This review of Fusarium ear blight (scab) of small grain cereals has shown that up to 17 causal organisms have been associated with the disease, which occurs in most cereal-growing areas of the world. The most common species were Fusarium graminearum (Gibberella zeae), F. culmorum, F, avenaceum (G, avenacea), F, poae and Microdochium nivale (Monographella nivalis). The disease was recorded most frequently under hot, wet climatic conditions where significant yield losses and mycotoxin accumulation in grain were reported. Possible sources of inoculum were reported as crop debris, alternative hosts and Fusarium seedling blight and foot rot of cereals. The mode of dispiersal of inoculum to ears remains unclear, but contaminated arthropod vectors, systemic fungal growth through plants, and wind and rain-splash dispersal of spores have been proposed. Infection of wheat ears was shown to occur mainly during anthesis, and it has been demonstrated that fungal growth stimulants may be present in anthers. Despite the importance of the disease, particularly during epidemic years, control methods are limited. Much effort has gone into breeding resistant wheat varieties and into improving our understanding of the possible mechanisms and genetic basis of resistance, with only moderate success. There are also surprisingly few reports of successful fungicidal or biological control of the disease in the field.

Fusarium ear blight (scab) in small grain cereals—a review

Detection and quantification of fusarium culmorum and fusarium graminearum in cereals using pcr assays

Fungal infections, especially those caused by opportunistic species, have become substantially more common in recent decades. Numerous species cause human infections, and several new human pathogens are discovered yearly. This situation has created an increasing interest in fungal taxonomy and has led to the development of new methods and approaches to fungal biosystematics which have promoted important practical advances in identification procedures. However, the significance of some data provided by the new approaches is still unclear, and results drawn from such studies may even increase nomenclatural confusion. Analyses of rRNA and rDNA sequences constitute an important complement of the morphological criteria needed to allow clinical fungi to be more easily identified and placed on a single phylogenetic tree. Most of the pathogenic fungi so far described belong to the kingdom Fungi; two belong to the kingdom Chromista. Within the Fungi, they are distributed in three phyla and in 15 orders (Pneumocystidales, Saccharomycetales, Dothideales, Sordariales, Onygenales, Eurotiales, Hypocreales, Ophiostomatales, Microascales, Tremellales, Poriales, Stereales, Agaricales, Schizophyllales, and Ustilaginales).

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Developments in Fungal Taxonomy

Accurate identification of fungal phytopathogens is essential for virtually all aspects of plant pathology, from fundamental research on the biology of pathogens to the control of the diseases they cause. Although molecular methods, such as polymerase chain reaction (PCR), are routinely used in the diagnosis of human diseases, they are not yet widely used to detect and identify plant pathogens. Here we review some of the diagnostic tools currently used for fungal plant pathogens and describe some novel applications. Technological advances in PCR-based methods, such as real-time PCR, allow fast, accurate detection and quantification of plant pathogens and are now being applied to practical problems. Molecular methods have been used to detect several pathogens simultaneously in wheat, and to study the development of fungicide resistance in wheat pathogens. Information resulting from such work could be used to improve disease control by allowing more rational decisions to be made about the choice and use of fungicides and resistant cultivars. Molecular methods have also been applied to the study of variation in plant pathogen populations, for example detection of different mating types or virulence types. PCR-based methods can provide new tools to monitor the exposure of a crop to pathogen inoculum that are more reliable and faster than conventional methods. This information can be used to improve disease control decision making. The development and application of molecular diagnostic methods in the future is discussed and we expect that new developments will increase the adoption of these new technologies for the diagnosis and study of plant disease.

