Targeting improved partial resistance for foliar diseases of wheat using a new yield loss response curve tool
Cultivar disease resistance ratings are familiar to growers as an indicator of a crop's potential for resisting severe disease and for providing valuable information to assist crop management. Less is known about actual yield losses and in particular the interplay between disease severity and cultivar resistance.
The Department of Primary Industries and Regional Development (DPIRD) led a collaborative national Grains Research and Development Corporation project to quantify this interplay. One of the outputs from this project was a disease yield loss tool to assist growers with selecting wheat resistance classes and ultimately varieties.
Interactive disease yield loss tool
A beta version of an interactive online tool has been developed to summarise and compare results for resistance categories MR (moderately resistant), MRMS (moderately resistant to moderately susceptible), MS (moderately susceptible), MSS (moderately susceptible to susceptible), S (susceptible) and SVS (susceptible to very susceptible) among key leaf diseases of wheat in Western Australia
The tool depicts potential yield impacts subject to the types of disease anticipated and the resistance category of varieties under selection or management. It provides another means of improving decisions about variety selection by being able to compare yield losses to various individual diseases by disease resistance category.
The results are presented disease by disease as yield response curves and yield loss summary tables, with an overall summary across diseases in the form of a box plot.
Using the familiar traffic light convention to summarise results across MR/MRMS, MS/MSS and S/SVS categories the box plots (Figure 1) depict statistically modelled yield losses estimated from project results derived in Western Australia (results include stripe rust data from NSW as this did not occur in WA in the experiment years).
The range of potential loss across these resistance categories caused by the various diseases indicates the potential value of factoring resistance into variety selection, depending on the disease risk scenario.
The interactive yield loss tool is currently configured to five wheat diseases: stem, leaf and stripe rusts, yellow spot and nodorum blotch and is based on a component of the national project led by DPIRD senior plant pathologist Dr Manisha Shankar. To access the new tool refer to Yield Loss to Disease Model.
Digging deeper with nodorum blotch
Nodorum blotch is specific to Western Australia and provides a useful example of visualising and summarising yield impacts of the disease under different varietal and disease intensity scenarios. Among the diseases tested, it was the most damaging of the commonly occurring foliar diseases of wheat in Western Australia (refer to Figure 1). One likely explanation for this is the ability of the pathogen to attack both leaves and glumes when conditions allow.
Manisha Shankar and her team generated disease response curves from the 4 yield loss project experiments (Figure 2), recently published in the international journal Plant Pathology. Partially resistant varieties slowed disease development rather than delaying disease onset. When it comes to yield impact what you see is what you get, as Manisha's team also found no evidence of differing disease tolerance among trialled varieties (note that 'tolerance' is the ability to maintain yield despite high infection). Partially resistant varieties did accumulate less disease and therefore suffered less yield loss. The wheat nodorum blotch relationship was non-linear.
At the highest leaf disease burdens, highly susceptible varieties lost between 2.3 to 2.7 t/ha. In contrast, a variety with higher levels of partial resistance expressed lower burdens of total disease and lost around 1.4 t/ha. The result indicates that partial resistance to nodorum leaf blotch reduced yield loss to around 40 to 47% of losses observed in susceptible varieties, contributing significantly to protecting yield in disease conducive situations.
Yellow spot is a common occurrence across the Western Australian grainbelt and can cost growers up to $30 per hectare in lost production and control costs. Amongst the diseases tested, yellow spot wasn’t as damaging as Nodorum blotch or Stem rust (refer to Figure 1) but could still cause significant yield losses in the most susceptible varieties.
In the three yield loss experiments for wheat yellow spot in WA the disease protected plots yielded between 3.3 to 3.5t/ha. In the plots with the highest disease burden, the most susceptible varieties lost 1.5 to 1.7t/ha in yield ((Figure 3). Varieties with the highest level of partial resistance reduced yield loss to 0.8 to 1.0t/ha, demonstrating that partial resistance is an important factor in protecting yield under high yellow spot disease pressure
Rusts (Stem, Leaf and Stripe Rust)
Rust can be a significant threat to wheat crops in Western Australia in some seasons and can cause significant yield losses. The yield response curves generated from the rust disease trials in WA are all linear, suggesting that yield loss is directly proportional to the amount of disease at any growth stage.
In the four yield loss experiments (Figure 4) the most susceptible varieties lost between 2.4 to 3t/ha at the highest disease as opposed to the varieties with the highest levels of partial resistance which lost between 0.3 to 1.1t/ha. This indicates that partial resistance to leaf rust reduces yield loss to 65% to 89% of the losses that were observed in the susceptible varieties. The variety Mace has been reclassified since these experiments and its rating has gone down from MS to MSS with the incursion of new pathotypes.
In the disease trials, the most susceptible varieties to stem rust lost between 2.5 to 3t/ha at the highest level of disease burden. In comparison, yield loss was reduced to 1.3t/ha in varieties with the highest partial resistance in the plots with the highest disease burden. This result indicates that partial resistance contributes significantly to protecting yield in disease conducive situations.
From the two stripe rust experiments conducted in Wagga Wagga in NSW, it was found that varieties with partial resistance to stripe rust reduced yield loss to 59% of the losses that was observed in susceptible varieties. At the highest disease burdens, the most susceptible varieties lost between 2.7 to 3.4t/ha whereas the varieties with the highest partial resistance lost between 1.1 to 1.4t/ha.
In all the diseases tested, partial resistance was effective in reducing yield loss. Resistance categories for wheat varieties are available for all these diseases in the new online tool.
For more information on current disease ratings refer to GRDC’s National Variety Trials (NVT) Disease Ratings.
Project acknowledgement: Yield Response Curves (YRC) project (DAW00245).
Project funder: GRDC.
Project partners: Department of Primary Industries and Regional Development Western Australia; Department of Agriculture and Fisheries, Queensland; Agriculture Victoria, Victoria; Department of Primary Industries, New South Wales.>