Assess cereal mildew risk and be prepared to act

UK - February 12, 2007

Although on average over a quarter of cereals in England and Wales are treated each spring with a specific mildewicide, it is a useful exercise to assess individual crops risk and be prepared to apply an effective protectant mildewicide, such as Fortress early at T0/T1 timing, warns Dow AgroSciences.

High risk conditions for powdery mildew are closely linked to the extent of inoculum available and weather conditions that favour spore movement.  "Mildew overwinters on volunteers or on autumn-sown crops, particularly on varieties with low resistance ratings and on early sown crops.  If the autumn and winter has been mild, as it has been this year, it means that the disease is ready and waiting to develop in the early spring once temperatures rise.  So the first risk factor to consider is the HGCA rating of the variety sown and of neighbouring varieties as well as the sowing date of that crop.  Earlier drilled crops are at higher risk from powdery mildew.  You should also take soil type into account, as mildew tends to be more prevalent in fertile organic soils," explains Liz Tomkinson, Fungicide Specialist for Dow AgroSciences.

"When temperatures rise in the early spring, the dormant fungus starts to grow and release spores.  The disease can develop over a wide range of temperatures from 6°C to 24°C but develops most rapidly around 15°C.  So you will need to take note of the recent temperature, particularly if it is between 13°C and 24°C.  The disease also flourishes when the relative humidity is around 95%, so dense crop canopies and light rainfall will favour its development.  Too much free water on the leaf surface will reduce spore release and germination," points out Ms. Tomkinson.

According to Liz, crops at risk tend to be lush and growing strongly, so other high risk factors include high nitrogen uptake and use of organic manures.  "Soils which are manganese deficient also tend to favour powdery mildew.  Growers and advisors are invited to use the assessment key when the crop is at late tillering in order to support their subsequent spray decisions.  Some winter wheats and barleys are already at mid-tillering, so it is timely to think about assessing risk very soon.  If the disease key results in a score in excess of 27 points, the crop is at high risk and an application of Fortress at a minimum of 0.2 l/ha should last at least six weeks, or a sequence of 0.15 l/ha followed by 0.15 l/ha in 3 to 4 weeks should be applied."

Dow AgroSciences advise that if the assessment results in points of 19 to 26 and the crop is at medium risk, Fortress can be applied at 0.15 to 0.2 l/ha.  If between 14 and 18 points a rate of 0.1 l/ha is advised, with the crop being reassessed 3-4 weeks later for a potential second application.

Cereal Powdery Mildew risk Assessment

Risk Category	Rating	Risk Category	Rating
Variety - HGCA rating		Recent Temperature
1-4 5-7 8-9	***** **** **	<5 C 6-12 C 13-24 C >25 C	* ** *** *
Nearby Crops		Recent Conditions
Same variety HGCA 1-4 Same variety HGCA 5-7 Same variety HGCA 8-9 Set-aside Different wheat variety/other	**** *** ** ** *	Heavy rain and soil waterlogging Drought Light rain Alternating wet and dry	* * *** ***
Previous Crop		Nitrogen Regime
Legumes/OSR Same as current crop Other	*** *** *	One N application Split N applications	*** **
Sowing date		Nitrogen Source
September October November	*** ** *	Organic manure Inorganic Nitrogen	**** **
Crop Canopy		Soil Type
Dense  - 0-10% soil visible Medium - 11-30% soil visible  Open - >30% soil visible	*** ** *	Organic Clay Sandy	***  **  *
		Soil Manganese Status
		Manganese deficient No soil manganese deficiency	***

Liz Tomkinson points out that it is early mildew, up to stem extension (Growth Stage 32), that causes the greatest impact on yield.  "Susceptible varieties can suffer up to 40% yield loss and losses of 10-15% are commonplace.  An infection reduces ear numbers, grain sites, grain size and quality.  An early (T0/T1) protectant treatment of Fortress will protect the leaves from infection for up to eight weeks, depending on the dose rate used.  Its early use delivers excellent levels of control and a high return on investment.  It is one of the most cost-effective mildewicides on the market."

Fortress contains 500 g/litre quinoxyfen, formulated as a suspension concentrate.  It is a systemic protectant fungicide giving protection against powdery mildew on new plant growth.  It is recommended on all varieties of winter and spring wheat, barley, oats, rye, triticale, durum wheat and sugar beet.  It has been accepted by the Brewing, Licencing and Retail Association for crops destined for processing.  In cereals the average dose rate is between 0.1-0.2 l/ha.  Dose rates are flexible according to the variety and length of persistence.  Fortress should be applied in the spring at first signs of disease up until Growth Stage 49.  It is rainfast in one hour and is packed in a 1 litre container.