Helminthosporium leaf blights: spot blotch and tan spot
E. Duveiller, H.J. Dubin

In recent years, Helminthosporium leaf blights (HLB), caused by both Bipolaris so-rokiniana and Pyrenophora tritici-repentis, have emerged as serious concerns for wheat cultivation in the developing world (Duveiller et al., 1998a). This situation arises from several factors. As a result of the better control of leaf and stem rust of wheat in the warmer environments, new biotic yield constraints have appeared, particularly in the marginal areas (Dubin and van Ginkel, 1991). Most importantly, due to the increasing food demand and to limited agricultural land, wheat is grown in more intensive cropping systems. An example is the rice-wheat rotation covering 12 million ha in South Asia. It replaced rice-fallow and has been used for the last 30 years. In this cropping system, there is a growing body of evidence that stress conditions are increasing the severity of leaf blights (Dubin and Bimb, 1994). Bipolaris sorokiniana is the prevailing pathogen, and disease severity is related to unbalanced soil fertility. Recent initiation of profitable technologies, such as reduced and conservation tillage, appear to change the aerial disease spectrum and require study (Dubin and Rajaram, 1996). Similarly, the increase in severity of tan spot due to P. tritici-repentis has been linked with the change in the variety composition and expansion of zero-tillage cultivation, especially in Brazil and Paraguay. This change is significant considering that the zero-tillage area in the State of Parana, Brazil, has increased from less than 50 000 ha in 1975 to over 350 000 ha in 1990 (Kohli et al., 1992). In advanced countries, tan spot is an important foliar disease in the prairies of Canada (Fernandez et al., 1998; Gilbert et al., 1998) and United States (McMullen and Hosford, 1989), and in Australia (Rees and Platz, 1992).

YIELD LOSSES AND DISEASE DISTRIBUTION

Yield losses due to HLBs are variable but are considered to be very significant. In farmer's fields, losses up to 20 percent have been reported, and in several areas it is the major biotic constraint that hampers growing wheat as a commercial crop (Duveiller and Gilchrist, 1994). In South Asia, Dubin and Bimb (1994) reported losses close to 30 percent in experimental studies. Disease severity increases dramatically with stresses favoured by plough pan, poor soil drainage or drought and late planting (Dubin and Rajaram, 1996). These conditions prevail in the Indo-Gangetic plains (Saari and Hettel, 1994), particularly in the poorest wheat-growing regions of India (e.g. eastern Uttar Pradesh, Bihar and West Bengal), the wheat belt of Bangladesh, the Nepali Terai and parts of China. Moreover, leaders from National Agricultural Research Systems (NARS) stressed the increasing need for a better control of leaf blights under optimum and irrigated conditions characterized by irrigated, low rainfall and temperate growing conditions (van Ginkel and Rajaram, 1993; CIMMYT, 1995). Yield losses assessment conducted on-station in Bhairahawa, Nepal, showed losses of 24 to 27 percent in highly susceptible varieties, and on-farm studies indicated losses up to 16 percent (Bhatta et al., 1998). In farmers' fields in Bangladesh, the average losses due to spot blotch was estimated to be 15 percent (Alam et al., 1998), and in Heilongjiang, China, losses of 15 percent have been reported in susceptible genotypes (Xiao et al., 1998). In Poza Rica, Mexico, a hot spot for screening for resistance to spot blotch, 49 to 90 percent yield losses have been found, but wheat is not grown as a commercial crop in this area (Duveiller et al., 1998b).

Tan spot was ranked as the most economically important wheat disease in North Dakota, United States (McMullen and Nelson, 1992). In the Mixteca region of Mexico, on-farm trials indicated that plots under zero-tillage and under severe natural infection by tan spot yielded 37 percent less than plots with one spray of propiconazole (Osorio et al., 1998). In Argentina, tan spot is recognized as a major leaf blight, and severity levels above 50 percent are not uncommon with potential yield losses of 10 to 20 percent (Annone, 1998).

