Project
title / Epidemiology of cereal stem base and ear blight pathogens
/ DEFRA
project code / CE0524

Department for Environment, Food and Rural Affairs CSG 15

Research and Development

Final Project Report

(Not to be used for LINK projects)

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Project title / Epidemiology of cereal stem base and ear blight pathogens
DEFRA project code / CE0524
Contractor organisation and location / Rothamsted Research, Harpenden, Herts. AL5 2JQ
Total DEFRA project costs / £ 310,543
Project start date / 01/04/99 / Project end date / 31/03/03
Executive summary (maximum 2 sides A4)
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CSG 15 (Rev. 6/02) 3

Project
title / Epidemiology of cereal stem base and ear blight pathogens
/ DEFRA
project code / CE0524

Cereal stem-base and ear blight diseases cause considerable losses in grain yield and quality (including production of mycotoxins in ear-blighted crops). The widespread use of fungicides to control these diseases represents a significant economic cost to the grower. Identifying the sources of inoculum of these diseases (e.g. in soil or in surface debris) and determining their relative importance will lead to improved disease management by eliminating or diminishing the effectiveness of that inoculum; this may be by, for example, better targeting of fungicides or more appropriate management of crop debris. Similarly, determining the relative importance of different dispersal routes from the inoculum source, e.g. as splash-dispersed conidia or wind-dispersed ascospores, will aid rational disease management. Crop debris, as well as being a potential source of inoculum (both as a consequence of previous infections and because it is a substrate for saprophytic colonisation by some pathogens), may contribute to suppression of disease, especially following non-inversion tillage. Identifying the conditions governing the suppressive effects of such debris and the mechanisms of suppression may allow it to be exploited in integrated disease management.

The aim of this project was, therefore, to gain a better understanding of the behaviour and relative importance of different inoculum sources, and methods of inoculum dispersal from those sources, of stem-base diseases of cereals, especially eyespot, and of ear blight caused by Fusarium species that contaminate grain with mycotoxins. This is strategic research, complementing the agronomy-based project at Rothamsted Research on interactions between cropping systems and soil-borne pathogens (CE0525), DEFRA-funded research on improving disease resistance in cereals (based at John Innes Centre) and more applied HGCA-funded projects or collaborations with industry to identify and reduce risks of mycotoxin contamination of grain. It addresses DEFRA’s overall objective to contribute to sustainable agriculture by developing an integrated approach to disease management.

Inoculum and inoculum sources of the group of ecologically related pathogenic fungi that cause stem-base and ear blight diseases in cereals were investigated. Although the primary inoculum is expected to originate from infected debris left from previous crops, either buried or on the surface, inoculum may also be available from debris outside the current crop, or from alternative, living plants within or outside the crop.

Despite the widespread availability of sexually produced ascospores of the eyespot fungus, T. yallundae, the evidence so far from this research is that asexual conidia are still likely to be the main inoculum for eyespot. Apothecia (the ascospore-producing structures) were found to develop on the grass, Holcus lanatus, but only in close proximity to wheat stubble on which apothecia had also developed. The greater importance of conidia as inoculum is supported by the continued prevalence of T. acuformis, which is not known to produce ascospores commonly. Recently reported reversals to predominance of T. yallundae in the UK and, particularly, France seem to be related more to the decline in the use of the selective fungicide, prochloraz, than to ascospore inoculum. Severe eyespot will develop where inoculum is available on fresh debris or old debris, ploughed up from the previous year. Fresh debris may be more infective, but it is likely that any differences will be lost in conditions very conducive to disease. Non-infested debris left on the surface, and possibly also cultivated-in, is likely to contribute to eyespot suppression. This is particularly the case where straw is chopped and incorporated (as is common practice nowadays) rather than baled and removed. This should be considered as an important component of eyespot management strategies.

