Julie M. Nicol

The challenges of maintaining wheat productivity: pests, diseases, and potential epidemics

Knowing pests and diseases that may cause injuries and are likely to affect plant health and quality is critical to minimizing the gap between attainable yield and actual yield. In this paper, we highlight concepts and strategies aimed at controlling major biotic constraints affecting wheat in intensive production systems and present emerging challenges, with a special attention to the developing world. Disease epidemics result from the combination of inoculum, favorable environment, and host susceptibility. Changes in cropping systems as a result of adoption of conservation agriculture may have serious implications. Necrotrophic pathogens such as those responsible for tan spot or septorias are likely to emerge, and Fusarium head blight may increase. However, resistance breeding combined with rotations, timely sowing, and irrigation or even fungicide utilization, if affordable, are part of integrated crop management practices that can minimize losses. In South Asia, the effect of spot blotch, a devastating foliar disease caused by Cochliobolus sativus, can be minimized by reducing physiological stress through timely sowing and adequate use of fertilizers, which demonstrates the complex relationships among crop physiology, disease resistance, and yield. Although some root rots that induce premature death of tillers in cooler high-yielding humid environments can be important, the dryland crown rot (Fusarium spp.), common root rot (C. sativus), and the cereal nematode (Heterodera spp. and Pratylenchus spp.) should not be ignored. These are all known to be much more damaging under suboptimal moisture (rainfed or supplementary irrigation), particularly where plant growth is stressed. Climate change is likely to modify the wheat disease spectrum in some regions, and pathogens or pests considered unimportant today may turn out to be potential new threats in future.

Wheat genetic resources enhancement by the International Maize and Wheat Improvement Center (CIMMYT)

The International Maize and Wheat Improvement Center (CIMMYT) acts as a catalyst and leader in a global maize and wheat innovation network that serves the poor in the developing world. Drawing on strong science and effective partnerships, CIMMYT researchers create, share, and use knowledge and technology to increase food security, improve the productivity and profitability of farming systems and sustain natural resources. This people-centered mission does not ignore the fact that CIMMYT’s unique niche is as a genetic resources enhancement center for the developing world, as shown by this review article focusing on wheat. CIMMYT’s value proposition resides therefore in its use of crop genetic diversity: conserving it, studying it, adding value to it, and sharing it in enhanced form with clients worldwide. The main undertakings include: long-term safe conservation of world heritage of both crop resources for future generations, in line with formal agreements under the 2004 International Treaty on Plant Genetic Resources for Food and Agriculture, understanding the rich genetic diversity of two of the most important staples worldwide, exploiting the untapped value of crop genetic resources through discovery of specific, strategically-important traits required for current and future generations of target beneficiaries, and development of strategic germplasm through innovative genetic enhancement. Finally, the Center needs to ensure that its main products reach end-users and improve their livelihoods. In this regard, CIMMYT is the main international, public source of wheat seed-embedded technology to reduce vulnerability and alleviate poverty, helping farmers move from subsistence to income-generating production systems. Beyond a focus on higher grain yields and value-added germplasm, CIMMYT plays an “integrative” role in crop and natural resource management research, promoting the efficient use of water and other inputs, lower production costs, better management of biotic stresses, and enhanced system diversity and resilience.

Pathogen population structure and epidemiology are keys to wheat crown rot and Fusarium head blight management

This paper summarises the key findings from recent research on the population genetics and epidemiology of Fusarium pathogens causing head blight and crown rot of wheat in Australia and how this information has enabled the screening and selection of wheat germplasm with improved resistance to Fusarium. By relating new findings to the current state of knowledge, the paper serves as a timely and critical review of the international literature. In Australia, both Fusarium pseudograminearum and F. graminearum can cause both crown rot and Fusarium head blight under artificial inoculation. However, the former species is more widespread and is predominantly associated with crown rot whereas F. graminearum is mainly associated with Fusarium head blight, with limited geographical distribution in and around the Liverpool Plains in northern New South Wales. Studies of population structure and genetics have revealed that both species are genotypically diverse with similar levels of genetic recombination despite Gibberella zeae, the teleomorph of F. graminearum, being homothallic and G. coronicola, the teleomorph of F. pseudograminearum, being heterothallic. A high-throughput and reliable crown rot bioassay has been developed and used to screen over 1500 wheat germplasms to select 17 lines with putative crown rot resistance. Key differences in pathogen biology and epidemiology between Australia and the USA have emerged from other recent collaborative studies, which show that macroconidia constitute the bulk of aerial Fusarium head blight inoculum in Australia, whereas ascospores are the dominant primary inoculum for Fusarium head blight worldwide. The limited spread of splash-dispersed macroconidia of F. graminearum probably explains the restricted geographical distribution of this species in Australia. Other research collaboration has compared the aggressiveness, mycotoxin production and genotypic polymorphisms of the pathogen population from Australia and the USA. These and other differences in pathogen adaptation emphasise that research outcomes from elsewhere must be tested for relevance before applying them to Australian farming systems.