SYMPTOMS

Typical symptoms caused by B. sorokiniana and P. tritici-repentis are found in regions where the spot blotch and tan spot diseases occur separately. However, it is often impossible, without the help of a microscope, to recognize the causal organism of a blight symptom in wheat-growing areas where both pathogens occur together, such as in the eastern plains of South Asia. Therefore, the term Helminthos-porium leaf blight is commonly used to refer to this disease complex.

Bipolaris sorokiniana is an aggressive pathogen that causes spot blotch, root and crown rots, node cankers and head and seedling blight (Zillinsky, 1983). Early lesions are characterized by small, dark brown lesions 1 to 2 mm long without chlorotic margin. In susceptible genotypes, these lesions extend very quickly in oval to elongated blotches, light brown to dark brown in colour. They may reach several centimetres before coalescing and inducing the death of the leaf. Fruiting structures develop readily under humid conditions and are generally easily observed on old lesions. If spikelets are affected, it can result in shrivelled grain and black point, a dark staining of the embryo end of the seed.

Typically, the first foliar symptoms of tan spot appear as small, light brown blotches that develop into oval-shaped, light brown, necrotic lesions bordered with a yellow halo (Schilder and Bergstrom, 1993). As lesions age, they coalesce and favour the early senes-cence of the entire leaf. Necrosis often begins near the tip and progresses towards the base of the leaf. Conidia are not always readily observable on samples collected in the field, and incubation on moist blotting paper may be required. However, samples collected in South Asia showed that conidiophores without spores can often be detected on lesions, suggesting that the disease is probably overlooked (Maraite et al., 1998).

THE PATHOGENS

Both pathogens are non-specific, as they lack clear physiological specialization, can infect several hosts (Misra, 1973; Krupinsky, 1992a) and thus can pass from one crop to another. Spot blotch refers to the disease caused by B. sorokiniana (Sacc. in Sorok) Shoemaker (syn. Helminthosporium sativum) (teleomorph: Cochliobolus sativus [Ito, Kuibayashi] Drechsler ex Dastur). In nature, the teleomorph of the fungus is only reported to occur in Zambia, where two different mating types must appear together (Raemaekers, 1991). Conidiophores are short, mostly single and bear one to six conidia. Conidia typically have five to nine cells, are ellipsoid, dark olive-brown, mostly straight to slightly curved with a thick wall and measure 60 to 120 x 12 to 20 µm (Zillinsky, 1983). Colonies on potato dextrose agar (PDA) are olivaceaous brown to black. In old cultures, the number of conidia is so abundant that the all culture turns black and shiny.

Pathogenicity studies on a global collection of B. sorokiniana monoconidial strains on a differential set of wheat entries showed that differences between strains existed but no clear host specialization was found (Hetzler et al., 1991). Similarly, Ruckstuhl (1998) found differences between groups of strains from South Asia, Mexico and Bolivia based on random amplified polymorphic DNA (RAPD) analysis but without any relationship to pathogenicity. These studies further support the data that B. sorokiniana forms a continuum of strains differing in aggressiveness but without clear physiological specialization (Maraite et al., 1998). Toxins produced by B. sorokiniana are non-specific and not directly associated with aggressiveness.

