Take-all wheat disease
Canterbury is home to some 90% of the countrys arable farmers. Take-all is a disease which can, in a bad year, lead to crop losses of 20% to100%. Crop rotation is used to reduce the chance of this disease. Government funding has provided the resources for fundamental research into crop rotations for soil borne diseases and the management of crops to minimise disease risks.
A MAF Sustainable Farming Fund (SFF) grant supported by the Foundation for Arable Research (FAR) has built on that research over the last three years, with the evaluation of a DNA test which will tell farmers what risk they have of take-all if they plant wheat. The information will help farmers plan crop rotations in a more sustainable way, both environmentally and economically. If they know, for example, that soil tests show there is little risk of take-all developing, they can take advantage of favourable wheat prices.
The DNA test is Australian and has led to good trans-Tasman collaboration. But the research that has been undertaken with the test in New Zealand is regarded as world leading in ecological research in soil borne diseases. The data from the research is being used to assist further research in the UK and Australia.
The term Take-all was first used in Australia more than 100 years ago to describe a severe seedling blight which destroyed entire stands of wheat. In the US, the Take-all disease is known as a root and crown rot. It generally occurs sporadically in wheat fields planted continuously to wheat or in fields infested with perennial grass weeds. The damage to wheat is related to the extent of root and basal stem colonisation by a soil-borne fungus. Wheat plants can withstand mild infections with only minimal yield loss. Mild infections do not result in obvious symptoms of the disease.
However, when symptoms become obvious, yield losses can be as high as 100%.
The most conspicuous field symptom of Take-all is the occurrence of stunted, yellow plants in circular patches during early stages of growth. Occasionally, dead plants can be found scattered throughout the stand. The roots of diseased plants are rotted and break when plants are pulled from the soil. Yield loss is attributed to the premature death of plants soon after heading and before grain filling. Prematurely killed plants produce bleached or whitened, empty heads. These white heads stand out in contrast to the normal green colour of the crop. White heads are usually empty of grain, but may contain only a few shrivelled kernels. During wet weather, the dead plants become covered with saprophytic fungi which mask the bleached white heads by turning them blackish in colour. Since several other soil-borne fungi and stem feeding insects can cause the white head symptom, positive diagnosis is made by examining the roots and lower stems of affected plants. When plants with white heads are pulled from the soil and the lower leaf sheaths are peeled off to expose the stem base, the stem base and roots near the crown are usually blackened due to the growth of the Take-all fungus.
The fungus causing Take-all is Gaeumannomyces graminis var. tritici. This fungus persists from year to year in infested wheat residues, and in infected volunteer wheat and weed grasses. Wheat plants become infected when their roots contact infested residues or living plants harbouring the fungus. The fungus spreads from residues to the root surface and from one root to the next by growth of "runner hyphae" through the soil.
Take-all is usually more severe in lighter soils, with higher pH, and low fertility. Generally, the earlier infection takes place, the more disease and the greater the yield loss at harvest. Thus, infections of young plants soon after planting result in the most severe yield losses. Moisture retaining, poorly drained soils or abnormally wet weather, especially in the second half of the growing season, favours the development of the disease. Take-all is reduced by maintaining adequate levels of N, P and K for crop growth. Crop rotation is the most practical, economical and successful means of control of Take-all.
The new DNA test is done on the soil, measuring the level of Take-all fungus in the soil before sowing, and this is used to predict the level of Take-all disease if a wheat crop is sown. For instance, in paddocks which have been used to grow barley for several years, fungus may be present. Barley is more resistant than wheat to Take-all, but can be infected, leading to increased levels of the fungus in the soil.
Take-all disease of wheat also shows a pattern known as Take-all decline, which describes the disease history of the same paddock sown year after year to wheat.
The first wheat crop usually has low levels of the disease, which rises in subsequent years. Eventually, naturally occurring micro-organisms that are antagonistic to the Take-all fungus also increase and the disease levels decline. Overseas, it is generally stated that disease levels increase until about the fourth consecutive wheat crop, then declines to stable levels, at which yield losses are not too high.
