Etienne Duveiller

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.

Lateral flow colloidal gold based immunoassay for the rapid detection of deoxynivalenol with two indicator ranges

A lateral-flow immunoassay using a colloidal gold-labelled monoclonal antibody was developed for the rapid detection of deoxynivalenol (DON). Different parameters, such as the amount of immunoreagents, type of the materials, composition of the blocking solution and of the detector reagent mixture, were investigated to provide the optimum assay performance. The experimental results demonstrated that such a visual test had an indicator range rather than a cut-off value. Thus, tests for DON determination with two different indicator ranges of 250-500 and 1000-2000 microg kg(-1) were designed. The method allowed detection of DON at low and high concentration levels, which could be useful for research and practical purposes. The assay applied to spiked wheat and pig feed samples demonstrated accurate and reproducible results. The applicability of the developed lateral-flow test was also confirmed under real field conditions. The test strips prepared in Belgium were sent to Mexico, where they were used for the screening of DON contamination in different bread wheat entries from Fusarium Head Blight inoculated plots. The results were compared with those obtained by ELISA and LC-MS/MS. A poor correlation between ELISA and LC-MS/MS was observed. Visual results of the dipstick tests were in a good agreement with the results of the LC-MS/MS method. Coupled with a simple and fast sample preparation, this qualitative one-step test based on the visual evaluation of results did not require any equipment. Results could be obtained within 10 min. The described assay format can be used as a simple, rapid, cost-effective and robust on-site screening tool for mycotoxin contamination in different agricultural commodities.

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.

Resistance to Helminthosporium Leaf blight and agronomic performance of spring wheat genotypes of diverse origins

Helminthosporium leaf blight (HLB), caused by a complex of Cochliobolus sativus (Ito & Kurib.) Drechsler ex Dastur and Pyrenophora tritici-repentis Died, is a serious disease of wheat (Triticum aestivum L.) in the warm lowlands of South Asia. Wheat cultivars grown in the area are either susceptible to HLB or possess low levels of resistance to it. A replicated field study was conducted in 1999 and 2000 at two sites in Nepal to determine the level of HLB resistance and other desirable traits in 60 wheat genotypes of diverse origin. The test genotypes were planted in main strips divided into two strips one of which was sprayed four times with Tilt™ (a.i. propiconazole) @ 125 g of a.i. ha−1. Four readings of HLB were recorded to calculate the area under the disease progress curve (AUDPC). Other traits under investigation included biomass yield (BY), grain yield (GY), 1000-kernel weight (TKW), harvest index (HI), days to heading (DH) and maturity (DM), plant height (PHT), and effective tiller number (ETN). Wheat genotypes differed significantly for all traits. Mean AUDPC values ranged from 45 to 1268. A few exotic genotypes were highly resistant to HLB. Losses in GY due to HLB ranged from 2 to 26%, and TKW was reduced by up to 33%. A few genotypes showed HLB tolerance, i.e., relatively smaller GY and TKW reductions despite high levels of HLB. In general, medium to late maturity and higher levels of HLB resistance and low to high GY and TKW characterized genotypes exotic to South Asia. Biplot analysis identified several genotypes that were HLB-resistant and agronomically superior. Results suggest it is possible to improve HLB resistance of local wheat cultivars based on selective breeding using this pool of germplasm.

Genetics of wheat–Pyrenophora tritici-repentis interactions

Tan spot, caused by an ascomycete fungus Pyrenophora tritici-repentis, is one of the most devastating foliar diseases of wheat. This fungus induces two distinct symptoms, tan necrosis and extensive chlorosis, on susceptible wheat cultivars. Besides causing average yield losses of 5–10%, tan spot also causes significant losses in grain quality by grain shriveling, red smudge, and black point. Conservation agriculture in combination with wheat monoculture involving cultivation of susceptible cultivars has resulted in frequent onset of tan spot epidemics worldwide. Development of new resistant wheat cultivars, in conjunction with crop rotation, will provide an effective, economical, and environmentally safe means of controlling tan spot. Presently, eight races of P. tritici-repentis have been identified worldwide based on the ability to induce necrosis and chlorosis symptoms on a set of differential wheat cultivars. P. tritici-repentis is a homothallic fungus having both sexual and asexual reproduction resulting in high genetic diversity worldwide. Both quantitative and qualitative mode of inheritance for resistance to tan spot of wheat has been reported. The tan spot fungus produces multiple host-specific toxins and host resistance is highly correlated to insensitivity to toxins. Genetic studies have further confirmed that wheat–P. tritici-repentis follows the toxin model of gene-for-gene hypothesis although other mechanism of host–pathogen interaction may exist and exploitation of all resistance phenomenon is to be adopted to develop durable resistant cultivars.

Variation in chlorophyll content, anatomical traits and agronomic performance of wheat genotypes differing in spot blotch resistance under natural epiphytotic conditions

Spot blotch caused by Cochliobolus sativus is a serious constraint to wheat (Triticum aestivum) yields in South Asia. A study was conducted at Rampur, Nepal, during 2002 and 2003 using five wheat genotypes (Sonalika, BL1473, Kanchan, Nepal 297 and NL 750) differing in spot blotch resistance to assess the effect of disease on loss of chlorophyll measured as soil plant analysis development (SPAD) over time characterised by the area under SPAD value decline curve (AUSDC) and the area under disease progress curve (AUDPC), and to investigate the association between disease severity and leaf anatomical traits. The SPAD value was measured using a Minolta SPAD-502 chlorophyll meter on three different dates in 10 randomly tagged flag leaves in each plot postanthesis to determine AUSDC. Eight anatomical characters were examined in the flag leaf. The AUDPC showed a significant negative correlation with the width of large vascular bundles, percentage of small vascular bundles with two girders and the number of large veins. The AUDPC was positively correlated with the distance between adjacent vascular bundles and leaf thickness. The SPAD and AUSDC values were higher in spot blotch resistant and tolerant genotypes. The findings underline the importance of mesophyll structure and chlorophyll content in spot blotch resistance in wheat. This is useful information for developing spot blotch resistant wheat.

