Gaeumannomyces tritici and G.avenae
- G. tritici attacks wheat, barley, rye and may attack some grass species, particularly couch grass. Oats are immune.
- G. avenae attacks oats, wheat, barley, rye and many grass species. This strain is rare.
Take-all attacks the roots of plants. However, it can infect plants at a low level without causing obvious symptoms. Moderate or severe infections cause roots to become blackened, rotten and have a ‘rat-tail’ appearance. In severe outbreaks, the base of plants may also blacken. The reduction in root activity restricts water and nutrient uptake. This slows canopy growth and causes yellowing and stunting in severe cases. Patches of stunted plants and whiteheads (bleached ears) form, which are usually first seen during grain filling. Generally, whiteheads contain small and shrivelled grains or, occasionally, no grain at all.
The take-all fungus is soilborne, with primary infection occurring in the autumn from inoculum in the soil, and from mycelium on infected roots and cereal debris. Secondary infection, where the disease spreads directly from root to root, usually occurs in the spring and summer. The disease then spreads from infected seedling roots to developing crown roots (where the roots and stem meet). As the disease progresses during the season, the root area lost to the disease increases and the ability of the plant to absorb water and nutrients from the soil declines. When root rotting is severe, plants are unable to absorb water and nutrients. As a result, the plants ripen prematurely, resulting in whiteheads and poor grain filling.
The disease survives the winter primarily as mycelium on infected roots or stubble debris, and can then spread to volunteer cereals, early autumn-sown crops and some grass weeds.
More secondary infection occurs when soils are warm and moist. Take-all development is encouraged by a warm winter followed by a wet spring/early summer. The pathogen is active in the soil when temperatures are above 10–12˚C.
Take-all is an important disease of UK winter wheat, especially as control with fungicides or varietal resistance is extremely limited. The disease is moderately important in winter barley. Spring sown cereals are not as badly affected.
The fungus is ubiquitous in UK arable land and reduces yield and grain quality. Estimates suggest the disease affects half of UK wheat crops, with average yield losses of 5–20%. However, more than half of the crop can be lost when disease is severe.
Rotational strategies are essential for control. Take-all is the major cause of ‘second wheat syndrome’ when yields of second wheat crops are frequently 10–15% less than those of first wheats. Take-all is usually most severe in the second to fourth successive cereal crop but yields generally recover, to some extent, in continuous cereals – ‘take-all decline’ (see below).
Severity depends on weather and other factors, such as soil type. Take-all causes most damage on light soils (sand, sandy loams and loams), where the fungus spreads more easily and the loss of active roots has a large effect on water and nutrient uptake. Take-all tends to be worse on chalky downland soils that are more drought prone. Whereas, well-structured, water-retentive soils, such as chalky boulder clays, result in less damage. However, even on these chalky boulder clay soils, losses of 10–20% are common in second and third wheat crops. On less well-bodied soils, yield losses can be much higher. It may even become uneconomic to grow second or subsequent wheat. However, risk can differ even within a soil series. For example, among chalky boulder clays, crops on the Ragdale series are more prone to take-all than those on the Hanslope series. Therefore, local knowledge is important when assessing risk.
Poor drainage and nutrient status also encourage the disease. Take-all is particularly encouraged by early sowing and light, puffy seedbeds. The effect of soil pH on take-all is complex, but infection in alkaline, light, soils tend to be most damaging. However, severe attacks can also occur in acid patches.
The weather also has a large influence on the impact of take-all. Dry conditions during grain filling exacerbate the damaging effects of the disease on root function.
Effect of soil moisture on yield in a severe take-all epidemic
A first wheat crop usually appears to remain healthy. However, moist soil builds up inoculum that impacts following crops. Favourable weather for disease development in successive years leads to severe 'take-all years'. More rarely, unfavourable weather over several years can result in crops in which take-all is almost absent.
Take-all is most prevalent in Eastern England, where cereal-dominated rotations are most common. Severe infections also occur in the West and South-West, but wetter weather during grain filling means the effects are often less severe.
