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Control thresholds for diseases

Field monitoring is an essential activity in order to optimize diseases management and apply IPM at farm level. Many countries have well-established control thresholds, which can be used as background for deciding whether or not to apply a fungicide. This guideline describes, how to do assessments and gives examples of thresholds recommended in different countries.

Eyespot (Oculimacula spp.) HGCA photos


>35 % attacked plants GS 30-32. The attack must have spread to the next to the outermost leaf sheath.

>20 % attacked tillers with penetrating lesions at GS 31-32.

15-20 % tillers with lesion at 25 cm crop hight

20 % tillers with penetrating lesions at GS 30-32

At GS 30-32< 10 % tillers with lesions: do not treat 10 to 35 % tillers with lesions: profitability variable, consider agronomic risk and weather conditions to decide.> 35 % tillers with symptoms: treatment recommended

40% severe attack at the milk-ripe stage calculated by the forecasting system SIMCERC 3; or 20-25% main tillers with lesions at GS 32-37

At GS 30-31: 25-30 % tillers with lesions

No specific threshold. Disease of minor importance

Eyespot development in crops is difficult to predict. The risk can be assessed based on visible presence of eyespot at stem extension or by using risk assessment including local experiences and weather data. Risk of disease development is relatively higher if the preceding crop is wheat and sowing was early. Minimal tillage has sometimes been found to reduce the risk compared to ploughing, but the trend might differ. There is a strong weather influence on disease development. Wet spring weather increases risk. In order to make a visible assessment, take out a sample of approximately 100 plants between GS 30 and 32. Rinse the plants under tap water and look at the plant basis to see if eyespot symptoms are present or not (see pictures). Only plants whose tillers have penetrating lesions (beyond the outer leaf sheath) should be included. Thresholds vary depending on the countries between 20 and 35% attacked plants.

 

Yellow (Stripe) rust (Puccinia striiformis) HGCA photos


>1 % plants with attack. GS 29-60 (S). >10 % plants attacked after GS 61-71 (S)

>1 % plants with attack or foci (S) GS 29-59. >10 % plants with attack (R)

At first symptoms.

1-2 % severity or foci present.

From GS 31: at first symptoms.Before GS 31: if spots are present and they are active.

First foci present.

At GS 30-31: 25-30 % tillers with lesions

First symptom occurrence on the upper 2 leaves.

Crops must be inspected carefully for small patches of infection (foci) before, and during, stem extension. Look out for the disease on all green parts between GS 29 and 60 and once the disease is seen in the crop, it is recommended to spray. The most recently emerged leaves always appear disease free between GS 32 and GS 39. However, the crucial final three leaves are at risk of infection as soon as they emerge. Keep a check on national ranking of the cultivars' susceptibility in order to be aware if you have a high or low risk situation. In very susceptible cultivars the treatments should be repeated after 2-3 weeks, before new symptoms appear. Autumn attacks may be found but are not generally found to have economic importance.

 

Brown rust (Puccinia triticina) HGCA photos


>25% plants attacked on the upper 3 leaves (S)>75 % plants attacked (R).

>10 % plants attacked on the upper 2 leaves between GS 37 and 59.

At first symptoms.

1-2 % on upper leaves.

From GS 32: at first symptoms on upper leaves.

30 % plants attacked on the upper 3 leaves between GS 37 and 61; or first foci present from GS 51.

Tillering: 10-15% leaves with lesions, Stem elongation: 10% stems with lesions, Heading: lesions on 2nd leaf and flag leaf.

First symptom occurrence on the upper 2 flag leaves.

Brown rust is usually more evenly distributed through a crop than yellow rust. However, pustules are tiny and easy to miss during early stages of disease development. It is important to spot the disease early as even low levels of visible infection on susceptible varieties may lead to a sudden epidemic in warm weather. Look out for the disease on all green parts between GS 30 and 71. The control threshold is approximately 30% measured as frequency of plants attacked assessing all green leaves. Autumn attacks may be found but are not generally found to have economic importance. In northern countries the disease appears normally quite late and is first considered as a problem from GS 37 onwards. Keep a check on national ranking of the cultivars' susceptibility in order to be aware if you have a high or low risk situation.