Tan spot (also known as yellow spot) is caused by P. tritici-repentis, which is the sexual stage of Drechslera tritici-repentis (Died.) Shoemaker (syn. H. tritici-repentis Drechs.). On the leaf, lesions are due to the anamorph of the pathogen, characterized by long multicellular spores, whereas the ascospores are formed in pseudothecia developing on the wheat residue. The anamorph is also found on stems, glumes, seeds and stubble. Septate conidiophores measure 80 to 400 x 6 to 9 µm and bear a single subhyaline one to nine septate conidium, which is 80 to 250 x 14 to 20 µm in size and easily recognized by its basal cell suggesting a snake head (Schilder and Bergstrom, 1993; Zillinsky, 1983). On PDA, the pathogen grows as dense, fluffy, greenish-grey mycelium without sporulating (Schilder and Bergstrom, 1993). On V8 agar medium, the mycelium is white to light grey. Conidia are produced after exposure to 12 to 24 hours near-UV light followed by 12 to 24 hours of darkness (Schilder and Bergstrom, 1993). Immature pseudothecia are formed in old cultures. The pseudothecia are black, erumptent, 0.2 to 0.35 mm in diameter with dark spines surrounding the short beaks and form on weathered stubble or straw. At maturity, they contain asci with eight ascospores measuring 40 to 60 long x 18 to 25 µm wide (Zillinsky, 1983). The mature ascospores are yellow-brown, oval-shaped, have three transverse septa and one or two longitudinal septa. Ascospores and conidia may both serve as primary inoculum.

Preliminary results of RAPD analysis showed polymorphism among strains, but no correlation could be established for these strain differences and geographic origin, pathogenicity or toxin production (Di Zinno et al., 1998). Similarly, virulence tests conducted by Shah and Fehrmann (1992) did not show presence of physiological races, although significant interaction could be detected between genotypes and isolates from four geographic locations. However, Lamari and Bernier (1989) grouped isolates on pathotypes based on their ability to induce tan necrosis toxin and extensive chlorosis (nec+, chl+), tan necrosis only (nec+, chl-), extensive chlorosis only (nec-, chl+), or neither necrosis nor chlorosis (nec-, chl-). Recently, Lamari et al. (1995) found a new virulence type in the nec-, chl+ pathotype of P. tritici-repentis and proposed to adopt a race classification system similar to the one used in cereal rust (Lamari and Sayoud, 1997). Strains inducing tan lesions produce a host selective necrosis inducing toxin in culture medium (Lamari and Bernier, 1989; Tomas et al., 1990; Orolaza et al., 1995; Tuori et al., 1995).

EPIDEMIOLOGY

Most Helminthosporium species are favoured by moderate to warm temperatures (18° to 32°C) and particularly by humid weather. Spot blotch is probably the most serious leaf blight disease of wheat in the ME5A mega-environment characterized by high temperature (coolest month greater than 17°C) and high relative humidity (see chapter "CIMMYT international wheat breeding" for a description of mega-environments; van Ginkel and Rajaram, 1993). However, it has been increasingly recognized as a problem in optimum and irrigated conditions also known as the ME1 mega-environment characterized by irrigated, low rainfall and temperate growing conditions (van Ginkel and Rajaram, 1993; CIMMYT, 1995). Tan spot occurs on all wheat-growing continents. It usually appears in more temperate environments than spot blotch; however, a wide range of climatic conditions are conducive to the disease, and it can be found both in ME1 and ME5 mega-environments.

The source of B. sorokiniana inoculum is infected seed, infected crop residues, volunteer plants, secondary hosts and free dormant conidia in the soil (Reis, 1991). However, the role of infected seed as a primary source of inoculum appears to be important, and according to Shaner (1981), it is the main source of inoculum of leaf blight pathogens. Transmission of the pathogen from infected seeds to plumules and coleoptile tips may reach an efficiency of 87 percent for B. sorokiniana (Reis and Forcelini, 1993). Immediately after planting, the fungus starts growing on the moistened seed, and just after emergence, as early as the first leaf stage, sporulation is induced in the presence of direct sunlight (Spurr and Kiesling, 1961). Conidia produced on the first leaves can be transmitted by rain splashes and wind, thus building up polycyclic epidemics. Conidia germination on the leaf surface can be completed within four hours. Appressoria form frequently at the juncture of the epidermal cell wall after eight hours. Hyphae from initially infected cells enter ad-jacent cells in 24 hours, and host cytoplasm becomes granular 24 hours after pathogenesis (Bisen and Channy, 1983). Although spot blotch severity is directly related to the duration of leaf moisture and high temperature in the field (Reis, 1991), little is known about the effects of environmental conditions on components of resistance that may allow a better measurement of small, quantitative differences of resistance among wheat genotypes (Duveiller et al., 1998b). Couture and Sutton (1978) indicated that spot blotch will develop when leaves remain wet for more than 18 hours with a mean temperature greater than 18°C. Therefore, sowing density is likely to affect disease severity, as dew and moisture may be higher or stay longer in the canopy, as a result of the favourable microclimate for disease development. The survival of conidia in the soil can reach up to 37 months (Reis et al., 1998); however, in the rice-wheat system it is not known if spores survive under the flooded paddy field or are able to infect the rice plant sufficiently to allow a green-bridge that may increase the load of primary inoculum in the subsequent wheat crop. The number of spores in the field will be maximized at crop maturity. High spore concentration, longer relative humidity and higher temperatures may overcome plant resistance and induce faster epidemics.

