Euphytica (2007) 157:417–430 DOI 10.1007/s10681-007-9380-z The challenges of maintaining wheat productivity: pests, diseases, and potential epidemics Etienne Duveiller Æ Ravi P. Singh Æ Julie M. Nicol Received: 12 September 2006 / Accepted: 2 February 2007 / Published online: 2 March 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Knowing pests and diseases that may adequate use of fertilizers, which demonstrates cause injuries and are likely to affect plant health the complex relationships among crop physiology, and quality is critical to minimizing the gap disease resistance, and yield. Although some root between attainable yield and actual yield. In this rots that induce premature death of tillers in paper, we highlight concepts and strategies aimed cooler high-yielding humid environments can be at controlling major biotic constraints affecting important, the dryland crown rot (Fusarium spp.), wheat in intensive production systems and pres- common root rot (C. sativus), and the cereal ent emerging challenges, with a special attention nematode (Heterodera spp. and Pratylenchus to the developing world. Disease epidemics result spp.) should not be ignored. These are all known from the combination of inoculum, favorable to be much more damaging under suboptimal environment, and host susceptibility. Changes in moisture (rainfed or supplementary irrigation), cropping systems as a result of adoption of particularly where plant growth is stressed. Cli- conservation agriculture may have serious impli- mate change is likely to modify the wheat disease cations. Necrotrophic pathogens such as those spectrum in some regions, and pathogens or pests responsible for tan spot or septorias are likely to considered unimportant today may turn out to be emerge, and Fusarium head blight may increase. potential new threats in future. However, resistance breeding combined with rotations, timely sowing, and irrigation or even Keywords Yield Á Rusts Á Foliar blights Á fungicide utilization, if affordable, are part of Fusarium Á Root rots Á Climate change integrated crop management practices that can minimize losses. In South Asia, the effect of spot blotch, a devastating foliar disease caused by Introduction Cochliobolus sativus, can be minimized by reduc- ing physiological stress through timely sowing and With world population increasing and food secu- rity projected to become more critical, increasing E. Duveiller (&) Á R. P. Singh wheat yield potential in the developing world Global Wheat Program, CIMMYT, El Bata´n, remains a high priority. High-production irrigated Apartado Postal 6-641, 06600 Mexico, D.F., Mexico areas will continue to play a major role in e-mail: [email protected] reaching this objective. However, whereas abso- J. M. Nicol lute yields are determined by genetic potential, CIMMYT, P.O. Box 39, Emek, 06511 Ankara, Turkey the level of diseases, pests, and other threats 123 418 Euphytica (2007) 157:417–430 determines actual yields (Cook and Veseth 1991). experiment stations or under controlled conditions, Light reducers, tissue consumers, stand reducers, generally using as a check, acommercialcultivar assimilate sappers, as well as senescence acceler- that has become susceptible. Although risks are ators are all ready to take advantage of the not undermined and must be properly calcu- situation (Gaunt 1995). Thus, knowing the patho- lated, crop health and actual losses in farmers’ gens, their ecology, distribution, virulence pat- fields are significantly different, particularly terns, and variability is important in minimizing when modern and broadly adapted resistant diseases and the gap between actual and attain- cultivars are cultivated, as is generally the case able yields. In environments characterized by in intensive production systems. Oerke et al. dense stands and high tiller density, foliar diseases (1994) suggested that the global average of caused by obligate or semi-biotrophic pathogens actual yield losses caused by all wheat diseases, with a high evolutionary rate (McDonald and including developed and developing countries, Linde 2002) are the most important yield con- was about 12.4% on an annual basis. Unfortu- straints. This is mainly because some airborne nately, in developing countries, precise data on parasites can migrate over long distances and are actual yield losses caused by diseases in farmers’ more likely to overcome genetic resistance than fields are often unavailable or are difficult to pathogen populations with a low evolutionary assess. In Kansas, a study covering 1976–2000 potential, which are confined to fields, i.e. soil- and including a total of 18 diseases reported borne pathogens. Their direct effect on grain-fill, annual losses of 10–22% (Bockus et al. 2001). particularly after anthesis via reduced light inter- During this period, leaf rust caused by Puccinia ception and radiation use efficiency, logically triticina was by far the most important disease, results in lower yields (Dubin 1996). causing on average 3.48% losses annually, fol- In this paper, we highlight efforts toward lowed by wheat streak mosaic virus (1.88%), and minimizing yield losses due to biotic constraints the Septoria complex (1.6%). In the same study, and revise concepts and strategies for controlling with the exception of leaf rust and outstanding diseases and pests mostly relevant to intensive outbreaks of Septorias (causing 5.8%, 3.5%, and irrigated wheat systems in the developing world. 