Matthew Cromey says he doesnt know whether Take-all decline will occur in continuous barley cropping, as against wheat. So we tested the soils in three paddocks where barley had been grown for about 20 years. Fungus inoculum levels were low in one paddock and medium in the others, which suggests that the inoculum has stablisied at a reasonably low level. Wheat test strips were then sowed in the paddocks. Take-all levels were very low, although the crops were sown in late spring, due to a very wet winter, and Take-all levels would normally be more severe in autumn-sown or early spring-sown wheat crops. Because of the late sowing we cannot be too strong in our conclusions, but it does look as though there is a level of natural biological control in the paddocks, which is keeping inoculum levels down and reducing Take-all disease levels in the crop.
Informtation from Matthew Cromey
Plant Pathologist
Crop & Food Research
The test
The soil is sampled prior to sowing (obviously, because the grower will use the result to aid in sowing decisions). Soil is sampled in a W pattern. Fifty cores per field are taken. Too few samples increases the sampling error by missing infected areas, or a few hot spots contributing too much to the average. Too many samples is too time-consuming. Fifty samples has been shown to be optimum. Samples are dried down and sent to Australia (the South Australian Research and Development Institute SARDI in Adelaide). The test was developed by SARDI and CSIRO, and it is marketed by Bayer as the Predicta-B test. We are now entering the implementation phase of the project in NZ and Bayer will make a decision in a year or two whether it is more efficient to continue testing in Australia, or licence it in NZ. The test is a quantitative PCR (DNA) test which measures the quantity of the take-all fungus in the soil. Risk thresholds have been established in Australia (categories are not detected, low, moderate, or high risk). The purpose of the first MAF SFF project was to evaluate the test under NZ conditions, and set appropriate risk thresholds for NZ. The test has shown promise for predicting take-all risk, so we are about to begin the implementation phase, where MAF SFF funds will be used to provide technical back-up during the tests implementation.
The take-all test only determines the level of the take-all fungus in the soil. We have shown that there is a correlation between these measurements and actual take-all levels in the wheat crops sown after the test. The test determines the risk of take-all, but there are other factors (soil type and management, crop and crop residue management, weeds, rotations, the climatic conditions in a particular growing season, etc) that will affect how much disease actually occurs. We have a related FRST project that is investigating the factors that influence disease development.
How big a problem and why not other controls
Take-all is an important disease of wheat in NZ and in other parts of the world. The biggest issue is that it can virtually destroy a crop which can have major consequences for a grower. It can also cause substantial losses in less-affected crops. The disease may not show itself until most of the costs have been spent, and so may result in a financial loss on the crop. Many growers avoid growing successive wheat crops because of the worry of take-all. This is reasonably effective, but reduces cropping options for growers.
Why is it such a big issue? Well, most arable crop diseases are controlled by a combination of disease resistance and strategic use of fungicides. Neither of these are available for adequate take-all control. There are no resistant cultivars. All can suffer severe take-all. Fungicides are almost always translocated upward in a plant. This means that seed treatments do not hang around in the root zones, and foliar fungicides never get there. A few fungicides may provide a limited degree of control but are not sufficiently effective where there is severe disease pressure.
Crop rotation is the usual means of control. However, grass weeds such a twitch (especially where herbicides are used) and wheat volunteers in a break crop can reduce the effectiveness of a break crop. The fungus survives in infected crop residues, but the presence of alternative hosts allows it to carry over. Overseas work has shown a phenomenon known as take-all decline. This is natural biological control that develops with continuous wheat cropping. We are investigating this in NZ, but have certainly found that even fourth year wheats can have severe take-all. The idea is that take-all will increase in severity over the first few years, then drop down to manageable levels, as long as wheat is grown. A year in another crop and take-all decline is lost. We are monitoring one field that has been in wheat for over 8 years, and, while it gets some take-all, still manages to yield well. We think this may be due to take-all decline, and are using it as a research site.
How would growers use the test?
1. to monitor how sustainable their rotations are. For instance, one of the growers in our group never growers wheat twice in a row. FAR (with MAF SFF funding) did a crop rotation trial on this property. One treatment was wheat after barley. This treatment had severe take-all and yielded less than half of other treatments. Rotations on this intensive arable growers property are allowing high populations of the take-all fungus to remain.
2. to select field where growers would like to grow a second wheat. Our research has shown that less than half of second wheats are at a moderate to high risk of take-all. The soil test would enable growers to grow second wheats in the least risky fields.
3. to see if grass or cereal weeds in a previous crop have enabled inoculum to be carried over. The presence of grasses and weeds does not necessarily lead to high take-all risk, but it can. The test will take out the uncertainty.
