Variation in mycorrhizal growth response among a spring wheat mapping population shows potential to breed for symbiotic benefit.

All cereal crops engage in arbuscular mycorrhizal symbioses which can have profound, but sometimes deleterious, effects on plant nutrient acquisition and growth. The mechanisms underlying variable mycorrhizal responsiveness in cereals are not well characterised or understood. Adapting crops to realise mycorrhizal benefits could reduce fertiliser requirements and improve crop nutrition where fertiliser is unavailable.

Determinants of optimal insecticide resistance management strategies.

The use of insecticides to control agricultural pests has resulted in resistance developing to most known insecticidal modes of action. Strategies by which resistance can be slowed are necessary to prolong the effectiveness of the remaining modes of action. Here we use a flexible mathematical model of resistance evolution to compare four insecticide application strategies: (i) applying one insecticide until failure, then switching to a second insecticide (sequential application), (ii) mixing two insecticides at their full label doses, (iii) rotating (alternating) two insecticides at full label dose, or (iv) mixing two insecticides at a reduced dose (with each mixture component at half the full label dose).

Extending the durability of cultivar resistance by limiting epidemic growth rates.

Cultivar resistance is an essential part of disease control programmes in many agricultural systems. The use of resistant cultivars applies a selection pressure on pathogen populations for the evolution of virulence, resulting in loss of disease control. Various techniques for the deployment of host resistance genes have been proposed to reduce the selection for virulence, but these are often difficult to apply in practice.

Changes in field dose-response curves for demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides against Zymoseptoria tritici, related to laboratory sensitivity phenotyping and genotyping assays.

Background: Insensitivity of Zymoseptoria tritici to demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides has been widely reported from laboratory studies, but the relationships between laboratory sensitivity phenotype or target site genotype and field efficacy remain uncertain. This article reports field experiments quantifying dose-response curves, and investigates the relationships between field performance and in vitro half maximal effective concentration (EC ) values for DMIs, and the frequency of the G143A substitution conferring QoI resistance.

Results: Data were analysed from 83 field experiments over 21 years.

Physiological Traits Determining Yield Tolerance of Wheat to Foliar Diseases.

Tolerance is defined as the ability of one cultivar to yield more than another cultivar under similar disease severity. If both cultivars suffer an equal loss in healthy (green) leaf area duration (HAD) over the grain filling period due to disease presence, then the yield loss per unit HAD loss is smaller for a more tolerant cultivar. Little is understood of what physiological and developmental traits of cultivars determine disease tolerance.

Identifying circumstances under which high insecticide dose increases or decreases resistance selection.

Insect management strategies for agricultural crop pests must reduce selection for insecticide resistant mutants while providing effective control of the insect pest. One management strategy that has long been advocated is the application of insecticides at the maximum permitted dose. This has been found, under some circumstances, to be able to prevent the resistance allele frequency from increasing.

Targeting Fungicide Inputs According to Need.

Fungicides should be used to the extent required to minimize economic costs of disease in a given field in a given season. The maximum number of treatments and maximum dose per treatment are set by fungicide manufacturers and regulators at a level that provides effective control under high disease pressure. Lower doses are economically optimal under low or moderate disease pressure, or where other control measures such as resistant cultivars constrain epidemics.

Emergence of Resistance to Fungicides: The Role of Fungicide Dose.

Resistance to antimicrobial drugs allows pathogens to survive drug treatment. The time taken for a new resistant mutant to reach a population size that is unlikely to die out by chance is called "emergence time." Prolonging emergence time would delay loss of control.

Dose and number of applications that maximize fungicide effective life exemplified by on wheat - a model analysis.

Two key decisions that need to be taken about a fungicide treatment programme are (i) the number of applications that should be used per crop growing season, and (ii) the dosage that should be used in each application. There are two opposing considerations, with control efficacy improved by a higher number of applications and higher dose, and resistance management improved by a lower number of applications and lower dose. Resistance management aims to prolong the effective life of the fungicide, defined as the time between its introduction onto the market for use on the target pathogen, and the moment when effective control is lost due to a build-up of fungicide resistance.