The leaf phase of the disease is the most obvious and more aggressive, and therefore efforts towards better disease resistance are concentrating on leaf blight. However, B. sorokiniana is also associated with common root rot. It is considered that losses due to infection of roots and sub-crown inter-nodes are significant and require attention (Dubin and Bimb, 1994). Spot blotch on above-ground plant parts is prevailing in the warmer wheat-growing areas, whereas root rot is causing more problems in regions such as Queensland, Australia (Tinline et al., 1988), the prairies of Canada (Ledingham et al., 1973) and Rio Grande do Sul in Brazil (Reis, 1991). Since crop rotation and environmental conditions influence inoculum potentials, research is needed on disease interaction with soil conditions and farmer-dependent factors, such as fertilizers, organic matter, water, plant residues and weeds. Nitrogen deficiency may play an important role in the development of leaf blights. Furthermore, the roles of phosphorus and potassium fertilization appear to be important as well. In general, it is recognized that leaf blight mainly caused by B. sorokiniana is increased under stress conditions. This is particularly important under late-sown conditions in South Asia, where terminal heat stress may speed up the senescence of green leaf tissue thus favouring the development of B. sorokiniana.

Tan spot has long been associated with fields in which relatively large amounts of straw remain on the soil surface through conservation practices (Bockus and Claassen, 1992). Residue is considered the main source of primary inoculum in areas of intensive wheat production in North America. Infected seed, other grasses and volunteer wheat constitute additional sources of inoculum, mostly in the form of conidia (Schilder and Bergstrom, 1993). Typically, the teleomorph is produced on the straw during the winter, and ascospores are produced in the following spring. Pseudothecia tend to discharge ascospores under humid conditions at night (Rees and Platz, 1980). The ascospores are shot out of the ascus over a short distance of several centimetres (Maraite and Weyns, 1982). It is followed by secondary infections on the upper leaves that are directly related to yield losses (Rees and Platz, 1983). As with spot blotch, the level of conidia in the field increases during the cropping season. In the United States, a recent study of tan spot based on collection of conidia and ascospores from wheat straw showed the higher recovery of conidia compared to ascospores, which indicates the importance of conidia in the epidemiology (Krupinsky, 1992b). Late-season conidial levels follow a diurnal pattern typical of graminicolous helminthosporia (Francl, 1997), and peak dispersal of conidia occurs during afternoon hours (Morrall and Howard, 1975; Platt and Morrall, 1980a, 1980b). It corresponds to conidiogenesis at night when humidity is high and liberation after leaf or other conidia-bearing surfaces are dried (Francl, 1997). Pyrenophora tritici-repentis can travel on the order of kilometres to tens of kilometres according to a deterministic forward trajectory model, suggesting that inter-field dispersal of viable conidia of P. tritici-repentis is a weighted factor in tan spot epidemics wherever wheat is grown intensively (Francl, 1997).