Fusarium foot rot is likely to be less amenable to suppression by chopped straw because of the ability of F. culmorum to colonise the straw. It may be possible to remove inoculum by ploughing-in, but this remains to be proven in the case of this fungus. Although fusarium foot rot is not usually a damaging disease in the UK, knowledge of the best means to control it is important because of its role, demonstrated here, as an inoculum source for ear blight, which is potentially a much greater threat. Fungicidal control of fusarium foot rot is difficult because of its late development, beneath an extensive leaf canopy. The availability of such fungicides would perhaps contribute to ear blight control. Ear blight caused by Fusarium graminearum is an increasing threat to wheat and other cereals in the UK and, as elsewhere, was found to be associated particularly with maize crops. Knowledge of the behaviour of F. graminearum in UK conditions, leading to recommendations on cultivations and on safe separation (spatial and temporal) for growing wheat and maize in the same arable systems, are essential to counter the threat that this fungus now poses.

CSG 15 (Rev. 6/02) 3

Project
title / Epidemiology of cereal stem base and ear blight pathogens
/ DEFRA
project code / CE0524
Scientific report (maximum 20 sides A4)
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CSG 15 (Rev. 6/02) 3

Project
title / Epidemiology of cereal stem base and ear blight pathogens
/ DEFRA
project code / CE0524

Introduction

The aim of this project was to gain a better understanding of the behaviour and relative importance of different inoculum sources, and methods of inoculum dispersal from those sources, of stem-base diseases of cereals, especially eyespot, and of ear blight caused by Fusarium species that contaminate grain with mycotoxins. This is expected to lead to the identification of alternative, non-chemical procedures for control of these diseases by reducing or eliminating inoculum, or to more effective targeting of fungicides. The potential for using natural disease suppression was also investigated since it may contribute usefully to an integrated disease management strategy.

Objective 1. To determine the role and significance of ascospores as inoculum for eyespot

Methods

Preliminary and controlled environment experiments

Newly collected isolates of eyespot fungi were tested for mating compatibility by culturing on pieces of barley straw (method in Dyer et al., 1993). Only isolates of Tapesia yallundae, and not T. acuformis, produced apothecia in 2-3 months in some pairings. Pairings of these were set up continually to produce ascospores for experimental inoculum. Apothecia of T. yallundae, produced artificially by the barley straw method, were placed outdoors in close proximity to a Burkard spore trap to test for production and release of ascospores. Weather conditions were recorded.

In a series of controlled environment experiments, seedlings of wheat cv. Hereward grown in pots (11-cm-diameter), at four plants per pot with five replicate pots, were inoculated with either ascospores or asexual spores (conidia) of T. yallundae at 101 to 105 ml-1. In early experiments, a 4-mm disc of sterile filter paper, soaked in a suspension of spores at the required concentration, was placed between the coleoptile and first leaf sheath of each plant and held in place with Parafilm. In later experiments, a 3-cm-long plastic tube, internal diameter 6.5 mm, was placed over each emerging seedling and 0.5 ml of spore suspension in 0.01% Tween 80 solution was dropped into the tube (cf. Bateman & Taylor, 1976). The latter method was found to give the most consistent results. Experiments were performed simultaneously in growth cabinets maintained with day/night temperatures of 10/5oC, 15/10oC or 20/15oC. The plants were assessed for eyespot severity after 6-7 wks.

Field experiments

Artificially-produced ascospore and conidial inoculum sources were compared in a designed (randomised block) field experiment. The inoculum sources (barley straw pieces bearing apothecia and culture plates producing profuse conidia) were placed in a winter wheat crop in November 1999. Eyespot was assessed on plants along transects from the inoculation points, as well as from non-inoculated control points, in April and July 2000.

A further field experiment with randomised block design was established. Winter wheat, cv. Hereward, was grown as a first cereal in 2000/01 in plots 36 m2, separated by at least 12 m of oats. The plots were inoculated with pairs of compatible mating types or with single strains of T. yallundae, grown on oat-grain medium, or were left non-inoculated. After harvest and straw removal, the plots were left as standing stubble and the rest of the field was sown with winter wheat, cv. Hereward. The incidence of apothecia on the standing stubble was recorded during the winter. Eyespot was assessed on plants along transects in the winter wheat crop from the eastern side of each stubble plot (i.e. the side that is commonly downwind) in spring and summer. Eyespot fungi were isolated from the plants on agar and their relationships to the parent mating types will be determined by PCR fingerprinting as part of an additional (MSc) project in summer 2003.

