The use of foliar fungicides for disease management in cereals

Nick Poole, Foundation for Arable Research (FAR), Lincoln, Canterbury, New Zealand

• Irrespective of whether foliar fungicides are curative or protectant they are better employed early on the plant tissue (leaf) to be protected.
• Flag leaf emergence (GS39) in wheat should be the primary focus of fungicide application in susceptible wheats, with application considered in the GS39-59 period for more resistant cultivars, subject to disease monitoring.

How do foliar fungicides work?

• Like many agrichemical inputs, fungicides do not create yield, they are applied to protect an inherent potential that would have been achieved in the absence of disease constraints.

Protectant activity

• Confers protection against future spore infection, and has no ability to control disease already present within the leaf.

Curative activity

• This is the ability of a fungicide to destroy disease after infection has occurred.
• Group C fungicides i.e. Tilt/Folicur/Triad.

Will fungicides protect a cereal leaf before it has emerged?

• Fungicides will cut down the inoculum pressure on leaves yet to emerge, they are not very effective at protecting un-emerged leaves directly. Fungicides move only in one direction once applied to the foliage, which is towards the leaf tip via the xylem vessels. There is no substitute for applying the foliar fungicide to the leaf you wish to protect.

Approximate yield contribution of the top three leaves in winter wheat and barley.

Source: HGCA (UK) 2000 Source: various

Management Strategies for Wheat

• Irrespective of whether it is barley or wheat when fungicides are employed timing is more important than product. Consider GS39 as the key timing for a single application to this crop.

• For more susceptible varieties – monitor at GS32 and GS33 and consider application if stripe rust is being reported widespread.

• There is no substitute for applying the fungicide to the leaf you wish to protect.

   

Management Strategies for Barley

• Unlike wheat it is far more difficult to pinpoint single fungicide timing.
• Monitor at GS30-31 for the presence of disease on the older leaves.

How do the new Strobilurin fungicides work in comparison

• Strobilurins used alone are superb protectants but weak curative materials.
• The protectant activity offered by these products is far more persistent and leads to enhanced green leaf retention at grain fill. A property not exhibited by the triazoles.

Where are Strobilurins likely to be cost effective?

• Most cost effective where the enhanced green leaf retention can be maintained by sufficient soil moisture. For example high yield potential scenarios of 5t/ha plus.
• Where these situations combine with more susceptible varieties Strobilurins in mixture with triaziles applied at the GS39-59 timing are more likely to give benefits over and above triazole fungicides.

Which is a better candidate for Strobilurin fungicides, wheat or barley?

• Current data suggests more application to barley than wheat.

Field Monitoring and Identification of Cereal Foliar Diseases Gordon Murray, NSW Department of Primary Industries)
When to survey crops

• Early (seeding to tillering) surveys required for yellow leaf spot and insect damage.
• For leaf diseases (except the rusts) to be severe in spring, they need to establish in the crop by late tillering or by the first node stage.
• Weekly inspections should occur when rust is known to be around and where susceptible crops are grown.
• Can go through one generation every 8 days in ideal conditions.
• Inspect susceptible varieties every 2-3 weeks in winter and then more frequently in early spring.
• Monitoring should be done on a regular basis depending on seasonal conditions and risks. In wet humid conditions, monitoring should be fortnightly.

Damaging diseases with long distance dispersal – Rusts

• Need for frequent inspections, it appears in “hot spots”.

Splash dispersal and less damaging diseases – Scald, YLS, Septoria, SFNB, etc

• Only requires localised monitoring along edges of paddocks, or where infected stubbles remain.
• If 10 random locations each with two plants reveal no symptoms, then the level of disease incidence is likely to be below 5% and therefore of no immediate concern.

Importance of understanding the biology and dispersal of pathogens

• You should be aware of the origins and dispersal mechanisms of the pathogens.

The rust diseases of winter cereals: diagnosis epidemiology and determining economic thresholds (Colin Wellings The University of Sydney, Plant Breeding Institute Cobbitty & Gordon Murray NSW Department of Primary Industries, Wagga Wagga
Disease Diagnosis
Disease symptoms and plant parts affected by the rust diseases of winter cereals in Australia

Further details and photographs of the rust diseases are found in “Wheat Rust: the Back Pocket Guide: published by GRDC

Disease Life Cycle

• The rust pathogens must infect the living host tissue to survive and reproduce spores.
• Several stripe and leaf rust samples were collected from seed increase plots indicating that rust pathogens have survived the summer period.
• Spores of the stripe rust pathogen will germinate at high humidity and temperatures from 5 to 15 C in as little as 6 to 8 hours.

Resistance Expression

• Host resistance to the cereal rust diseases can be divided into two broad categories.
• Seedling resistances can be detected from the emergence of the seedling leaf, and remain effective throughout the life cycle of the plant.
• A second category is adult plant resistance (APR) where resistance is expressed at various growth stages ranging from the second leaf through to flowering.
• A problem with APR is the variable onset of expression.
• It is important to monitor cereal rust pathogen populations.
• The occurrence of stipe rust pathotype 134 E16 A+ in eastern Australia caught most of us by surprise, particularly when it was clear that this pathotype was virulent for at least one as yet uncharacterised APR gene.

Rust sample collection

• In the light of the discussion in this paper, it is important to confirm disease diagnosis and to monitor pathotype change in rust pathogen populations. Rusted leaf tissue should be samples with fresh sporulating lesions; 2-3 leaves should be folded together once; placed in a paper envelope; details of location, variety, collector’s contacts should be recorded and the envelope dispatched without delay to:

Australian cereal Rust Survey ,
Plant Breeding Institute
Private Bag 11
Camden NSW 2570

Epidemiology of stripe rust of wheat

• Stripe rust lives year round on living wheat plants.
• The disease develops most rapidly in mild temperatures (15-200C), and virtually stops as temperatures rise much over 200C.

Economic thresholds for stripe rust of wheat

For wheat maturing under average spring temperatures, and for stripe rust beginning at different stages of crop growth, the expected losses are:
Losses in %

Losses generally have to exceed about 10% for spraying to be economic.

Management Strategies for 2004

Action from tillering to grain filling

• Be alert for reports of stripe rust development during the season
• Monitor crops
• If stripe rust develops in the area, be prepared to spray
• Get advice on the best spray strategy for the particular crop
• If stripe rust (or the other rusts) is behaving differently to expectation, get samples to your local agronomist/adviser.

Monitoring

• Growers can assist by checking their crops frequently.
• During August crops should be monitored at least every 14 days, while in September and October monitoring should be weekly
• Several products are available for spraying.
• Generally the less expensive ones stop further disease devilment about 7 days after spraying and protect the plant for 3 weeks after spraying
• The more expensive stop rust about 5 days after spraying and protect for about 4 weeks.
• Control depends on acting early in an epidemic.
• There is little point in spraying before stripe rust is observed
• As a rule of thumb: Collect 100 tillers randomly, count “1” for a tiny pustule or the whole leaf covered. When the count is about 35 infected leaves per 100 leaves, the stripe rust level is about 1% leaf area diseased. When stripe rust is seen as well developed “hot spots” in the crop, the average level is about 1% leaf area
• Spray before the level of stripe rust exceeds 5% of leaf area.

Yellow spot and Septoria tritici blotch epidemiology – commercial impact and strategies for management (Greg Platz, Northern Barley Improvements Program, Queensland Dept of primary Industries and Fisheries

• Yellow spot and Septoria tritici blotch reduce green leaf area.

Both diseases are stubble-borne and require free moisture for infection development