Pyrenophora tritici-repentis spores germinate and infect wheat over a wide range of temperatures when the leaves are wet for a specified period. Severe spotting will occur on susceptible varieties if leaves are wet for 12 hours, but 18 to 24 hours may be needed on more resistant genotypes (McMullen and Hosford, 1989). Resistance to tan spot is partly modulated by temperature (Lamari and Bernier, 1994) and nitrogen availability (Huber et al., 1987; Duveiller, unpublished data). Krupinsky et al. (1998) showed that at a low nitrogen level, the severity of leaf spot diseases was higher for zero-tillage than for conventional tillage. They showed that when significant differences were evident between nitrogen treatments, higher levels of diseases were generally associated with the low nitrogen levels as compared to the higher nitrogen levels (Krupinsky et al., 1998).

In South Asia, the incidence of B. soro-kiniana prevails over the occurrence of P. tritici-repentis (Dubin and Bimb, 1994; Nagarajan and Kumar, 1998). However, in Nepal P. tritici-repentis prevailed in the Terai area until the late 1980s. In recent years, the situation has reversed, and B. sorokiniana has become the most important and frequent pathogen (Bhatta et al., 1998). The reasons for this change are not clearly identified but appear to be associated with climate. However, surveys conducted over several years in several countries of South Asia indicated that symptoms caused by P. tritici-repentis might be overlooked (Maraite et al., 1998).

CONTROL OPTIONS

An integrated approach

The best way to control Helminthosporium diseases is through an integrated approach (Dubin and Duveiller, 2000; Mehta, 1993). It includes the use of a variety of resistance sources, such as hexaploid wheat from Brazil and China (some of which is rate-limiting), alien genes and synthetic wheats. In addition, appropriate management practices that enhance the health of the plant populations, in general, are critical. Cooperation of pathologists, breeders and agronomists will be necessary to ensure sustainable control of this group of diseases. Economic feasibility of recommended practices has to be determined as part of the research. Options for controlling tan spot and spot blotch include disease-free seed, seed treatment with fungicides, proper crop rotation and fertilization, cultural practices in order to reduce inoculum sources, the use of chemicals and the research of disease resistance. The latter offers the best long-term control at no cost for the farmer and is ecologically safe.

Seed health

In Brazil, it is recommended not to plant seed lots with more than 30 percent black point in order to limit spot blotch (Reis, 1991). Seed treatment may prove to be appropriate, although the inoculum remaining on secondary hosts or in the soil may reduce the treatment efficiency. Seed treatments with phytoalexin inducer appeared to provide good protection to wheat seedlings against B. sorokiniana infection (Hait and Sinha, 1986). Seed treatment with fungicide will help protect germinating seed and seedlings from fungi causing seedling blights. Fungicide seed treatments include: captan, mancozeb, maneb, thiram, pentachloronitrobenzene (PCNB) or carboxin guazatine plus, iprodione and triadimefon (Stack and McMullen, 1988; Mehta, 1993). Seed-borne inoculum of P. tritici-repentis can be controlled with seed-applied fungicides, such as guazatine and guazatine + imazalil, but other chemicals are also effective (Schilder and Bergstrom, 1993).

Rotations and crop management

Clearing or ploughing in the stubble, grass weeds and volunteer cereals reduce inoculum as does crop rotation (Diehl et al., 1982). Reis et al. (1998) indicate that eradicant fungicide treatment of the seed and crop rotation with non-host crops can control spot blotch. In the rice-wheat system of South Asia, little work has been done on the epidemiology of HLB and how management of the rotation crops affects spot blotch and tan spot, except as noted earlier. More quantitative information is required on the role of alternate rotations, soil and plant nutrition, inoculum sources and climate. In the rice-wheat system, there is a need for timely planting of wheat, better stand establishment and root development, increased soil organic matter, sufficient levels of macro- and micronutrients, and water and weed management (Hobbs et al., 1996; Hobbs and Giri, 1997). Crop rotation and organic manures will play a major role in HLB. This should favour beneficial soil organisms as well as better plant nutrition. In the rice-wheat system, it will be necessary to break the rotation with other crops to make it more sustainable, and this should help reduce disease problems in general. The use of oilseed rape in South Asia is common in mixture with wheat or in rotation. Since rape is known to have some fungitoxic effects upon decay, its effects on HLB would need research (Dubin and Duveiller, 2000). In the HLB complex, rotations would need to be sufficiently long to reduce the amount of soil inoculum. Cook and Veseth (1991) note that the kind of rotation crop may not be so important to root health as the length of time out of wheat. The rotation crops and length of rotation would have to be studied in relation to HLB.