7.4% losses in 1983, 1984, and 1995, respec- Although soilborne pathogens are more fre- tively), barley yellow dwarf virus (BYDV) (4.5% quently found in rainfed systems, they should and 3.5% losses in 1976 and 1987, respectively) not be overlooked because water availability may or tan spot (3.5% losses in 1980 and 1987), all not always be satisfactory. Similarly, viruses are diseases taken individually caused less than 3% often overlooked. An exhaustive list of various losses annually, and in general not more than wheat diseases commonly found in most agricul- 0.1–2% (Bockus et al. 2001). An estimate tural systems along with details for their identi- calculated on 22 developing countries growing fication and control can be found in specialized more than 100,000 ha of wheat indicated an compendia (Wiese 1987; Wilcoxson and Saari area-weighted average yield loss over 10 years 1996; Duveiller et al. 1997). Taking a long-term due to leaf rust of around 3.7% (Marasas et al. perspective, we also draw attention to emerging 2004) which is in agreement with above data. challenges resulting from environmental and cli- Spot blotch caused by Cochliobolus sativus is mate change. reported to prevail on about nine million hect- ares of wheat grown after rice in the Indo- Gangetic Plains and yield losses of 20% have Actual yield losses been observed (Duveiller 2004a). However, most likely only 40% of this area is actually suffering Research papers on wheat diseases frequently losses in the range of 5–15% every year, report impressively high potential yield losses depending on the level and duration of dew and suggest that sizable areas of wheat are at and heat stress (Sharma and Duveiller 2004). It risk to specific diseases or pests. High yield loss should be underlined that a disease only occurs figures are often obtained in trials conducted on if inoculum is present, environment (climate, 123 Euphytica (2007) 157:417–430 419 soil, and cropping system) favorable, and the The strategies to limit damages caused by host susceptible. Also, the relationship between wheat rusts, especially in the last decades, fol- diseased leaf area or any disease scoring scale lowed the principles of a coordinated anticipatory and yield is not straightforward because the breeding approach (McIntosh and Brown 1997; physiological implication of symptom assessment Singh and Rajaram 2002; Singh et al. 2002). The and effect on yield is not always proportional or methodology has been particularly successful linear. The extent of damage may vary signifi- against leaf and yellow rusts and is summarized cantly according to the host physiological growth hereafter. Pathogen monitoring through scouting stage at the onset of an epidemic and often of commercial fields and observing trap nurseries depends more on the green leaf area duration at relevant hot spot or favorable locations along than the amount of disease area estimated with with limited sampling for race analysis allows the the help of a visual scale. early detection of new races and confirms the prevalence of major existing races. Enhanced information about the effectiveness and genetic basis of resistance in important wheat cultivars is Obligate parasites with high evolutionary obtained together with the testing of new wheat potential germplasm developed by International Agricultural Research Centers (IARCs), National Agricultural The wheat rusts Research Systems (NARS) and Advanced Re- search Institutes (ARIs) at relevant hot spot sites. The rust diseases of wheat have historically been Diverse resistance sources are used for germ- one of the major biotic production constraints plasm enhancement. Breeding for durable resis- worldwide (Saari and Prescott 1985). Stem (or tance based on the accumulation of additive black) rust caused by Puccinia graminis has been minor genes through use of race nonspecific (slow under control since the semidwarf spring wheats rusting) resistance is emphasized. As defined by of the Green Revolution came to occupy most of Caldwell (1968), slow rusting is a type of resis- South and West Asia in the 1960s. Leaf (or tance in which disease progresses at a retarded brown) rust caused by P. triticina and stripe (or rate. It results in intermediate to low disease yellow) rust (Chen 2005) caused by Puccinia levels against all pathotypes of a pathogen. It is striiformis continue to pose a major threat to also characterized by adult plant resistance wheat production over large areas, particularly in despite compatible infection type at seedling Asia. In this continent, leaf and stripe rust could stage. In the process, DNA marker-assisted affect production on approximately 60 (63%) and selection (MAS) should be utilized when feasible.