The Evolution of Fungicide Resistance Resulting from Combinations of Foliar-Acting Systemic Seed Treatments and Foliar-Applied Fungicides: A Modeling Analysis.

For the treatment of foliar diseases of cereals, fungicides may be applied as foliar sprays or systemic seed treatments which are translocated to leaves. Little research has been done to assess the resistance risks associated with foliar-acting systemic seed treatments when used alone or in combination with foliar sprays, even though both types of treatment may share the same mode of action. It is therefore unknown to what extent adding a systemic seed treatment to a foliar spray programme poses an additional resistance risk and whether in the presence of a seed treatment additional resistance management strategies (such as limiting the total number of treatments) are necessary to limit the evolution of fungicide-resistance.

Mixtures as a fungicide resistance management tactic.

We have reviewed the experimental and modeling evidence on the use of mixtures of fungicides of differing modes of action as a resistance management tactic. The evidence supports the following conclusions. 1.

Governing principles can guide fungicide-resistance management tactics.

Fungicide-resistance management would be more effective if principles governing the selection of resistant strains could be determined and validated. Such principles could then be used to predict whether a proposed change to a fungicide application program would decrease selection for resistant strains. In this review, we assess a governing principle that appears to have good predictive power.

Fungicide resistance risk assessment based on traits associated with the rate of pathogen evolution.

Background: A new fungicide resistance risk assessment method is described, based on traits (of pathogens, fungicides and agronomic systems) that are associated with rapid or slow occurrence of resistance. Candidate traits tested for their predictive value were those for which there was a mechanistic rationale that they could be determinants of the rate of resistance evolution.

Results: A dataset of 61 European cases of resistance against single-site-acting fungicides was assembled.

The emergence of resistance to fungicides.

Many studies exist about the selection phase of fungicide resistance evolution, where a resistant strain is present in a pathogen population and is differentially selected for by the application of fungicides. The emergence phase of the evolution of fungicide resistance--where the resistant strain is not present in the population and has to arise through mutation and subsequently invade the population--has not been studied to date. Here, we derive a model which describes the emergence of resistance in pathogen populations of crops.

EVIDENCE FOR REDUCED SEXUAL REPRODUCTION OF ZYMOSEPTORIA TRITICI FOLLOWING TREATMENT WITH FLUXAPYROXAD AND IMPLICATIONS FOR INITIAL INFECTION OF WHEAT CROPS.

Zymoseptoria tritici (previously Mycosphaerella graminicola) is the causal agent of septoria tritici leaf blotch (STB), a globally important fungal disease of bread, feed and durum wheat. Airborne ascospores originating from over-winter crop residues are considered to be the primary source of initial infection. The active ingredient fluxapyroxad (BASF) belongs to the chemical group of carboxamides and is a succinate dehydrogenase inhibitor (SDHI) fungicide.

Evaluation of a matrix to calculate fungicide resistance risk.

Background: In the European Union, assessments of resistance risk are required by the regulatory authorities for each fungicide product and are used to guide the extent of anti-resistance strategies. This paper reports an evaluation of a widely used 'risk matrix', to determine its predictive value. Sixty-seven unique cases of fungicide resistance in Europe were identified for testing the risk assessment scheme, where each case was the first occurrence of resistance in a pathogen species against a fungicide group.

Optimal fungicide application timings for disease control are also an effective anti-resistance strategy: a case study for Zymoseptoria tritici (Mycosphaerella graminicola) on wheat.

Strategies to slow fungicide resistance evolution often advocate early "prophylactic" fungicide application and avoidance of "curative" treatments where possible. There is little evidence to support such guidance. Fungicide applications are usually timed to maximize the efficiency of disease control during the yield-forming period.

Accounting for the economic risk caused by variation in disease severity in fungicide dose decisions, exemplified for Mycosphaerella graminicola on winter wheat.