Epidemic modelling

Newly collected and archived data from a long-term field experiment to study the effects of repeated use of eyespot-controlling fungicides on, and interactions with, populations of Tapesia spp. were accessed. Using these and controlled environment studies, disease progress following infection from conidia of both Tapesia spp. was modelled in relation to thermal time.

Apothecia on wild grasses

Wild grasses on farmland at Rothamsted were examined for apothecia of Tapesia spp. during the winters of 2002 and 2003.

Results and Discussion

Determining the viability of ascospores and infectivity in different conditions

In outdoor tests using artificially produced apothecia, most ascospore release was short-lived (1-2 days), apparently as the apothecia dried, with slight resurgences following rainfall.

In controlled environments, disease increased in severity at increasing concentrations of ascospores or conidia. Conidia usually caused more disease than ascospores at the same concentration, probably a consequence of the greater size of the former. Optimum temperature ranges (15-20oC day/10-15oC night) were similar for ascospores and conidia. Conidia and ascospores are produced naturally during the winter in similar temperature conditions, although the period of conidial production, since it can begin earlier and includes secondary production on growing plants, is more prolonged. A large proportion of eyespot in cereal crops results from infection by T. acuformis, which appears to produce apothecia and ascospores only rarely (Dyer et al., 2001). Since conidia are also slightly more infective than ascospores, it is likely that conidial inoculum makes the major contribution to disease. Ascospore production contributes to genetic recombination, particularly in T. yallundae, and adds to the disease inoculum in wet weather during the period of apothecial ripening. Ascospores, being smaller than conidia, may also increase the dispersal capability of the fungus (but see below).

Determining ascospore dispersal and infectivity from artificial inoculum sources in the field

Samples taken in April showed more eyespot from ascospore inoculum than from other sources within 1 m of the source; there was still considerable infection up to 5 m from the sources, suggesting abundant natural background inoculum. This was confirmed by sampling in July, when severe disease but no treatment differences and no disease gradients were found. Conditions in 2000, when the samples were taken, were exceptionally conducive to eyespot generally. Artificially produced apothecia are evidently not ideal for epidemiological experimentation. Conditions for natural production in situ of apothecia on infected stubble were therefore created as part of the following field experiment.

Determining ascospore dispersal and infectivity from inoculum sources created naturally in the field

In the first year, similar amounts of eyespot developed in all inoculated wheat plots but the disease did not become severe. There was less eyespot in non-inoculated plots; this eyespot presumably developed from inoculum surviving from earlier cereals on the site. Apothecia were numerous on stubble in the following February in plots inoculated with two mating types or left non-inoculated, but were scarce in plots inoculated with a single mating type. This is new evidence that prior infection can exclude later infection by different strains of T. yallundae. In contrast, modelling of our population data has suggested that T. yallundae and the other eyespot fungus, T. acuformis, are tolerant of each other and not mutually exclusive (Bierman et al., 2002, and below). Disease gradients in the surrounding wheat, when determined in April, suggested a short-range (up to 1-2 m) boost to eyespot where two complementary mating types were applied. If this was caused by ascospore release, most of the rest of the disease is likely to have been background inoculum from the open field rather than a consequence of dispersal from the plots. A less distinct short-range peak in the July sample was not obviously related to application of inoculum of complementary mating types. For this reason, the population samples taken along the transects will be genetically fingerprinted to determine any parental origins. On the evidence so far, ascospore inoculum does not add to the dispersal capability of T. yallundae within a wheat field. It may add, in smaller amounts, to the inoculum provided by conidia: conidia are produced in large numbers in similar conditions and are relatively more infective (see above).

Modelling eyespot epidemics

Disease progress following infection from conidia of both species of eyespot fungus was modelled in relation to thermal time using data from controlled environment and field studies (Bierman et al., 2002). T. yallundae grew more quickly through successive leaf sheaths and reached the stem first, whilst T. acuformis appeared to develop more effectively on stems. They were shown to occupy similar niches yet avoid competition. This implies that fungicides that selectively control one species (usually T. yallundae, which is more sensitive to most triazole fungicides) do not contribute to an increase in the other species as a result of increased niche availability. This is consistent with our previous observations on the consequences of repeated use of prochloraz, which appeared to select for T. acuformis as a result of the range of sensitivities to prochloraz in that species (Bateman, 2002).