 

Powdery mildew (Blumeria graminis f. sp. tritici) HGCA photos


>10 % plants attacked from GS 29 in susceptible cultivars. >25 % plants attacked from GS 29 in resistant cultivars. After GS 40 no further mildew treatments are recommended.

>10 % plants attacked from GS 30 in susceptible cultivars (S) >25 % plants attacked from GS 30 in resistant cultivars. After GS 40 no further mildew treatments are recommended.

After GS 31-32 with lesions on leaf 3

Average of 3-5 % severity on upper leaves.

Susceptible varieties: > 20 % of F3, or F2 or F1, with symptoms (5% of each leaf attacked). Other varieties:>50 % of F3, or F2 or F1, with symptoms (5% of each leaf attacked).

60% plants attacked on the upper 3 leaves from GS 35 to 61, in some regions (e.g. Bavaria) with reference to only one of the upper leaves depending on the GS.

Tillering: 70% plants with first disease symptoms, Stem formation: 10% plants with first disease symptoms, or, Heading: first disease symptoms on 2nd leaf, flag leaf and head.

10-12 mildew spot uniformly distributed on the upper flag leaves.

Powdery mildew is generally most important at early growth stages in spring. Risk is often linked to specific regions and soils where the farmers from experience need to be very alert. The risk is often considered to be high on late sown crops, near hedges and on sandy soil. Look out for the disease on all green parts between GS 29 and 55. The control threshold early in the season is low in susceptible cultivars. Effective control using specific mildewicides requires treatments at low disease levels. Autumn attack may be found but is not considered to have economic importance. Control is not regarded to be economical after heading. Keep a check on national ranking of the cultivars susceptibility in order to be aware if you have a high or low risk situation. Mildew is so visible that the likely amount of damage may be overestimated. Low levels of attack as often seen around heading on the lower part of the crop are regarded not to have economic importance.

 

Septoria leaf blotch (Zymoseptoria tritici) HGCA photos


4 days with more than 1 mm rain from GS 32 to 71 (Susceptible cultivars), 5 days with more than 1 mm rain between GS 37 and 71 (Moderate resistante cultivars). Or, attack on 3rd leaf from GS 45 to 60 (all cultivars).

20-30 mm or 4-5 precipitation days counted from GS 32.

After GS 32 + rain. No specific threshold.

No specific threshold.

From GS 32> 20 % of the third leaf visible with symptoms. (the last leaf emerged is counted if visible).

30% (in some regions 40%) plants attacked on the upper 4 leaves from GS 32 to 37 or 10 % from GS 39 to 61; additional use of meteorological data required.

Tillering: 30-50% leaves with lesions or 1% leaves with fruiting bodies, Stem elongation: infected 15-20% area of 2nd leaf or 1% leaves with fruiting bodies, Heading: infected 10% area of flag leaf or 1% leaves with fruiting bodies.

No specific threshold

Septoria is often easy to find during winter and early spring. The severity depends on precipitation, and due to a long latent period (3 weeks) the control threshold is linked more to rain events than to symptoms in the crop. If several rain events occur during elongation (from GS 32), the risk is considered high. In more resistant cultivars the threshold is higher and the count of rainy days can be delayed to GS 37. Occasionally the disease appears despite few rain events and therefore the 3rd leaf can also be used to decide whether to spray or not during heading and flowering. Attacks of economical importance are very common in major regions with regular precipitation events during elongation of the crop. This gives to a large extent rise to more or less routine treatments.

 

Tanspot (Pyrenophora tritici-repentis) HGCA photos


>75 % plants attacked at GS 31-32 (S). >25 % plants attacked at GS 33-60 (S). >50 % plants attacked at GS 61-71 (S).

>75 % plants attacked at GS 31-32. >25 % plants attacked at GS 33-59. High risk only when minimal tillage and precrop is wheat.

At first symptoms.

No threshold. Disease of little importance.

From GS 39: at first symptoms on upper leaves.

5-10% plants attacked on the upper 3 leaves between GS 32 and 65; or first attack in cases of minimal tillage and previous crop being wheat.

No information.

No specific threshold. Disease of minor importance.

Tan spot appears particularly in intensive wheat production where minimal tillage is practised. The disease appears typically in early spring when temperatures above 10ºC appear. If more than 10% of plants show attack from GS 35 onwards, control is recommended. In fields with a high level of debris the risk is high.

 
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Revised 09.06.2017