The emergence of cereal fungal diseases and the incidence of leaf spot diseases in Finland

Fungal plant pathogens causing cereal diseases in Finland have been studied by a literature survey, and a field survey of cereal leaf spot diseases conducted in 2009. Fifty-seven cereal fungal diseases have been identified in Finland. The first available references on different cereal fungal pathogens were published in 1868 and the most recent reports are on the emergence of Ramularia collo-cygni and Fusarium langsethiae in 2001. The incidence of cereal leaf spot diseases has increased during the last 40 years. Based on the field survey done in 2009 in Finland, Pyrenophora teres was present in 86%, Cochliobolus sativus in 90% and Rhynchosporium secalis in 52% of the investigated barley fields. Mycosphaerella graminicola was identified for the first time in Finnish spring wheat fields, being present in 6% of the studied fields. Stagonospora nodorum was present in 98% and Pyrenophora tritici-repentis in 94% of spring wheat fields. Oat fields had the fewest fungal diseases. Pyrenophora chaetomioides was present in 63% and Cochliobolus sativus in 25% of the oat fields studied.


Introduction
Plant diseases are an outcome of the long-term interaction between plants and pathogens. Plants have evolved defence mechanisms while pathogens have evolved effectors to overcome plant defences (Stuke-nbrock and McDonald 2009). Changes in cropping systems and in climate are likely to maintain the plant-pathogen interactions (Gregory et al. 2009) and are assumed to play a key role in the emergence of infectious plant diseases (Anderson et al. 2004).
The term 'emerging disease' refers to a recent disease on a new host and/or in a new area, or it is a disease that has recently become important due to an increase in virulence (Giraud et al. 2010). A recent example of a new disease is Ramularia leaf spot on barley . Examples of pathogens with increased virulence are the Ug99 pathotype of wheat stem rust (Singh et al. 2008) and a new type of Puccinia striiformis (causal agent of stripe rust) adapted to warmer climates (Hovmøller et al. 2008). A change in virulence may result from plant disease control methods as in the case of the rapid emergence and spread of QoI resistance in Mycosphaerella graminicola populations (Torriani et al. 2009) or Puccinia triticina isolates able to overcome the resistance gene Lr9 (McCallum et al. 2010).
Cereal cultivation has a long history in Finnish agriculture. There are records of barley, oat, rye, and wheat cultivation from the 14 th century. Barley was the main cereal until the end of the 19 th century, when it was replaced by oat (Grotenfelt 1922). The acreage of barley cultivation began to increase rapidly again at the end of 1950 ( Figure 1) (Tike 2010). The total agricultural area in Finland in 2010 was 2.3 million hectares, of which cereals were cultivated on almost half the area (1.0 million ha), barley being the main one (0.45 million ha) (Matilda 2010a). A total of 4.2 milliard kilos of grain were produced in 2008, over half of which was used to feed livestock (Matilda 2010b).
Plant diseases cause significant yield losses in cereal crops and reduce grain quality, threatening food safety (Gregory et al. 2009). Oerke (2006) estimated that despite current crop protection practices, the average global yield loss in wheat attributable to plant pathogens is about 10%. In Finland, the average yield increase from fungicide treatments in field trials over four years was 11% in barley, and 13% in spring wheat ). However, environmental conditions, amount of inoculum, host susceptibility, host physiological growth stage and timing of the epidemic, all affect the degree of damage significantly (Duveiller 2007).
Reports on causal agents of cereal fungal diseases in Finland have existed since the end of the 19 th century (Karsten 1868). Liro (1917) and Grotenfelt (1922) informed farmers of the importance of destroying smut and other disease-causing fungi from cereal seed. Since then, the methods to control plant diseases have developed considerably  1910 1924 1929 1934 1939 1944 1949 1954 1959 1964 1969 1974 1979 1984 1989 1994 1999 2004 2009 Other and have concentrated on various important plant diseases at different times. Currently, 73% of Finnish farmers treat cereal fields with fungicides annually (Mäenpää 2010). The sale of fungicides in agriculture (in active ingredients) increased from 18 tonnes to 158 tonnes between 1953 and 2008 (Evira 2010). At the same time, there has been substantial development in disease resistance breeding (Jalli 2010). Pest surveys recognise the changes of pest status in time and also identify the indigenous and non-indigenous pest species. In Finland, the pest surveys are made on weeds (Salonen et al. 2011) and on insects (Vappula 1962). Previous survey on cereal leaf spot pathogens in Finland was conducted by Mäkelä (1975a,b). The aim of this study was to evaluate the current state of cereal leaf diseases in Finland with reference to historical and current research data. The questions addressed were: i) which cereal pathogen species have been identified in Finland; and ii) what is the incidence and distribution of the main leaf spot diseases in the Finnish spring cereal fields? The objective was to get a base for better understanding on the possible risks caused by plant pathogens in the Finnish cereal production. The updated information on the incidence of pathogens and their distribution is in the key role in advisory services, in developing novel durable plant protection methods and in disease resistance breeding strategies for future needs.

