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Cultural practices impact on disease development

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


Resistance genes Varieties with moderate resistance genes are known, and help to reduce disease levels. [25,33]

Previous crop Wheat and other cereals increases the risk for attack. Non-cereal crops such as oilseed rape, etc reduce the risk. [38]

Sowing date Early sowing is known to increase disease risk. Late sowing is seen to decrease the disease level as epidemic generally gets delayed. When wheat is sown after wheat it is recommended if possible and practical to delay the sowing time to minimize the risk. [38]

Tillage Ploughing can increase the risk - thought to be due to increased N-mineralization coupled with deeper drilling. Direct drilling can reduce disease levels as plants have a more open habit with greater air movement. Ploughing can preserve crop debris and then increase the risk once it is brought back to the surface. [38]

Debris and volunteers Debris may directly influence disease levels as disease as both ascospores and condiospores are released from crop debris in the autumn.

Nitrogen level High nitrogen amounts increase to some extent the susceptibility of the crop.

Nitrogen strategy No information available

Crop density High crop density stimulates development as the humidity increases in a dense crop stand. [28]

Landscape No information available

Soil type No specific differences seen in some countries. Other countries see some differences. [38]

Weather Dry weather reduces the risk as the disease particularly during elongation the crop as the crop escape the attack by fast growth. Infection occurs at temperatures above 5 °C and during wet periods.

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


Resistance genes Varieties with good resistance are known, and help to reduce disease levels. Many specific genes are used and described but also non-specific resistance genes are known to be of importance. [2,23,40]

Previous crop High proportions of susceptible varieties and infected wheat in the previous year increases the risk of attack as high levels of inoculum potentially can survive to the next season. [16]

Sowing date Early sowing is known to increase disease level in autumn. Late sowing in the autumn has been seen to increase disease level in spring, as the very young plants in spring generally are more susceptible than early sown crops. [16]

Tillage No information available.

Debris and volunteers Debris does not directly influence disease levels as mildew is an obligate parasite. Fields with volunteers are an important source of inoculum as it serves as a green bridge for the spread of the disease between seasons.

Nitrogen level High nitrogen amounts increase the susceptibility of the crop due to high nitrogen concentrations in leaf tissues, easier penetration in plants and possibly due to denser crop with higher levels of humidity.

Nitrogen strategy No information available

Crop density High crop density stimulates yellow rust development as the humidity in the crop increases disease development.

Landscape No information available

Soil type No information available

Weather Severe frosts during the winter will reduce the inoculum and help to reduce disease levels. However, within plants the fungus can survive at very low temperatures. In the spring in cool moist weather the fungus starts to grow and produces active sporulating lesions. Temperature at 10-15 °C and relative humidity of 100% are optimal for spore germination, penetration and production of new spores. [12,16]

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


Resistance genes Varieties with good resistance are known, and help to reduce disease levels. Many specific genes are used and described but also non-specific resistance genes are known to be of importance. [40]

Previous crop High proportions of susceptible varieties and infected wheat in the previous year increase the risk for attack as high levels of inoculum potentially can survive to the next season.

Sowing date Early sowing is known to increase disease level in autumn. Late sowing in the autumn has been seen to increase disease level in spring, as the very young plants in spring generally are more susceptible than early sown crops.

Tillage No information available.

Debris and volunteers Debris does not directly influence disease levels as mildew is an obligate parasite. Fields with volunteers are an important source of inoculum as it serves as a green bridge for the spread of the disease between seasons.

Nitrogen level High nitrogen amounts increase the susceptibility of the crop due to high N-content of leaves, easier penetration in plants grown at high N levels but also due to denser crop with higher levels of humidity, which favours the epidemic.

Nitrogen strategy No information available

Crop density Dense crops likely to favour the disease as higher levels of humidity favour the disease.

Landscape No information available

Soil type No information available

Weather Severe frosts during the winter will reduce the inoculum and help to minimize the disease level. Mild winter and warm spring and summer weather stimulate attack. Temperatures between 15 and 22 °C accompanied by 100% relative humidity are needed for sporulation and spore germination.

