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Decision processes for barley fungicide application

Spraying a barley crop with a fungicide is, at best, an informed decision based on environmental conditions, expected disease pressure and the use of a susceptible variety. At worst, it is a shot in the dark because fungicide application is primarily an insurance spray: a way to limit the future development of a disease. But researchers at Agriculture and Agri-Food Canada (AAFC) are developing spray decision tools to help take some of the guesswork out of fungicide application in barley.

Barley leaf disease can be costly, but so can spraying with a fungicide. Photo By Bruce Barker.

“Unfortunately, fungicides represent an added input cost and whether they are needed can be difficult to evaluate, given the limited risk assessment tools currently available,” says research scientist, Kelly Turkington with AAFC at Lacombe, Alberta. “There is a general lack of objective, scientifically sound methods of risk assessment and fungicide decision support tools for cereal producers.”

However, Turkington says that understanding the diseases of concern and the factors influencing their development can help when making fungicide spray decisions. He lays out a suggested sequence of steps that may be followed to assess the risk of leaf disease development.

Of course, selecting a variety with the best disease resistance package is critical in helping to minimize barley disease losses, but disease resistance may be rendered ineffective by changing pathogens or may not always be available in suitable varieties. If a variety is susceptible, then Turkington suggests that the flow chart may be a potential starting point when trying to decide on whether to spray or not. The flow chart is based on a combination of common sense and an evaluation of risk factors for disease development.

Call in the scouts
The first step is to note any emerging leaf disease problems when weed scouting in June. Although fungicide application is not recommend at this stage, the appearance of symptoms will indicate the potential for leaf disease development later in the growing season.

“There may be a tendency to trivialize the occurrence of a small amount of leaf disease during weed scouting activities in June, however, barley leaf spot diseases have the potential to build up to damaging levels in relatively short periods of time,” explains Turkington.

Under favourable conditions, barley leaf pathogens can complete and repeat their life cycles every seven to 14 days depending on the particular disease and prevailing weather conditions. Even a very small amount of disease observed in June could develop into a problem, especially when a highly susceptible variety is grown, and there is potential for favourable weather conditions to promote rapid disease development.

Are the frogs croaking?
What is the weather forecast? That is the next step in assessing disease risk, along with continued field scouting. The presence of a disease in sufficient quantity along with the occurrence of favourable weather and the presence of a suitable host are all needed for disease to occur. However, Turkington explains the relative importance of each of these components will vary depending on the particular plant disease and agricultural region.

Figure 1. A suggested sequence of steps that may be followed to assess the risk of leaf disease in cereals when considering fungicide application.

For leaf diseases like scald and net blotch of barley, the most important components of the disease triangle are the host and environment. The host is important for offering disease resistance, with the potential for rapid disease development greater for a susceptible variety than a resistant one. The environment is important, as it will determine the extent to which leaf pathogens will cycle and build on the developing barley crop.

Typically, the risk of disease increases with frequent showers, high relative humidity and/or heavy dews when growing a susceptible variety with a high yield potential. In barley, fungicide application tends to be most economical for seed growers and malt barley
growers. Most feed barley varieties tend to have better leaf disease resistance packages and as a consequence, their response to fungicide may be limited compared with more susceptible varieties.

Protection of the upper leaves, especially the two leaves immediately below the barley head will be important when a foliar fungicide is applied for disease control. Turkington says that it is important to keep in mind that fungicide application is mainly effective at preventing the pathogen from infecting the host plant. If extensive development of leaf disease, covering more than 10 to 20 percent of the flag and penultimate leaves, has already occurred on the upper leaves of the canopy, the ability of fungicides to eradicate well-established infections is limited, at best.

Assessment of the prevalence and severity of leaf disease is important when the crop is coming into the flag leaf stage. The appearance of low to moderate levels of disease on leaves in the lower and middle parts of the barley canopy will indicate a potential risk for infection of the upper canopy leaves, which are important for yield and grain filling. If scouting at the flag leaf emergence stage indicates the presence of low to moderate leaf disease in the lower and middle canopy of a susceptible variety, coupled with the occurrence of favourable weather conditions, then fungicide application to protect the upper canopy leaves may be warranted.

“Even if the potential for disease is there, you also have to consider the potential costs and benefits of fungicide application as part of any spray decision,” cautions Turkington.

On the yield side of the equation, under favourable environmental conditions, and with a susceptible barley variety and the expectation of moderate to severe disease levels, yield increases of 15 to 30 percent may be expected from fungicide application. However, with less favourable conditions for disease development, yield increases may only be in the five to 10 percent range. Assuming a barley price of approximately $2.00 per bushel, yield increases in the range of nine to 10 bushels per acre would be needed to cover the cost of fungicide and application. This could possibly be reduced to the range of six to seven bushels per acre with some of the rebate programs offered by pesticide manufacturers.

“Unless barley prices are over $2.00 to $3.00 per bushel and projected yields are over 80 bushels per acre, the additional cost of fungicide would likely not be economical,” says Turkington.

At the end of the day, the decision whether to spray a fungicide will depend upon localized conditions and individual field circumstances. However, careful assessment of economics, as with any crop input, in conjunction with a thorough understanding of the conditions that favour disease development will help make the spray decision easier. -end-

Yield loss equations
Researchers have developed yield loss estimates for scald in barley. They found that yield loss was proportional to the area of the flag leaf and second last leaf infected by disease, according to the following equation:

Yield loss (percent) = {2/3 of the area of the flag leaf infected (percent) + 1/2 of the area of the second last leaf infected} divided by 2.

For example, the yield loss with 30 percent of flag leaf infected and 50 percent of second last leaf infected would be {30 x 2/3 + 50 x 1/2} divided by 2 = 22.5 percent.

Although this equation was developed specifically for scald in barley, it can probably also be used to roughly estimate the yield losses due to other leaf diseases in wheat or barley. -end-
Source: Alberta Agriculture, Food and Rural Development.