Material and methods
The first part of this study, based on literature survey, is an evaluation of the earliest records on cereal fungal pathogens diagnosed in Finland. The earliest available references on pathogen occurrence and collection year were surveyed. The literature is published mainly in Finnish. In this study, the pathogen names were changed to common names recommended by Species Fungorum (2010).
In the second part, we studied the incidence and distribution of cereal leaf spot diseases in spring barley, spring wheat and oats in Finland in 2009. The disease survey was co-organized with a weed distribution study (Salonen et al. 2011) and by ProAgria Rural Advisory Centres in order to cover the main cereal production area (Fig. 2). The total number of investigated fields was 107 in barley, 84 in spring wheat, and 64 in oats. The selected fields represented the most common cereal cultivars grown in Finland under both ploughed and reduced tillage systems, as well as for conventional and organic farming.
Leaf samples were collected between flowering and milk ripening growth stages from mid-July till early August. For 10 sites per field, 10 leaves were collected from 10 randomly selected plants; 100 leaves per field in total. The aim was to collect the oldest green leaves, usually the second or third leaf from the apex.
The leaves were dried and pressed between tissues immediately after collection. The visual symptoms were recorded for all dried leaves. The pathogen identifications were made using the agar plate method (barley and oat) or a PCR-test method (wheat). The oat and barley leaf tissues with lesions were surface sterilized in 50% ethanol for 15 s and in 2% NaOCI for 30 s, and then rinsed with distilled water and placed on water agar in sterile Petri dishes. Petri dishes were kept under near UV light and a 12:12 h L:D photoperiod at 18° C for 7-14 days. The pathogens were identified using a light microscope. From wheat samples, Stagonospora nodorum (stagonospora leaf blotch), Pyrenophora tritici-repentis (tan spot) and Mycosphaerella graminicola (septoria tritici leaf blotch) were identified using a specific PCR-test method as described by Jalli et al. (2011).

Literature survey
Based on the literature study, 57 plant pathogens causing cereal diseases in Finland have been reported to date (Table 1). In 25 cases, the data for collection year were not available. Reports were published

Field survey
The causal agents of fungal leaf spot diseases identified in the 2009 field survey were: Pyrenophora teres (net blotch), Cochliobolus sativus (spot blotch) and Rhynchosporium secalis (scald) in barley, Stagonospora nodorum, Pyrenophora tritici-repentis and Mycosphaerella graminicola in spring wheat and Pyrenophora chaetomioides (oat leaf spot), Cochliobolus sativus, and Stagonospora avenae in oats. Minor powdery mildew infections were identified on wheat and barley, and yellow rust infection was observed in one wheat field. Two of the 107 barley fields studied had no identified plant pathogens on the sampled leaves, and 46 fields carried all three barley pathogens. Disease symptoms were observed on 95% of the collected leaves. P. teres was present in 86 % (Fig.  3a), C. sativus in 90% (Fig. 3b) and R. secalis in 52% (Fig. 3c) of the investigated fields.
Based on the PCR-test, only one of the studied 84 spring wheat fields had none of the tested pathogens. No visual symptoms were observed in that field either. A positive PCR-result was obtained from 15% of the samples that had no clear visual symptoms. Five of the fields were infected by all three pathogens. Disease symptoms were observed on 92% of the collected leaves. S. nodorum was present in 98% (Fig. 3d), P. tritici-repentis in 94% (Fig. 3e) and M. graminicola in 6% (Fig. 3f) of the investigated fields.
Oat fields had the fewest fungal diseases. 26% of the tested fields had no fungal infection. 27% of the studied oat leaves were totally symptomless. P. chaetomioides was present in 63% (Fig. 3g), C. sativus in 25% (Fig. 3b), and S. avenae in 3% of the investigated fields.

Discussion
The main outcome of the literature study was the list of plant pathogens identified on cereals in Finland. Mention of the first report is missing in most cases, except for the most recently identified diseases, Fusarium langsethiae and Ramularia collo-cygni. Some of the papers reported the first observation of the sexual stage of the pathogen, including Phaeosphaeria avenaria f.sp. avenaria and Phaeosphaeria nodorum.
