the source of primary infection. However, a survey conducted in the early 2000’s failed to detect oospores in the majority of WA vineyards. Vineyards in the Margaret River and Swan Valley regions of WA having experienced downy mildew infection during the 2014/15 season were assessed for oospores. All vineyards were found to have downy mildew oospores. To determine mating type ratios 11 isolates were co-inoculated on leaf discs and oospores were detected in three crosses including a cross between the same isolate. There is the possibility a single mating type of P. viticola exists in WA with the capacity for secondary homothallism or selfing.

Despite P. viticola being present in WA since 1997/98 an economic assessment of the impact of the disease on WA viticulture had not been undertaken. A bioeconomic model was developed linking weather with spread, infection of the disease and production costs and revenue to estimate the cost of P. viticola over time. It was found P. viticola costs the WA viticulture industry AUD $7.3 million per year with cumulative costs over a 30 year period to be AUD $140 million.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
United Nations SDGs: Goal 12: Responsible Consumption and Production
Supervisor(s): Burgess, Treena, Cook, David and Wicks, Trevor
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