Molecular diagnostics for fungal plant pathogens

The re-emergence of fusarium head blight throughout the world and especially in Western Europe prompted a survey of the situation in the Netherlands. To allow for a high throughput screening of large numbers of samples, a diagnostic PCR method was developed to detect the most common species of Fusarium occurring on wheat. Seven primer pairs were tested for their ability to identify isolates of Fusarium avenaceum, F. culmorum, F. graminearum, F. poae, F. proliferatum and Microdochium nivale var. majus and M. nivale var. nivale. Each primer pair only generated a PCR product with the corresponding Fusarium species and all PCR fragments had different molecular sizes. This allowed the generation of these amplicons using a mixture of all seven primer pairs. The robustness of this multiplex PCR encouraged us to screen a large series of isolates collected in 2000 and 2001. In both years 40 fields were sampled leading to a collection of 209 isolates from 2000 and 145 isolates from 2001. The results of the multiplex PCR demonstrated that F. graminearum was the most abundant species in the Fusarium complex on wheat in both years. This is in sharp contrast to reports from the 1980s and early 1990s, which found F. culmorum as the predominant species. Primers derived from the tri7 and tri13 genes, which are implicated in the acetylation and oxygenation of the C-4 atom of the backbone of the trichothecene molecule, were used to discriminate between deoxynivalenol and nivalenol (NIV) producers. The populations of F. culmorum and F. graminearum both showed a slight increase in NIV-producers in 2001.

Major changes in Fusarium spp. in wheat in the Netherlands

Development of a PCR assay to identify and quantify Microdochium nivale var. nivale and Microdochium nivale var. majus in wheat

The pathogenicity of Microdochium nivale var majus and var nivale was tested on wheat, rye and oat seedlings using both visual disease scoring and quantitative PCR measurements In an individual inoculation trial at 10°C var majus and var nivale were strongly pathogenic towards wheat and rye, with var nivale causing slightly greater disease in rye At this temperature only var nivale caused significant disease of oats In a further experiment M nivale was inoculated as a series of mixtures of the two varieties and incubated at 15°C The ratio of the varieties present in the inoculum and present at harvest was analysed by quantitative PCR and this enabled a coefficient of selection to be calculated for the varieties on each host M nivale var majus showed a weak selective advantage over var nivale on wheat (033 ± 008) and oat seedlings (035 ± 0016) and M nivale var nivale showed a strong selective advantage over var majus on rye seedlings (092 ± 026) The isolates were also compared for sensitivity to benzoxazolinone (BOA), a hydroxamic acid compound derived from rye leaves M nivale var majus was found to be significantly more sensitive to BOA than M nivale var nivale, indicating a possible mechanism for the selective advantage of var nivale growing on rye This is the first substantiated indication of a significant difference in host preference between Microdochium nivale var majus and var nivale

Evidence for differential host preference in Microdochium nivale var. majus and Microdochium nivale var. nivale

Analysis of variation within Microdochium nivale from wheat : evidence for a distinct sub-group

Fusarium moniliforme, the imperfect stage of the ascomycete Gibberella fujikuroi, is an economically important pathogen with a very wide host range. The genetic characteristics of isolates of the fungus collected from different regions of Ghana from maize, rice and sorghum were determined using the random amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) techniques. The pathogenicity of the isolates was also compared on maize and rice. DNA fingerprints detected as RFLPs of ribosomal DNA and RAPDs separated the isolates into discrete groups which were generally host-related. The possibility of a sub-structuring of the maize population of the fungus into tissue-related subgroups was suggested by the results. A dendrogram of the relatedness of the isolates is presented. However, the pathogenicity of the isolates on rice, measured by their ability to cause ‘bakanae’ symptoms, did not resolve the isolates into the clearly defined groups suggested by the genetic studies, and maize isolates of the fungus could cause ‘bakanae’ symptoms to the same extent as rice isolates. Similarly, some isolates identified as rice-type isolates caused as much shoot stunting in maize as maize isolates. However, the effects of the isolates on root growth of maize seedlings showed a broad correlation with the defined genetic groups, with maize isolates of the fungus showing the greatest tendency to cause root stunting

H. N. Rezanoor

Papers

Detection and quantification of fusarium culmorum and fusarium graminearum in cereals using pcr assays

Development of a PCR assay to identify and quantify Microdochium nivale var. nivale and Microdochium nivale var. majus in wheat

Evidence for differential host preference in Microdochium nivale var. majus and Microdochium nivale var. nivale

Analysis of variation within Microdochium nivale from wheat : evidence for a distinct sub-group

Variation in the Fusarium section Liseola: pathogenicity and genetic studies of isolates of Fusarium moniliforme Sheldon from different hosts in Ghana