Take-all decline occurs because microbial organisms antagonistic to the take-all fungus build up in soil, reducing the rate of secondary spread. If successive cereal crops are grown, take-all will generally build up during the second or third years. However, take-all will almost invariably decline during subsequent years, until it reaches a stable position when yields are acceptable. The yields of long-term cereals, where take-all decline is established, are never as high as those of first wheats but may allow the crop to continue to be grown economically.
Take-all decline can develop in successive crops of wheat, barley or triticale. The disease becomes most severe in winter wheat, and the consequent take-all decline is robust, conferring protection on wheat and the less susceptible cereals. However, where decline occurs under such crops, it will not protect winter wheat. Therefore, growing wheat after a barley (or triticale) crop often leads to severe take-all.
Effective take-all decline, established after a long run of susceptible crops, will not be completely lost after a one-year break or fallow. Longer or frequently repeated breaks will result in the complete loss of take-all decline.
The above figure illustrates how take-all develops in successive wheat crops. It shows primary and secondary phases of infection, as well as relative yields. Crop debris, which contains inoculum, starts to decay soon after harvest. The rate of decay is an important determinant of amounts of disease in second and subsequent crops.
Wheat 1 (first wheat)
Little inoculum in soil after a break. This results in few primary infections to initiate secondary infection. Thus, there is likely to be little disease during grain filling. However, high levels of take-all inoculum may be left in the soil.
Wheat 2 (second wheat)
Residual inoculum leads to more primary and, hence, secondary infection. Second wheats are more severely diseased that first wheats and inoculum builds
Wheat 3 (third wheat)
After two susceptible wheat crops, amounts of soil inoculum are higher than in wheat 2. This ensures more primary infection, secondary infection and inoculum.
Wheat 4 (continuous wheat)
The disease becomes less severe because take-all decline reduces the rate of secondary infection. The timing of decline is variable and may not appear until the fifth crop.
- Second, third or fourth cereal crop
- Light or alkaline soils
- Warm winter and wet spring/early summer, which encourages take-all development
- Dry summer, which exacerbates take-all symptoms and increases yield loss
- Poor drainage, low nutrient status, early sowing and light, puffy seedbeds
- Cereal volunteers and grass weeds, especially couch and bromes, in break crops that carry the disease through
Complete control of the disease is not possible. Manage disease severity and impact through cultural control approaches.
A one-year break from susceptible crops will usually prevent the disease causing problems in the subsequent cereal crop. Any broad-leaved crop will form an effective break, as will oats, unless the rare oat-attacking strain of the fungus is present. However, a one-year break does not eliminate the fungus. Where the preceding cereal crop suffered a lot of disease, significant inoculum may persist through the break, allowing infection of a susceptible crop. Cereal volunteers and grass carriers also undermine the effectiveness of a break.
Preceding a short sequence of cereals with a two-year ryegrass ley encourages antagonistic fungi to develop. This usually delays the onset of severe take-all by a year, allowing an extra wheat crop to be grown with reduced risk of severe take-all. However, a one-year ryegrass cover may increase severity. A catch crop of a nitrogen-demanding crop (e.g. white mustard), sown after wheat volunteers have been destroyed, may further reduce take-all survival
A firm seedbed discourages fungal growth. However, on heavy soils, over-compaction can hinder root growth and exacerbate the damaging effects of the fungus on root function. Ploughing buries most of the take-all inoculum, which is in the top 10 cm of soil at harvest, and brings less infective soil to the surface. This reduces early infection and gives the plant time to establish before its roots reach more infective soil. However, consolidation after ploughing is important to achieve a firm seedbed. Minimum tillage of first wheat stubble leaves highly infective soil near the surface. This allows more rapid infection of a second wheat but leaves a firmer seedbed than ploughing. Tillage method does not consistently affect take-all in longer cereal sequences.