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


Resistance genes Varieties with good resistance are known, and help to reduce disease levels. Many specific genes are used and described but also non-specific resistance genes are known to be of importance. [30]

Previous crop If volunteers have been removed the impact is small.

Sowing date Early sowing is known to increase disease level in autumn, but this rarely have impact on disease levels in spring. Late sowing in the autumn has been seen to increase disease level in spring, as the very young plants in spring generally are more susceptible than early sown crops. [14]

Tillage Ploughing has been found to increase the risk of mildew compared with minimal tillage. It is the increased mineralization of nitrogen following ploughing, which stimulates a more severe attack. [27]

Debris and volunteers Debris does not directly influence disease levels as mildew is an obligate parasite. Fields with volunteers are an important source of inoculum as it serves as a green bridge for the spread of the disease between seasons.

Nitrogen level High nitrogen use increases the susceptibility of the crop due to higher N concentration in leaves, easier penetration of the fungus. Possibly also due to denser crop with higher levels of humidity, which stimulates the epidemic. [34,35]

Nitrogen strategy Spilt strategies of N are less likely to encourage high disease levels compared to single applications of a single high level. [34,35]

Crop density High crop density stimulates mildew development as the humidity in the crop favours disease development. Overlapping in headlands often has higher levels of attack. [28]

Landscape The attacks are known to be more severe near hedges and in low and humid parts (black soils) of the field. [4]

Soil type Sandy soils are known to stimulate the disease development. This is often related to manganese deficiency which makes the crop more prone to mildew, It might also be related to the crop being more exposed to stress on these soils or higher levels of leaf wetness due to higher differences between plant and soil temperatures. Stress in the form of drought can also increase the risk of mildew. [14]

Weather As temperatures rise in the spring, dormant mycelium starts to grow and spores are quickly produced. The disease is not very temperature dependant although 15 C is optimal with relative humidity above 95%. Free water inhibits spore germination. Under dry conditions spores can be formed in about seven days.

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


Resistance genes Varieties with good resistance are known, and help to reduce disease levels. Specific genes are known and described but also non-specific resistance genes are known to be of importance. [5]

Previous crop High proportions of wheat in the crop rotation increase the proportion of inoculum and risk for attack. In areas with lots of wheat the level of ascospores will be high.

Sowing date Early sowing is known to increase disease level in autumn, which again can result in higher disease levels in spring and summer. Late sowing can decrease disease levels as the epidemic is generally delayed. [28]

Tillage Ploughing has been found to increase the risk of septoria compared with minimal tillage. This might be related to an increased N-mineralization following ploughing which can stimulate a more severe attack. [27]

Debris and volunteers Debris may directly influence disease levels as ascospores are released from crop debris in the autumn. Volunteers are not important as source of inoculum as they will typically be destroyed before the attack becomes visual.

Nitrogen level High nitrogen amounts increases to some extend the susceptibility of the crop. The effect is not believed to be of major importance within commercially used rates (120-200kg/ha). [34,35]

Nitrogen strategy Spilt strategies have been seen to reduce the attack compared with single applications. [34,35]

Crop density Low crop density stimulates septoria development as the disease is spread up the crop by rain splash, which is more effective in thinner crops. Dense crops may reduce rain splash but have in some trials been found to increase the risk, possibly due to higher humidity in the crop. [28]

Landscape No information available

Soil type No information available

Weather Dry weather reduces the risk as the disease needs 48 hours of humidty to stimulate development. Optimal temperatures are 15-20°C

The disease 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


Resistance genes Varieties with moderate resistance are known, and help to reduce disease levels. Few specific genes are described for this disease. [27]

Previous crop Wheat as previous crop increases the risk of attack as high levels of inoculum potentially can survive to the next season on debris. [27]

Sowing date Disease will in most regions in Europe first develop in spring as ascospores need to ripen and spread. This normally takes place in April.