4. to decide on crop management options where there is some risk of take-all. Growers can try to manage crops to reduce the risk of take-all. For instance, later sowing will help (giving infected residues longer to break down), and seed treatments and foliar sprays may help with the disease pressure is not too high.
A MAF Sustainable Farming Fund (SFF) grant supported by the Foundation for Arable Research (FAR) has built on that research over the last three years, with the evaluation of a DNA test which will tell farmers what risk they have of take-all if they plant wheat. The information will help farmers plan crop rotations in a more sustainable way, both environmentally and economically. If they know, for example, that soil tests show there is little risk of take-all developing, they can take advantage of favourable wheat prices.
The DNA test is Australian and has led to good trans-Tasman collaboration. But the research that has been undertaken with the test in New Zealand is regarded as world leading in ecological research in soil borne diseases. The data from the research is being used to assist further research in the UK and Australia.
The term Take-all was first used in Australia more than 100 years ago to describe a severe seedling blight which destroyed entire stands of wheat. In the US, the Take-all disease is known as a root and crown rot. It generally occurs sporadically in wheat fields planted continuously to wheat or in fields infested with perennial grass weeds. The damage to wheat is related to the extent of root and basal stem colonisation by a soil-borne fungus. Wheat plants can withstand mild infections with only minimal yield loss. Mild infections do not result in obvious symptoms of the disease.
However, when symptoms become obvious, yield losses can be as high as 100%.
The most conspicuous field symptom of Take-all is the occurrence of stunted, yellow plants in circular patches during early stages of growth. Occasionally, dead plants can be found scattered throughout the stand. The roots of diseased plants are rotted and break when plants are pulled from the soil. Yield loss is attributed to the premature death of plants soon after heading and before grain filling. Prematurely killed plants produce bleached or whitened, empty heads. These white heads stand out in contrast to the normal green colour of the crop. White heads are usually empty of grain, but may contain only a few shrivelled kernels. During wet weather, the dead plants become covered with saprophytic fungi which mask the bleached white heads by turning them blackish in colour. Since several other soil-borne fungi and stem feeding insects can cause the white head symptom, positive diagnosis is made by examining the roots and lower stems of affected plants. When plants with white heads are pulled from the soil and the lower leaf sheaths are peeled off to expose the stem base, the stem base and roots near the crown are usually blackened due to the growth of the Take-all fungus.
The fungus causing Take-all is Gaeumannomyces graminis var. tritici. This fungus persists from year to year in infested wheat residues, and in infected volunteer wheat and weed grasses. Wheat plants become infected when their roots contact infested residues or living plants harbouring the fungus. The fungus spreads from residues to the root surface and from one root to the next by growth of "runner hyphae" through the soil.
Take-all is usually more severe in lighter soils, with higher pH, and low fertility. Generally, the earlier infection takes place, the more disease and the greater the yield loss at harvest. Thus, infections of young plants soon after planting result in the most severe yield losses. Moisture retaining, poorly drained soils or abnormally wet weather, especially in the second half of the growing season, favours the development of the disease. Take-all is reduced by maintaining adequate levels of N, P and K for crop growth. Crop rotation is the most practical, economical and successful means of control of Take-all.
The new DNA test is done on the soil, measuring the level of Take-all fungus in the soil before sowing, and this is used to predict the level of Take-all disease if a wheat crop is sown. For instance, in paddocks which have been used to grow barley for several years, fungus may be present. Barley is more resistant than wheat to Take-all, but can be infected, leading to increased levels of the fungus in the soil.
Take-all disease of wheat also shows a pattern known as Take-all decline, which describes the disease history of the same paddock sown year after year to wheat.
The first wheat crop usually has low levels of the disease, which rises in subsequent years. Eventually, naturally occurring micro-organisms that are antagonistic to the Take-all fungus also increase and the disease levels decline. Overseas, it is generally stated that disease levels increase until about the fourth consecutive wheat crop, then declines to stable levels, at which yield losses are not too high.
Matthew Cromey says he doesnt know whether Take-all decline will occur in continuous barley cropping, as against wheat. So we tested the soils in three paddocks where barley had been grown for about 20 years. Fungus inoculum levels were low in one paddock and medium in the others, which suggests that the inoculum has stablisied at a reasonably low level. Wheat test strips were then sowed in the paddocks. Take-all levels were very low, although the crops were sown in late spring, due to a very wet winter, and Take-all levels would normally be more severe in autumn-sown or early spring-sown wheat crops. Because of the late sowing we cannot be too strong in our conclusions, but it does look as though there is a level of natural biological control in the paddocks, which is keeping inoculum levels down and reducing Take-all disease levels in the crop.