Apparently, sound management recommendations may antagonize specific diseases as in the case of tan spot. Tan spot has been controlled largely by cultural practices, such as rotation with non-host crops and removal or burial of stubble (Rees and Platz, 1992). Bockus and Claassen (1992) observed that rotation to sorghum was as effective as ploughing for control of tan spot, and under certain conditions, crop rotations as short as one year controlled tan spot. In South Asia, recent work by Hobbs and Giri (1997) indicates that minimum tillage may be the best way to reduce turnaround time from rice to wheat and thus permit the planting of wheat more timely. Since this probably increases inoculum of tan spot, it highlights the need for integration of disciplines to determine how best to achieve attainable yields.

Fungicides

Although pesticide use should be minimized, fungicides have proven useful and economical in the control of tan spot (Loughman et al., 1998) and spot blotch (Viedma and Kohli, 1998). The triazole group (e.g. tebuconazole and propiconazole) especially has proven to be very effective for both HLBs, and their judicious use should not be overlooked. However, it may provide acceptable control but not always economic return in commercial grain production. This is dependent on the price received for the wheat, the price of the fungicide and the percent yield increase from using the fungicide. Situations will differ significantly according to geographical areas and cropping conditions. Spot blotch in particular is a very aggressive disease, and under a favourable environment, spraying at one- to two-week intervals for as long as necessary may be needed to maintain the disease under control.

Breeding for resistance

Studies of the inheritance of resistance to spot blotch some years ago indicated monogenic or oligogenic dominant resistance, and recent work indicates this, as well as polygenic resistance (Adlakha et al., 1984; Velazquez, 1994; Sharma et al., 1997a). Heritability estimates by Sharma et al. (1997b) indicate moderate to high levels of heritability. Table 17.1 indicates recent results on the number of resistance genes in some key germplasm.

Novel sources of resistance for spot blotch are available in the synthetic hexaploid wheats using different sources of Aegilops tauschii (syn. Triticum tauschii) and durum wheats (Villareal and Mujeeb-Kazi, 1996; Mujeeb-Kazi, 1998). Very promising results have been reported recently. The resistances come from the D genome of Ae. tauschii. Further research is being done to characterize the resistance involved.

TABLE 17.1
Number of resistance genes for Bipolaris sorokiniana in some wheats

^a Chyria 1 and Cugap are derived from Agropyron curvifolium.
Source: Velazquez, 1994; Sharma et al., 1997b.

Work at the International Maize and Wheat Improvement Center (CIMMYT) and in Bangladesh, India and Nepal has recently shown moderate levels of resistance to HLB in diverse germplasm of hexaploid wheat and in wheat wide-crosses (Table 17.2). No source of resistance gives high resistance in early-maturing genotypes, but careful crossing and selection methods should permit obtaining good resistance levels (Dubin and Rajaram, 1996; Dubin et al., 1998). Recent procedures using three-way crosses in Nepal are providing acceptable resistance and adapted progeny. The first two crosses may be with resistance from Brazil, China or other areas, or of alien origin. The third cross is with a well-adapted cultivar that has modest HLB resistance. Care is taken not to be too stringent in the selection for HLB resistance until the F4 generation since some of these resistances are rate-limiting and polygenic. Leaf rust resistance, earliness, stay-green effect and plump, healthy seed are requisites for acceptance of the progeny (Bhatta et al., 1998). Since all screening is carried out in the adult plant and in the field, resistance for both spot blotch and tan spot are obtained. This is necessary since they both commonly occur together in this area. It has been found that lateness is correlated with higher levels of resistance, but early selections can be obtained and need to be used as parents to obtain HLB-resistant and early parents as needed in South Asia (Dubin et al., 1998).