Bacterial Sheath Rot of Wheat Caused By Pseudomonas-fuscovaginae in the Highlands of Mexico

Angular, blackish brown lesions, 10 or 20 cm in length, bordered with purple-black angular area 1-2 mm wide and showing a grayish center, were frequently observed on leaf sheaths of bread wheat, durum wheat, and triticale at the booting stage in the central highlands of Mexico, 2,249-2,640 m above sea level. Fluorescent, strictly aerobic bacteria identified as Pseudomonas fuscovaginae (based on their positive reaction for Kovacs oxidase and arginine dihydrolase but negative for esculin hydrolysis and nitrate reduction) were isolated from these lesions (...)

Genetic analysis of resistance to bacterial leaf streak caused by Xanthomonas campestris pv. undulosa in bread wheat

SummaryThe inheritance of resistance to bacterial leaf streak or black chaff of wheat was studied under field conditions, with an artificial epidemic of Xanthomonas campestris pv. undulosa. A complete series of crosses between five parents, differing in reaction to X. c. pv. undulosa, was generated. Disease was recorded at two different stages of growth. No evidence of cytoplasmic effect was found from the comparison between reciprocal F1 crosses. The study of the F3 generations attested that five genes were involved in resistance to bacterial leaf streak. Separate analyses carried out for the two scoring dates were mutually consistent: genotypes, number of genes, and their action and relative importance were verified. The genes differed in strength of expression of resistance. One of the two strongest genes, Bls1, is present in all three superior parents, Pavon 76, Mochis T88 and Angostura F88. Resistance was not complete, and proved to be stable over the season.

Genetic Diversity of the Mating Type and Toxin Production Genes in Pyrenophora tritici-repentis

Pyrenophora tritici-repentis, the causal agent of tan spot on wheat, is a homothallic loculoascomycete with a complex race structure. The objectives of this study were to confirm the homothallic nature of the pathogen, characterize mating type diversity and toxin production genes in a global collection of strains, and analyze how these traits are associated between each other and with existing races. The pseudothecia production capacity, race identification, mating type locus (MAT), internal transcribed spacer, and glyceraldehyde-3-phosphate dehydrogenase regions were analyzed in a selection of 88 strains originating from Europe, North and South America, North Africa, and Central and South Asia. Some (60%) strains produced pseudothecia containing ascospores, independent of their origin. Race identification obtained using the multiplex polymerase chain reaction targeting host-selective toxin (HST) genes was consistent, overall, with the results based on the inoculation of a set of differential wheat cultivars and confirmed the predominance of race 1/2 strains ( approximately 83%). However, discrepancies in race identification, differences from the reference tester strains, and atypical ToxA profiles suggest the presence of new races and HSTs. The MAT1-1 and MAT1-2 coding regions are consecutively arranged in a single individual, suggesting putative heterothallic origin of P. tritici-repentis. Upstream from the MAT is an open reading frame of unknown function (ORF1) containing a MAT-specific degenerate carboxy-terminus. The phylogenetic analysis of the MAT locus reveals two distinct groups, unlinked to geographical origin or ToxA profile. Group I, the best-represented group, is associated with typical tan spot lesions caused by races 1, 2, 3, and 5 on wheat. It is more homogenous than group II encompassing race 4 strains, as well as isolates associated primarily with small spot lesions on wheat leaves or other hosts. Group II could contain several distinct taxa.

Comparison of immunofluorescence and two assays for detection of Xanthomonas campestris pv. undulosa in seeds of small grains

A monoclonal antibody specific for Xanthomonas campestris pathovars in the translucens group, including X. c. pv. undulosa, the causal agent of bacterial leaf streak or black chaff in wheat and triticale, was used in immunofluorescence and a dot-immunobinding assay to detect pathogens in aqueous seed extracts. These techniques were compared to dilution plating of seed wash water as potential routine seed-indexing methods. Twenty-six cereal seed lots were harvested from plants with different levels of natural disease incidence grown in three locations in Mexico. Among the 26 lots analyzed, pathogen-free seed came from clean fields principally in Ciudad Obregon, where conditions are less favorable for its development. Pathogen populations detected with dilution plate ranged from 1.3 X 10(4) to 5.3 X 10(6) cfu/g of air-dry weight. Populations were recovered from seed lots that had been stored for 3 yr. Some discrepancies in pathogen detection occurred among the different methods used. Seed lots with high pathogen populations were consistently identified with all three methods. Differences in detection thresholds were found with seed wash water as compared to reference strain from pure culture. Apparently a substantial number of dead cells were present in wash water from some seed lots. Immunofluorescence was more sensitive than dot-immunobinding assay and produced more reproducible results. It is proposed as a standard for indexing germ plasm. Because of the high variation in data, direct counts of fluorescent bacterial cells under UV microscopy will probably not be necessary for identifying infected seed. It is recommended that immunofluorescence-positive seed lots should not be considered for sowing in areas favorable for disease development.