The most recently identified cereal pathogens in Finland, F. langsethiae on barley and oats, and R. collo-cygni on barley, are considered to be emerging pathogens also in Europe Torp and Nirenberg 2004). F. langsethiae may cause serious problems, mainly because of its toxin production. Since 2004, it has been detected in Finnish barley and oat fields annually (Parikka, P., MTT, personal communication). In contrast, even though R. collo-cygni is recognized as an important pathogen of barley in Northern Europe and New Zealand , and has been identified in Finnish barley field already since 2001, it has not yet caused any epidemics in Finland (Jalli, unpublished data).
As Mäkelä (1975b) concluded, the time of identification of a pathogen on cereals does not correlate with its history on wild plants in Finland. Most of the leaf spot disease causal agents, like Stagonospora species and R. secalis, have been isolated from numerous cultivated and wild grass species (Mäkelä 1974;Mäkelä 1975b). Plant pathogens are assumed to have co-evolved from their original host to their modern hosts during domestication thousands of years ago. This is shown, for example, with M. graminicola (Stukenbrock et al. 2007) and R. secalis (Zaffarano et al. 2008). Therefore, caution is needed when drawing conclusions on emergence of a specific pathogen. Instead, based on the literature, we can estimate which plant  pathogens have been significant in Finnish cereal production at different times. Even though the year of emergence of different fungal species on cereals in Finland is usually missing, the significance of different diseases at various times is illustrated in an interesting way in the literature. In the early 20 th century, the literature concentrated on smut and rust diseases while towards the end of the century the focus was more on leaf spot diseases. In general, the earliest publications reported most about identification and taxonomy of fungi while the later ones informed on applied research results on the importance and control methods of the diseases. Some of the papers published already at the beginning of the 20 th century are relevant today, such as that published by Pesola (1927) on the resistance of spring wheat cultivars to yellow rust. Currently yellow rust and host-plant resistance are again subjects of special interest in Europe and other parts of the world (Hovmøller 2008). There is a similar phenomenon with stem rust. After the heavy stem rust epidemics in Northern Europe in 1951 (Jamalainen 1953) this disease was rare or of minor importance in Finnish cereal fields until recent years (Jalli, unpublished data).
The disease survey, made on spring cereals in 2009, represents an opportunity to compare the changes in incidence and distribution of leaf spot diseases in Finland. Forty years ago, in 1971-1973, a similar type of survey was made for cereals (Mäkelä 1974;Mäkelä 1975a: Mäkelä 1975b. The surveys were made on the same crops covering the same cultivation areas. In barley and in oats, the causal agents were identified with methods comparable to each others. For barley, the frequency of fields infected by leaf spot diseases increased during the last 40 years. The incidence of P. teres infected fields has increased from 60 to 86%, C. sativus infected fields from 30 to 90%, and in R. secalis infected fields from 30 to 52%. In contrast, Pyrenophora graminea that was found in 30% of the fields in 1971-1973 was absent in the fields studied in 2009. P. teres, C. sativus and R. secalis survive in straw debris and are favoured by barley monoculture, while P. graminea is controlled using healthy or dressed seed (Mathre 1997). During the past 40 years, the barley growing area has increased 50% from 403 500 ha to 600 700 ha. At the same time, the total cultivated area has decreased from 2 526 400 ha to 2 026 800 ha (Matilda 2010a), which means that the possibilities for crop rotation have lessened. Barley monoculture together with the increase in no-or reduced tillage cultivation area in Finland (MAVI 2009) represent a risk for increase in straw-borne pathogen incidence. However, the data presented in this paper illustrate only the incidence, not the severity of different pathogens.
Efficient control methods, the active use of fungicides (Mäenpää 2010) and the moderate resistance level of Finnish barley cultivars (Kangas et al. 2009), all play a role in keeping the level of yield losses to a minimum. However, at the same time, large effective population sizes pose a risk of cancelling out efficient control methods (McDonald and Linde 2002). There is evidence that sexual recombination may also be influenced by climate change and milder winters (Garret et al. 2006). A recombinant pathogen can rapidly assemble new combinations of virulence alleles (McDonald and Linde 2002). In addition to surveys on the incidence of plant pathogens, surveys on virulence are needed when studying the emergence of plant pathogens. In Finland, no change in the virulence of P. teres f. teres has been observed during the last 15 years (Jalli 2010).
In spring wheat, the frequency of both S. nodorum and P. tritici-repentis infected fields has increased over 40 years. The number of S. nodorum infected fields has risen from 60-70 to 98% and P. tritici-repentis infected fields from 30 to 94%. In the study by Mäkelä (1975b), M. graminicola was only observed in winter wheat. In this study, M. graminicola was identified on spring wheat in Finland for the first time. The positive samples originated from South-West and Southern Finland.