Global Knowledge And Its Application For The Integrated Control And Management Of Nematodes On Wheat

Importance of cereals and wheat nematodes in the world is revised. Distribution of cereal nematodes, species and pathotypes includes root lesion, cereal cyst nematodes and other cereal parasitic species. Life cycle, symptoms of damage and yield losses are also revised for root knot, stem and seed gall nematodes. Integrated control of cereal nematodes and some chemical, biological and cultural practices, including grass free rotations and fallowing with cultivation, are discussed. The effects of time of sowing, crop rotations and cultivation of resistant/tolerant varieties are also revised.

Root and Crown Rot Fungi Associated with Spring, Facultative, and Winter Wheat in Turkey

The objective of this study was to determine the distribution frequency of the fungi associated with wheat (Triticum aestivum) crowns and roots in cereal producing areas of Turkey through a targeted survey of 518 commercial fields over a 2-year period. More than 26% of the fields had one or more of the fungal species commonly reported as part of the dryland root rot complex, Fusarium culmorum (14%) > Bipolaris sorokiniana (10%) > F. pseudograminearum (2%). The fungi considered to be part of the high rainfall root rot complex were found at very low frequencies: 2% for Gaeumannomyces graminis and 3% for Pythium spp. Species of Rhizoctonia were found in 22% of the fields. Several Fusarium species considered to be less or nonpathogenic to cereals were also found in high frequencies at 11% (F. oxysporum, F. chlamydosporum), 10% (F. sporotrichioides), and 8% (F. avenaceum and F. solani). The mostly random distribution of cereal root-rotting species across the survey area suggests the fungi are not distributed in any distinct agroecological relationship. As a result, the relative economic importance of a given species on wheat will be determined by a number of factors, such as their fungal pathogenicity, host susceptibility/tolerance, and the seasonal conditions. Results from this study suggest that there are a wide range of fungal species associated with root and crown tissues of wheat.

Variation in reproductive capacity and virulence on different genotypes and resistance genes of Triticeae, in the cereal cyst nematode species complex

Two sets of experiments in 1998 and 1999 studied the (a)virulence status of 14 populations of Heterodera avenae, and two populations each of H. filipjevi and H. latipons towards the resistance genes and genotypes of Triticeae, presently known to be resistant to H. avenae, the main species of the cereal cyst nematode (CCN) species complex. Susceptible controls were Triticum aestivum cv. Arminda and Triticum turgidum cv. Cham1. Even in controlled experimental conditions, host responses differed between replicates in the same test and between consecutive tests for unexplained reasons. In spite of that, it has been confirmed that several of these CCN populations differ in their intrinsic capacity to reproduce, and this has to be taken into account in resistance screening. Consequently, the (a)virulence status of a CCN population is better established by a qualitative demarcation between resistant and susceptible plant genotypes based on an average of one cyst limit per plant. The matrix inferred from such a classification of host responses of pooled 1998 and 1999 data showed a complex distribution of avirulent and virulent phenotypes in these CCN populations which seems to arise from a mosaic-like evolution. The more or less wide virulence spectrum shown by this complex of populations and species is discussed in relation to their phylogenetic relationships. Correspondence factor analysis of the pooled data demonstrated a geographical demarcation between several populations of H. avenae and the representatives of H. filipjevi and H. latipons for their (a)virulence to either Cre1 or Cre3 genes and T. aestivum AUS4930 genotype. This could reveal local or regional selection of new pathotypes, more particularly in H. avenae. Genetic introgression to improve resistance to these nematodes in both bread and durum wheats is discussed for regional and global applications.