Tillage Tillage is found to have a major impact on the disease. Increasing amounts of straw and debris increase the amount of inoculum. Ploughing will minimize the disease risk to a very low level. [26,27]

Debris and volunteers Debris from a previous crop of wheat left on the surface will increase the risk of tan spot as a source of inoculum for both ascospores and condiospores. [26,27]

Nitrogen level No information available

Nitrogen strategy No information available

Crop density No information available

Landscape No information available

Soil type No information available

Weather Weather conditions, which stimulate the breakdown of debris will help to reduce the inoculum. Warm and humid summers stimulate disease development. Optimum temperatures are between 20-28°C accompanied by long periods of dew or rain ( 18 hours or more)

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.

 

Fusarium head blight (Fusarium spp)

 HGCA photos


Resistance genes Varieties with good resistance are known, and may help to reduce disease levels. Several non-specific genes are used and described e.g. Fhb1 from Chinese spring wheat. Different types of resistance are described: Resistance to initial infection (type I), resistance to pathogen (type II), ability to degrade mycotoxins (type III and IV), or resistance to grain infection (type V). Tall cultivars are often seen to be less susceptible (longer distance for inoculum to spread). Compact heads are known to increase the risk of attack. Open flowering increase the risk of infection. [1,7,8,22,32,36,41]

Previous crop Maize as previous crop has been found to increase the risk of fusarium head blight. Wheat has also been found to potentially increase the risk in some regions. [14,36]

Sowing date Not found to be of specific importance

Tillage Ploughing decreases the risk by removing inoculum. Minimal tillage significantly increases the risk when wheat follows maize or wheat. [3,31]

Debris and volunteers Crop debris on the surface increases the risk of disease development. [3,27,36,39,42]

Nitrogen level No information available

Nitrogen strategy No information available

Crop density No information available

Landscape No information available

Soil type No information available

Weather Wet and humid conditions during heading and flowering stimulate attack (GS 51-69). [36,42]

Fusarium

 

Take-all (Gaeumannomyces graminis var. tritici ) HGCA photos


Resistance genes There are no varieties with specific resistance genes. Different wheat varieties have been found to build up different amounts of take-all inoculum in the soil, when grown as first cereal crop. [17]

Previous crop The disease is usually most severe in second, third or fourth successive cereal crops, but generally declines in importance in continuous cereals. Oats and broad leaved crops like oilseed rape as the previous crop will reduce the risk of take all. [13,17]

Sowing date Early sowing is known to increase disease risk. Late sowing is seen to decrease the disease level as the epidemic is delayed. When wheat is sown after wheat it is recommended to delay the sowing time to minimize the risk. A crop sown in ideal conditions is better than one where soil structure is poor. [9,19,37]

Tillage Tillage is found sometimes to have a major impact on the disease development. Increased levels are sometimes seen following ploughing compared with non-inversion tillage, but sometimes the opposite can take place. It relates to factors like soil compaction, water content, etc. Light puffy seedbeds can encourage the development of the disease. In short sequences of cereals, ploughing generally has an advantage. [13,17]

Debris and volunteers Debris from a previous crop of wheat left in the field will increase the risk. Cereal volunteers and grasses can be carriers of the disease and e.g. make oil seed rape less effective as a break crop. [18]

Nitrogen level Reduced levels of N can increase the risk of attack as the crop has limited sources to develop root systems. Ammonium sulphate consistently has given less disease compared with ammonium nitrate, urea and ammonium chloride fertilisers. [19]

Nitrogen strategy Early applications of N in February/March, followed by the main dressing in April will help to reduce the severity on the roots. [17]

Crop density No information available

Landscape No information available

Soil type Take all causes most damage on light soils (Sand, Sandy loams and loams), particularly if they are alkaline in nature. Crops grown on more sandy soils are more likely to develop take all as plants are more likely to suffer from drought stress. Poor drainage increase risk. [17]

Weather Weather conditions which stimulate disease development is warm and moist autumns and winters. Wet springs and dry summers.

Take-all

 
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Revised 09.06.2017