Informtation from Matthew Cromey
Plant Pathologist
Crop & Food Research
The test
The soil is sampled prior to sowing (obviously, because the grower will use the result to aid in sowing decisions). Soil is sampled in a W pattern. Fifty cores per field are taken. Too few samples increases the sampling error by missing infected areas, or a few hot spots contributing too much to the average. Too many samples is too time-consuming. Fifty samples has been shown to be optimum. Samples are dried down and sent to Australia (the South Australian Research and Development Institute SARDI in Adelaide). The test was developed by SARDI and CSIRO, and it is marketed by Bayer as the Predicta-B test. We are now entering the implementation phase of the project in NZ and Bayer will make a decision in a year or two whether it is more efficient to continue testing in Australia, or licence it in NZ. The test is a quantitative PCR (DNA) test which measures the quantity of the take-all fungus in the soil. Risk thresholds have been established in Australia (categories are not detected, low, moderate, or high risk). The purpose of the first MAF SFF project was to evaluate the test under NZ conditions, and set appropriate risk thresholds for NZ. The test has shown promise for predicting take-all risk, so we are about to begin the implementation phase, where MAF SFF funds will be used to provide technical back-up during the tests implementation.
The take-all test only determines the level of the take-all fungus in the soil. We have shown that there is a correlation between these measurements and actual take-all levels in the wheat crops sown after the test. The test determines the risk of take-all, but there are other factors (soil type and management, crop and crop residue management, weeds, rotations, the climatic conditions in a particular growing season, etc) that will affect how much disease actually occurs. We have a related FRST project that is investigating the factors that influence disease development.
How big a problem and why not other controls
Take-all is an important disease of wheat in NZ and in other parts of the world. The biggest issue is that it can virtually destroy a crop which can have major consequences for a grower. It can also cause substantial losses in less-affected crops. The disease may not show itself until most of the costs have been spent, and so may result in a financial loss on the crop. Many growers avoid growing successive wheat crops because of the worry of take-all. This is reasonably effective, but reduces cropping options for growers.
Why is it such a big issue? Well, most arable crop diseases are controlled by a combination of disease resistance and strategic use of fungicides. Neither of these are available for adequate take-all control. There are no resistant cultivars. All can suffer severe take-all. Fungicides are almost always translocated upward in a plant. This means that seed treatments do not hang around in the root zones, and foliar fungicides never get there. A few fungicides may provide a limited degree of control but are not sufficiently effective where there is severe disease pressure.
Crop rotation is the usual means of control. However, grass weeds such a twitch (especially where herbicides are used) and wheat volunteers in a break crop can reduce the effectiveness of a break crop. The fungus survives in infected crop residues, but the presence of alternative hosts allows it to carry over. Overseas work has shown a phenomenon known as take-all decline. This is natural biological control that develops with continuous wheat cropping. We are investigating this in NZ, but have certainly found that even fourth year wheats can have severe take-all. The idea is that take-all will increase in severity over the first few years, then drop down to manageable levels, as long as wheat is grown. A year in another crop and take-all decline is lost. We are monitoring one field that has been in wheat for over 8 years, and, while it gets some take-all, still manages to yield well. We think this may be due to take-all decline, and are using it as a research site.
How would growers use the test?
1. to monitor how sustainable their rotations are. For instance, one of the growers in our group never growers wheat twice in a row. FAR (with MAF SFF funding) did a crop rotation trial on this property. One treatment was wheat after barley. This treatment had severe take-all and yielded less than half of other treatments. Rotations on this intensive arable growers property are allowing high populations of the take-all fungus to remain.
2. to select field where growers would like to grow a second wheat. Our research has shown that less than half of second wheats are at a moderate to high risk of take-all. The soil test would enable growers to grow second wheats in the least risky fields.
3. to see if grass or cereal weeds in a previous crop have enabled inoculum to be carried over. The presence of grasses and weeds does not necessarily lead to high take-all risk, but it can. The test will take out the uncertainty.
4. to decide on crop management options where there is some risk of take-all. Growers can try to manage crops to reduce the risk of take-all. For instance, later sowing will help (giving infected residues longer to break down), and seed treatments and foliar sprays may help with the disease pressure is not too high.