TABLE 17.2
Some wheat lines with useful field resistance to Bipolaris sorokiniana in South Asia

^a AUDPC = area under disease pressure curve.
^b R = resistant; MR = moderately resistant; S = susceptible.
Source: Dubin et al., 1998.

At CIMMYT, directed crosses are made between high-yielding parents from target countries and distinct sources of spot blotch resistance. Early generations are shuttled between the Mexican highlands and coastal plains area for adaptation and disease resistance. As a special part of its disease resistance programme, spot blotch resistance is screened at Poza Rica in the Mexican tropics in generations F4 to FLines showing slow disease development, good finish and plump, healthy grain are selected. The best lines are recycled in the programme (van Ginkel and Rajaram, 1998). Recent studies on estimating levels of HLB in the field indicate that the area under disease progress curve (AUDPC) gives the most precise information but that percent HLB or double digit scale can be useful as well, depending on the goals of the study (Dubin et al., 1998; Duveiller et al., 1998b; Franco et al., 1998).

Since diversity rather than uniformity in nature is the norm, some form of mixtures may be useful in reducing the HLBs. Crop and cultivar mixtures are not uncommon in South Asia and would be acceptable. Data from Nepal indicate that appropriate cultivar mixtures can reduce HLB as well as increase yields (Sharma and Dubin, 1996). Care would have to be taken in making the mixture to ensure compatible components, and more research is needed to see how long the mixture persists before a new one should be made.

Tan spot resistance is available in bread wheat from many areas, Brazil in particular, and generally appears to be oligogenic and recessive (Rees and Platz, 1992; Shabeer and Bockus, 1990; Wilson and Loughman, 1997). Resistance is incomplete and can easily be detected qualitatively based on infection type. However, intercrossing of apparently unrelated resistance sources suggests a considerable part of the resistance in these sources is common (Rees and Platz, 1992). Screening for resistance to tan spot under natural infection is not always considered reliable enough for routine field screening (Brûlé-Babel and Lamari, 1992). Artificial infection can be obtained by spreading infected straw or by spraying known concentrations of inoculum and covering with plastic tents to maintain humidity (Lamari and Bernier, 1989). However, these methods cannot be applied for large nurseries. First, it is difficult to produce a considerable amount of inoculum in the laboratory for field inoculation, and secondly, it is critical that field inoculation is followed by conditions of high field humidity. Also, P. tritici-repentis cultures in the laboratory result in production of inocula containing mixtures of conidia, conidiophores and hyphal fragments complicating inoculum quantification and disease assessment based on percentage of diseased leaf area. As a result, rating of reaction types as opposed to percent infection is sometimes preferred (Lamari and Bernier, 1989; Brûlé-Babel and Lamari, 1992). None-theless, in Mexico, the most favourable and practical screening conditions for the assessment of tan spot resistance were under natural epidemic in the Mixteca region, where wheat is grown in monoculture and under zero-tillage (Osorio et al., 1998). The repeatability of experiments conducted under controlled conditions should increase the accuracy of genetic resistance evaluation (Zhang et al., 1998). Also, the estimate of infection efficiency should prove useful in the identification of virulent isolates of P. tritici-repentis and should lead to improved identification of resistance to tan spot (Evans et al., 1996).

Breeding and the use of different sources of resistance, especially durable types, are major contributions to sustainable management of the Helminthosporium diseases, but as noted, it must go hand in hand with other methods of management for the most reliable and long-lasting control.

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