Fusarium culmorum is a single phylogenetic species based on multilocus sequence analysis

Fusarium culmorum is a major pathogen of wheat and barley causing head blight and crown rot in cooler temperate climates of Australia, Europe, West Asia and North Africa. To better understand its evolutionary history we partially sequenced single copy nuclear genes encoding translation elongation factor 1-α (TEF), reductase (RED) and phosphate permease (PHO) in 100 F. culmorum isolates with 11 isolates of Fusarium crookwellense, Fusarium graminearum and Fusarium pseudograminearum. Phylogenetic analysis of multilocus sequence (MLS) data using Bayesian inference and maximum parsimony analysis showed that F. culmorum from wheat is a single phylogenetic species with no significant linkage disequilibrium and little or no lineage development along geographic origin. Both MLS and TEF and RED gene sequence analysis separated the four Fusarium species used and delineated three to four groups within the F. culmorum clade. But the PHO gene could not completely resolve isolates into their respective species. Fixation index and gene flow suggest significant genetic exchange between the isolates from distant geographic regions. A lack of strong lineage structure despite the geographic separation of the three collections indicates a frequently recombining species and/or widespread distribution of genotypes due to international trade, tourism and long-range dispersal of macroconidia. Moreover, the two mating type genes were present in equal proportion among the F. culmorum collection used in this study, leaving open the possibility of sexual reproduction.

Effects of the root-lesion nematode, Pratylenchus thornei, on wheat yields in México

Tolerance and resistance to P. thornei of seven CIMMYT-derived cultivars of wheat (Triticum aestivum), selected on soil naturally infested by Pratylenchus thornei and released from 1960-1997, were evaluated under optimum and limited irrigation conditions in northwestern Mexico. Replicated trials were grown in a naturally infested site in two seasons. Differences in yield, agronomic traits and nematode numbers were recorded in paired field plots, untreated or fumigated with dazomet to control nematodes. With optimum irrigation the nematode did not affect wheat yield but, with limited irrigation where plants were under water-stress, yield loss of all cultivars was comparable to that of the intolerant, susceptible control cv. Warigal (29%). There was a significant negative linear relationship (Y = –3.69 x + 3117) between initial nematode density (x) and grain yield (Y) under limited irrigation. None of these cultivars was resistant and post-harvest P. thornei populations had increased from three to 15 times the initial estimates. There was no relationship between date of release of cultivars and their responses to the nematode. There is a need for sources of resistance to be identified and incorporated into the germplasm.

Effect of the cereal cyst nematode, Heterodera filipjevi, on wheat in microplot trials

The cereal cyst nematodes, Heterodera avenae , H. filipjevi and H. latipons , are considered to be the most economically important species of cyst-forming nematodes on wheat. Two trials were conducted in 2006 and 2007 to determine the impact of different initial population levels of H. filipjevi (0, 2.5, 5, 10 and 20 eggs and second-stage juveniles (J2) (g soil) −1 ) on wheat cv. Sardari in microplots under natural field conditions in Iran. The results showed that increasing the initial populations of H. filipjevi significantly reduced several growth parameters of wheat (plant height, root dry weight, aerial shoot dry weight and grain yield) compared with the untreated controls. The final population of H. filipjevi increased with increasing initial population, while the reproduction factor ( R f ) declined as the initial population increased but was greater than 1 in all treatments. Regression analysis showed a significant negative relationship between the initial populations and grain yield. The modelling clearly demonstrated that H. filipjevi is economically important on winter wheat in Iran. Grain yield loss was demonstrated even at the lowest population density and reached a maximum loss of 48% with an initial population density of 20 eggs and J2 (g soil) −1 . The aerial shoot yield loss was as great as 40% in both years.

Development of two species-specific primer sets to detect the cereal cyst nematodes Heterodera avenae and Heterodera filipjevi

Twelve Heterodera species are of major economic significance in wheat and barley. Of these, H. avenae, H. filipjevi and H. latipons are among the most important ones, and sometimes coexist. The identification of Heterodera species using morphological characteristics is time consuming, requires specialized skill and can be imprecise, especially when they occur mixed in field populations. Molecular techniques can provide a more accurate way for nematode identification. This study reports the results of experiments targeting the mitochondrial cytochrome oxidase subunit 1 (COI) gene to develop species-specific primers that could be used for the identification of H. avenae and H. filipjevi. The COI gene of 9 Heterodera spp. and Punctodera punctata was partially sequenced and the resultant sequences were aligned to find unique sites suitable for the design of primers. The alignment showed variability between H. avenae, H. filipjevi and other Heterodera species. Two sets of species-specific primers were identified for the identification of both species and the conditions for their use in PCR were optimised. The specificity of the designed primers was checked by comparison with one population of P. punctata and populations of 14 other Heterodera species, nine populations of H. avenae and 10 populations of H. filipjevi originating from different countries. To test the sensitivity, the PCR was run with DNA extracted from five second-stage juveniles (J2) of H. avenae or five J2 of H. filipjevi mixed with DNA extracted from varying numbers of J2 of H. latipons. It was possible to detect as few as five J2 of H. avenae or H. filipjevi among 100 J2 of H. latipons. The two primers sets allow the detection of H. avenae and H. filipjevi where they occur in mixed populations with other Heterodera spp.