During the last 40 years, the wheat cultivation area has increased by 25%, spring wheat covering 90% of the wheat growing area (Matilda 2010a). S. nodorum was considered the main wheat pathogen in Finland till 2004 when the first epidemics of P. tritici-repentis were observed . There is evidence that one reason for rapid increase of P. tritici-repentis is the practice of wheat monoculture combined with no-tillage cultivation (Huusela-Veistola et al. 2008). Moreover, P. tritici-repentis has been recorded in 50% of the studied Finnish Agropyron (Elymus) repens samples (Mäkelä 1971), a weed present in 30% of the spring cereal fields in Finland (Salonen et al. 2011). P. tritici-repentis is a fungus that became pathogenic on wheat only recently in comparison with other leaf spot diseases. Friesen et al. (2006) provided evidence that a gene encoding a virulence factor was transferred from S. nodorum to P. triticirepentis probably in the beginning of 1950s. This horizontal gene transfer significantly enhanced virulence and led to the emergence of a new disease of wheat.
M. graminicola has been relatively common on winter wheat tillers in early spring, but the disease has rarely developed further under Finnish conditions (Kurtto et al. 2005). However, since 2006, M. graminicola has also been observed on the upper leaves of winter wheat later in the growing season (Jalli et al. 2007). Furthermore, in other parts of Europe, the importance of Septoria tritici blotch, caused by M. graminicola, has increased particularly during the last decade, being currently the most important wheat disease in Europe (Kema et al. 2008). Contrary to P. tritici-repentis, M. graminicola is an ancient pathogen of wheat. The genetic data provide evidence that the domestication of an agricultural crop was accompanied by the domestication of a wheat-adapted fungal pathogen from a population infecting wild grasses in the Middle East approximately 10,500 years ago (Stukenbrock et al. 2007). The recent growth in the incidence of M. graminicola may partly be explained by the changes in climate as well as the increase in wheat growing area. Similar phenomenon has been observed with potato late blight epidemics in Finland. Earlier outbreaks of the potato late blight epidemics were associated with a climate more conducive to the disease and the lack of rotation (Hannukkala et al. 2007).
In oats, the incidence of P. avenae (observed in about 60% of the oat fields both in 1971-1973 and 2009) has remained stable during the past 40 years. Also C. sativus was moderately common in oats already in the previous study. In contrast to wheat and barley, the cultivation of oat has decreased 35% since 1970s (Matilda 2010a).
The information on the incidence of cereal leaf diseases in Finland is based on a survey made in one cropping season and reflects the situation under that season's environmental conditions. The PCRbased diagnostic method to identify wheat pathogen species is very sensitive compared with the agar-plate method used by Mäkelä (1975b). However, it is evident that the cereal leaf spot diseases have more importance today than they had 40 years ago. In Mäkelä's studies both S. nodorum and M. graminicola were considered to be of minimum importance and were observed most commonly in the southern parts of the country, while in our studies S. nodorum and P. tritici-repentis were common at all locations. P. teres, C. sativus and R. secalis were of minor significance in barley until the early 1970s while the seed-borne pathogen P. graminea was dominant in barley fields (Mäkelä 1972). Now the situation has become reversed. The low incidence of P. graminea is related to the increase in barley seed treatment. In 2009, 46% of the cereal seed was fungicide treated in Finland. Approximately 75% of the farmers use fungicide treated barley seed (Markkula, A., Syngenta, personal communication).
As a plant disease is an interaction of several components, the role of different factors in the complex function may only be discussed. A common factor affecting the plant pathogens' environment, besides climatic conditions, is the increased cultivation of susceptible host plants. Huusela-Veistola and Jauhiainen (2006) concluded that the risk of pea moth infestation increases if regional cropping area and frequency of pea cropping increases. Planting large areas of susceptible cultivars would increase the infection pressure exerted on other cultivars (Bingham et al. 2008). The wheat cultivars grown in Finland are only moderately tolerant (Kangas et al. 2009) to leaf spot diseases and the resistance level of barley has increased only during the last 15 years (Jalli 2010).
Our field study concentrated on fungal leaf spot diseases of cereals. However, there is evidence for the increased importance of Fusarium head blight (Parikka et al. 2010) and rusts (Jalli, unpublished data) on wheat, barley and oats. Stem diseases are important cereal pathogens in Finland (Hannukkala 1985). Also, the insect-transmitted virus diseases are becoming increasing relevant in Finnish cereal production. A recently emerged virus disease is wheat yellow dwarf virus (WYDV) on winter wheat (Lemmetty and Huusela-Veistola 2005). Even though novel pathogen species emerge rarely, the existing hosts and pathogens continuously interact, which may lead to novel virulence if no precautions are taken. Research on pathogen-host interactions, monitoring, risk assessments, multiprotection using cultural practices, crop rotation, durable resistant plants, fungicide used only when needed, and global cooperation among researchers and breeders, are the main strategies to avoid, or to minimize the impact of plant diseases in cereal production.