Epidemiology of Fusarium head blight on small-grain cereals

Abstract

Fusarium head blight (FHB) is one of the most serious diseases affecting wheat and barley worldwide. It is caused by Fusarium graminearum along with F. culmorum, F. avenaceum and other related fungi. These fungi also produce several mycotoxins. Though the disease results in reduced seed quality and yield, the toxins which may accompany the disease are often a more serious problem. Pathogen inoculum is usually very abundant, however production and dispersal of inoculum are weather-sensitive processes. An abundance of colonized substrate (i.e. maize or cereal debris) in a region contributes to airborne inoculum throughout the area. Local residues beneath the cereal crop (i.e. from previous crop) may have a less obvious effect, particularly in regions where long-distance dispersal is likely due to wind conditions. The host is most susceptible to infection at anthesis and shortly thereafter. A warm, moist environment characterized by frequent precipitation or heavy dew is highly favorable to fungal growth, infection and development of disease in head tissues. As the fungus grows, it produces mycotoxins which are water-soluble and may be translocated between tissues or leeched from source tissues. Important epidemiological issues have arisen recently and include an apparent shift in prevalence of Fusarium species on infected heads in Europe toward F. graminearum; and the presence of multiple chemotypes and aggressiveness variants within a species in a region.

Introduction

Fusarium head blight (FHB), also called ear blight or scab, is a major fungal disease affecting several gramineous hosts including wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). The disease occurs throughout much of the world and is associated with several Fusarium spp. including Fusarium graminearum Schwabe (teleomorph: Gibberella zeae (Schwein.) Petch) and F. culmorum (W.G. Smith) Sacc., as well as F. poae (Peck) Wollenw., F. avenaceum (Fr.) Sacc., F. sporotrichoides Scherb., and Microdochium nivale (Fr.) Samuels & I.C. Hallett. While F. graminearum is often the prevalent species in many of the affected regions, the others can be highly pathogenic or are frequently found in association with the disease. These fungi are capable of producing a number of trichothecene mycotoxins including deoxynivalenol (DON) and nivalenol (NIV), as well as zearalenone (ZEA) and moniliformin (MON), all of which have a range of toxicity to animals (Desjardin, 2006, Leslie and Summerell, 2006, Rotter et al., 1996).

Fusarium head blight of wheat appears as blighted head and peduncle tissues which turn brown or tan and senesce prematurely. Symptoms often are accompanied by evidence of the fungus, which may include purple-black perithecia and/or pink sporodochia on heads, especially on glumes. Seed from FHB-affected fields often is shrunken, or shriveled with a characteristically bleached appearance (Andersen, 1948, Goswami and Kistler, 2004, McMullen et al., 1997, Parry et al., 1995). The disease results in direct economic losses including reduced grain yield and quality (aborted or shriveled seed, reduced seed size), and indirect loss due to contamination by mycotoxins leading to rejection or downgrading of grain at marketing (Parry et al., 1995, Snijders, 1990, Sutton, 1982). Additional outcomes have included socially destabilizing economic impacts within FHB-affected regions (McMullen et al., 1997, Nganje et al., 2004, Windels, 2000).

Epidemics of FHB (like those of many other disease systems) are strongly influenced by local and regional environment, host factors such as physiological state and genetic make-up, and pathogen factors including adaptation and virulence.

The objective of this review is to outline the current state of knowledge for FHB and highlight a number of epidemiological issues. A further objective is to discuss some of the pressing issues that have emerged more recently or that have proven especially difficult to deal with experimentally.