Volunteer cereals and grass weeds
Cereal volunteers (not oats in most of the UK) and some grassweeds, particularly couch and barren brome, carry the take-all fungus through break crops. In first wheats, the risk of take-all increases in proportion to the density of volunteers or weeds. Early destruction of volunteers and grass weeds reduces the risk to first winter wheats. Take-all may, however, survive or even multiply on couch rhizomes after foliage has been killed. Take-all decline, like the fungus itself, diminishes during a break from wheat.
All varieties are susceptible to take-all. However, some are more tolerant. Although not tested in the AHDB Recommended List, second wheat yields can provide some guidance on varieties for second and subsequent sowings. However, yield responses reflect take-all tolerance, eyespot resistance and ability to withstand the stress of root loss associated with the disease. There is evidence of good levels of resistance in some triticale varieties.
Take-all inoculum falls relatively rapidly after harvest. Consequently, delayed drilling reduces the disease in second wheats. However, any benefit from later sowing may be lost if volunteers or susceptible grasses are allowed to grow between crops. Earlier drilling of first wheats increases the risk that damaging take-all will develop in second wheats.
Climatic conditions, before, during and after sowing, affect risk. Soil temperature during the autumn is particularly influential. The pathogen is active when soil temperatures are above 10–12˚C. Ideally, delay sowings of second wheats until soil temperatures are at or below 12˚C. Drilling dates for continuous cereals should be the same as for second wheats. Early drilled crops are particularly prone to severe attacks, so sow first wheat crops first, followed by long-term wheat and third wheat last.
|Drilling date||6 Oct||18 Oct||31 Oct||13 Nov|
|Final take-all index||52.5||46.0||44.0||21.1|
The effect of drilling date on take-all severity and yield of a second wheat
Take-all can develop more quickly and become more severe at higher seed rates. This allows more primary and secondary infection, because of greater root density. Reducing seed rates can increase yield where take-all is severe. This may not be practical with later-sown at-risk crops.
Take-all destroys roots, reducing a crop's ability to take up nitrogen and other nutrients. Timing of nitrogen applications is important. Apply nitrogen earlier to crops at risk of take-all. An early application (around 60kg N/ha) in February/March followed by the main dressing in April is better than a single dressing or later applications. Because uptake by crops with severe take-all is less efficient, the disease may increase leaching losses. Take-all may be less severe with ammonium sulphate than with ammonium nitrate, urea or ammonium chloride fertilisers. However, in well-buffered UK soils, the effect is small.
Phosphate-deficient soils (less than 15mg/kg of soil – Index 1 or less) favour take-all. Phosphate should be at 20mg/kg soil (Index 2) before the start of a cereal sequence and any deficiency rectified, ideally before the break crop, to reduce take-all risk in the second cereal.
Manganese, potash and sulphur deficiencies are linked to increased take-all severity. Correct manganese and potash deficiencies before sowing. However, severe take-all sometimes occurs even when supplies of these nutrients are adequate.
No foliar applied fungicides control take-all . Seed treatments only provide partial control:
Wheat 1 (first wheat)
Only treat if there is a known risk (e.g. many cereal volunteers or couch and barren brome weed pressures).
Wheat 2 (second wheat)
Only treat if there is a known risk (e.g. take-all regularly occurs on the farm or the crop is drilled very early).
Wheat 3 (third wheat)
Only grow a third wheat if the second wheat appeared healthy during grain filling and yielded well. Where a third wheat is grown, use a seed treatment. However, expect reduced yields.
Wheat 4 (continuous wheat)
Treat at-risk crops. However, treatment may delay the take-all decline phenomenon.
- Extending the rotation is the most effective way to manage take-all
- Seed treatments only give partial control
- Provide good drainage, consolidated seedbeds and adequate nutrition, especially where second, third, or fourth wheats are grown
- Continuous wheats are not affected so much because of take-all decline. However, yields never return to those of first wheats
- Control cereal volunteers and grass weeds in break crops and avoid a green bridge between successive cereal crops
- Avoid very early drilling of first and second wheats. Aim to drill second and subsequent wheats after mid-October
- Varietal resistance and foliar fungicides offer little control