A method is presented to calculate economic optimum fungicide doses accounting for the risk aversion of growers responding to variability in disease severity between crops. Simple dose-response and disease-yield loss functions are used to estimate net disease-related costs (fungicide cost plus disease-induced yield loss) as a function of dose and untreated severity. With fairly general assumptions about the shapes of the probability distribution of disease severity and the other functions involved, we show that a choice of fungicide dose which minimizes net costs, on average, across seasons results in occasional large net costs caused by inadequate control in high disease seasons.

The usefulness of fungicide mixtures and alternation for delaying the selection for resistance in populations of Mycosphaerella graminicola on winter wheat: a modeling analysis.

A fungicide resistance model (reported and tested previously) was amended to describe the development of resistance in Mycosphaerella graminicola populations in winter wheat (Triticum aestivum) crops in two sets of fields, connected by spore dispersal. The model was used to evaluate the usefulness of concurrent, alternating, or mixture use of two high-resistance-risk fungicides as resistance management strategies. We determined the effect on the usefulness of each strategy of (i) fitness costs of resistance, (ii) partial resistance to fungicides, (iii) differences in the dose-response curves and decay rates between fungicides, and (iv) different frequencies of the double-resistant strain at the start of a treatment strategy.

Quantifying the non-fungicidal effects of foliar applications of fluxapyroxad (Xemium) on stomatal conductance, water use efficiency and yield in winter wheat.

The active ingredient fluxapyroxad belongs to the chemical group of carboxamides and is a new generation succinate dehydrogenase inhibitor (SDHI) in complex II of the mitochondrial respiratory chain. It has strong efficacy against the key foliar diseases of winter wheat in the UK: Septoria leaf blotch, yellow stripe rust and brown rust. Fluxapyroxad is marketed under the brand name of Xemium, was launched in 2012 and is available in the UK as a solo product (Imtrex) for co-application with triazoles, in co-formulation with epoxiconazole (Adexar), or in a three way formulation with epoxiconazole and pyraclostrobin (Ceriax).

Foliar pathogenesis and plant water relations: a review.

As the world population grows, there is a pressing need to improve productivity from water use in irrigated and rain-fed agriculture. Foliar diseases have been reported to decrease crop water-use efficiency (WUE) substantially, yet the effects of plant pathogens are seldom considered when methods to improve WUE are debated. We review the effects of foliar pathogens on plant water relations and the consequences for WUE.

The evolution of plant pathogens in response to host resistance: factors affecting the gain from deployment of qualitative and quantitative resistance.

Disease resistance genes are valuable natural resources which should be deployed in a way which maximises the gain to crop productivity before they lose efficacy. Here we present a general epidemiological model for plant diseases, formulated to study the evolution of phenotypic traits of plant pathogens in response to host resistance. The model was used to analyse how the characteristics of the disease resistance, and the method of deployment, affect the size and duration of the gain.

Delaying selection for fungicide insensitivity by mixing fungicides at a low and high risk of resistance development: a modeling analysis.

This study used mathematical modeling to predict whether mixtures of a high-resistance-risk and a low-risk fungicide delay selection for resistance against the high-risk fungicide. We used the winter wheat and Mycosphaerella graminicola host-pathogen system as an example, with a quinone outside inhibitor fungicide as the high-risk and chlorothalonil as the low-risk fungicide. The usefulness of the mixing strategy was measured as the "effective life": the number of seasons that the disease-induced reduction of the integral of canopy green area index during the yield forming period could be kept <5%.

Blumeria graminis interactions with barley conditioned by different single R genes demonstrate a temporal and spatial relationship between stomatal dysfunction and cell death.

Hypersensitive response (HR) against Blumeria graminis f. sp. hordei infection in barley (Hordeum vulgare) was associated with stomata "lock-up" leading to increased leaf water conductance (g(l)).

Epidemiological analysis of take-all decline in winter wheat.

Take-all dynamics within crops differing in cropping history (the number of previous consecutive wheat crops) were analyzed using an epidemiological model to determine the processes affected during take-all decline. The model includes terms for primary infection, secondary infection, inoculum decay, and root growth. The average rates of root production did not vary with cropping history.