There were several reports of stripe rust in Victorian wheat crops this season, despite the limited green bridge over the previous summer and a dry spring. Stripe rust reports were closely followed by questions about the economic benefits of using a fungicide for control. This article aims to look at the important factors that determine the economic benefits of fungicide control of stripe rust.
There are three factors that are important to consider when determining the benefits of fungicide control of stripe rust. They are the time when the stripe rust epidemic starts, the resistance rating of the variety and the expected environmental conditions during the remainder of the growing season.
- Know the resistant rating of the wheat variety grown and check the resistance rating each year using the latest Cereal Disease and Variety Guide
- Monitor crops regularly (at least every 2 weeks) for stripe rust, even in varieties rated as resistant.
- Use a proactive stripe rust management program when growing varieties like Mace (MSS) in the Susceptible categories, even in seasons not conducive to rust development.
Timing of stripe rust epidemic
The earlier that a stripe rust epidemic starts the greater the potential for crop loss. A low level of stripe rust late in the season (i.e. post flowering) will have a minimal impact compared to an early occurrence in a crop (i.e. pre-flag leaf emergence). When stripe rust appears early in a season growers need to be proactive with their disease management.
Poor responses to fungicide control can occur when spraying is delayed until the stripe rust epidemic is well developed and the disease is already covering 10 to 15% of leaf area. Therefore, successful economic control of stripe rust requires the fungicide to be applied early in the epidemic, before the level of infection reaches 1% leaf area affected (this is when approximately 35 to 40 leaves per 100 have stripe rust), regardless of growth stage.
Knowing the resistance rating of a particular wheat variety is important because it’s the resistance of a variety that determines the speed at which stripe rust can develop in a crop.
Wheat varieties vary in their resistance to stripe rust. Varieties are rated for resistance using a nine point rating scale that has been adopted as a national standard by the grains industry. The resistance rating scale and definitions for rust ratings are available from NVT Online. Resistance ratings for individual wheat varieties are available from the state department Cereal Disease and Variety Guides, which are published each year.
The relationship between the leaf area affected by stripe rust and the resistance rating is not linear. There is an exponential increase in the leaf area affected by stripe rust moving from Resistant varieties to more Susceptible varieties. Therefore, there can be large differences in the amount of stripe rust observed on a variety even if the resistance rating appears similar, for example between an a Moderately Susceptible to Susceptible (MSS) variety like Derrimut compared to a Susceptible to Very Susceptible (SVS) variety like Mace (Figure 1).
In this example, from the 2014 NVT trials at Diggora (Figure 1), Derrimut has < 5% infection at the time of assessment on 6 October 2014 and would not need spraying as it is rated Moderately Susceptible to Susceptible (MSS) to stripe rust. Varieties like Derrimut (MSS) have Adult Plant Resistance (APR). APR becomes effective between tillering (GS 20) and booting (GS 49), and slows down the development of the stripe rust. On the other hand, Mace is rated as Susceptible to Very Susceptible (SVS) because it has no effective resistance to stripe rust in eastern Australia. With stripe rust covering 80% of the leaf area on Mace (SVS), it is already too late to expect an economic return from spraying.
*Ratings valid at time of publication – check current ratings.
Area under the disease progress curve
Another way to look at stripe rust development in relation to a varietal resistance is the area under the disease progress curve (Figure 2). In this example from 2013, the rate of stripe rust development is compared in seven wheat varieties from different resistance categories i.e. Resistant (green), Intermediate (yellow) and Susceptible (red).
Varieties rated as Resistant experience little rust and do not need spraying for stripe rust as they are able to delay epidemics developing until late in the season (Figure 2). The one proviso is when there is a breakdown in resistance to the major gene that protects varieties rated as Resistant. In this situation, a variety can go from being Resistant to Susceptible within a season. This is why it is important to check crops regularly for stripe rust, even if the variety grown is rated as Resistant.
While the benefits of growing a Resistant variety to stripe rust are clear cut, there is some confusion on the benefits of growing an Intermediate variety compared to growing a Susceptible variety. The difference between stripe rust development for varieties in different resistant categories is illustrated in Figure 2 and Table 1.
Table 1. Comparison of stripe rust development for Intermediate and Susceptible varieties.
|Variety||Resistance||7 October||14 October||18 October||21 October|
This data shows the rate of increasing infection of the SVS variety Mace and the MSS variety Derrimut. Both start with an affected leaf area of less than 1%, but the rate of increase is more dramatic for Mace. In the two weeks since the epidemic started (7 Oct) stripe rust increased from less than 1% to 50% in Mace, but only 15% in Derrimut.
In this example, the implication of growing Mace (SVS) compared to Derrimut (MSS) is the window of opportunity to spray for stipe rust. For Mace this window is extremely narrow. If stripe rust was not detected in the Mace crop on the 7th October and controlled, then stripe rust would have increased to 10% leaf area affected in a week. If it took a further four days to purchase a fungicide and spray the crop, stripe rust would have covered 20% of the leaf (18 Oct) and been extremely difficult to control. Growing the MSS variety Derrimut gives a wider 10 day window to detect stripe rust and apply a fungicide before the stripe rust reached a critical level.
Stripe rust can develop throughout the growing season. For infection to occur, the stripe rust fungus requires temperatures of less than 18°C (optimum 6-12°C) with a minimum of three hours of leaf-wetness (for example, dew) for new infections to occur. Once an infection is established, the fungus can survive short periods of temperatures as high as 40°C. Cool, wet conditions late in the growing season will favour stripe rust development.