Development of disease pressure during the last 30 years

The most important diseases in winter wheat are Septoria leaf blotch (Septoria tritici) fol-lowed by powdery mildew (Blumeria graminis), stripe rust (Puccinia striiformis), leaf rust (Puccinia triticina), tan spot (Drechslera tritici-repentis) and fusarium head blight (Fusarium spp.). For the period up to 1985 only limited data exist to describe the disease pressure in the individual season. Focus will therefore be on the last 10-20 years.

Treatment Frequency Index (TFI)

generally been very flat. This has been shown in many trials annually (Jørgensen et al., 2008;

Pedersen, 2007). The economic optimum in resistant cultivars was a Treatment Frequency Index (TFI) between 0.25 and 0.5. On more susceptible cultivars, the optimum TFI was 0.5-0.9, and a 2-spray programme gave more flexibility with regard to the dose needed at the 2nd application. Figure 6.6 shows an example of calculated net yield gain in winter wheat in resis-tant (a) and susceptible cultivars (b) for selected strategies and two grain prices. The results are based on data from the National Field Trials in the Sjælland region 1999-2003.

Figure 6.6 Calculated net yield gain in winter wheat in resistant (a) and susceptible cultivars (b) for selected strategies and two grain prices. The legends for fungicide strategies are ranked according to the most beneficial solutions. B: GS 32-36, C: GS 37-50, D: GS 51-64. Data based on 5 years trial data from the National Field Trials carried out by DAAS-region Sjælland.

6.3.2 Conclusion: Impact of low dose strategies on yield potential

The actual gross yield is not optimized using fungicides as focus has been based on net yields rather than gross yield. Many years’ trials have shown that the best net yields are obtained by using reduced rates, typically between 0.4 and 0.75 depending on the year and level of resis-tance in the cultivar. The data have shown that the gross yield has been reduced by 4-5 dt/ha in the last 10-15 years due to the reduced treatment frequency policy in Denmark. From the point of view of the farmers’ economy the low dose policy is, however, still believed to be a sensible policy.

6.4 Development of disease pressure during the last 30 years

The most important diseases in winter wheat are Septoria leaf blotch (Septoria tritici) fol-lowed by powdery mildew (Blumeria graminis), stripe rust (Puccinia striiformis), leaf rust (Puccinia triticina), tan spot (Drechslera tritici-repentis) and fusarium head blight (Fusarium spp.). For the period up to 1985 only limited data exist to describe the disease pressure in the individual season. Focus will therefore be on the last 10-20 years.

Variability of disease pressure is closely linked to climate and the susceptibility of cultivars grown. Figure 6.1 indicates the yield response found across the time periods. This figure does not indicate any major changes in yield responses looking over the whole period, but as de-scribed previously the annual variation is a result of disease pressure, cultivars as well as fun-gicides used.

6.4.1 Link between resistance level in cultivars and grown cultivars

One way to investigate changes in disease risk over the years is to combine existing informa-tion on grown cultivars in specific years with their actual resistance ranking using an index. In the computer program Crop Protection Online (CPO www.planteinfo.dk ) all the varieties are assigned indices from 0-3 ranging from not susceptible to very susceptible (www.sortinfo.dk, Hagelskjær & Jørgensen, 2003).

Indexing varieties in susceptibility groups first started in 1999. Figure 6.7 shows the resis-tance index in winter wheat to the four leaf diseases that fungicide treatments can be aimed at.

The resistance index is indicated as the average group in CPO (group 0-3) weighted with dis-tribution of sold seed of the varieties that have been grown in Denmark. Therefore, the resis-tance index changes when changes occur in the distribution of varieties or if changes occur in the virulence profile of the pathogens (infection races). It can be noted that the resistance in-dex has been rising (= falling susceptibility) against yellow rust and mildew, fairly unchanged variety resistance against Septoria (however, rising resistance over the last few years (variety Ambition)) but falling resistance level against brown rust.

1998 2000 2002 2004 2006 2008 2010

Resistance Index [CPO weighted by area]

0,0 0,5 1,0 1,5 2,0 2,5

Mildew Septoria Brown rust Yellow rust

Figure 6.7 Resistance index in winter wheat to the four leaf diseases that fungicide treatments can be aimed at. (see text).

Variability of disease pressure is closely linked to climate and the susceptibility of cultivars grown. Figure 6.1 indicates the yield response found across the time periods. This figure does not indicate any major changes in yield responses looking over the whole period, but as de-scribed previously the annual variation is a result of disease pressure, cultivars as well as fun-gicides used.

6.4.1 Link between resistance level in cultivars and grown cultivars

One way to investigate changes in disease risk over the years is to combine existing informa-tion on grown cultivars in specific years with their actual resistance ranking using an index. In the computer program Crop Protection Online (CPO www.planteinfo.dk ) all the varieties are assigned indices from 0-3 ranging from not susceptible to very susceptible (www.sortinfo.dk, Hagelskjær & Jørgensen, 2003).

Indexing varieties in susceptibility groups first started in 1999. Figure 6.7 shows the resis-tance index in winter wheat to the four leaf diseases that fungicide treatments can be aimed at.

The resistance index is indicated as the average group in CPO (group 0-3) weighted with dis-tribution of sold seed of the varieties that have been grown in Denmark. Therefore, the resis-tance index changes when changes occur in the distribution of varieties or if changes occur in the virulence profile of the pathogens (infection races). It can be noted that the resistance in-dex has been rising (= falling susceptibility) against yellow rust and mildew, fairly unchanged variety resistance against Septoria (however, rising resistance over the last few years (variety Ambition)) but falling resistance level against brown rust.

1998 2000 2002 2004 2006 2008 2010

Resistance Index [CPO weighted by area]

0,0 0,5 1,0 1,5 2,0 2,5

Mildew Septoria Brown rust Yellow rust

Figure 6.7 Resistance index in winter wheat to the four leaf diseases that fungicide treatments can be aimed at. (see text).

Data from the national monitoring network can also be used as a source of information, when one investigates the changes in disease risk – see Figure 6.8 (Nielsen 2008, Nielsen & Jensen 2008, Jørgensen et al., 2008; www.lr.dk/regnet ). The septoria attacks vary annually but no specific trend in disease risk has been observed. The septoria attacks may seem severe, but it must be noted that registrations are made using percentage observations with > 25% attacked plants. Many plants are often attacked by Septoria, but the percentage coverage varies consid-erably over the years, even though there are 100% attacked plants.

1998 2000 2002 2004 2006 2008 2010

Percent observation with >25% attacked plants

0 20 40 60 80 100

Mildew Septoria Brow rust Yellow rust

Figure 6.8 Data from the plant production advisers’ monitoring network in 1999-2008

(www.lr.dk/regnet) in which registrations are made in untreated plots in the wheat varieties most commonly grown. The threshold for including data is >25% of plants attacked.

1980 1985 1990 1995 2000 2005 2010

Percent attack at GS71-75

0 20 40 60 80

Mildew Septoria Yellow rust

Figure 6.9 Data from the registration trials (DJF) in 1983-2007, in which assessments are made in untreated plots in commonly used wheat varieties. The data indicate a general increase in Septoria (from the early part of the 1980s and to the end of the 1990s) and a stable or reduced level of mildew and yellow rust.

Data from the national monitoring network can also be used as a source of information, when one investigates the changes in disease risk – see Figure 6.8 (Nielsen 2008, Nielsen & Jensen 2008, Jørgensen et al., 2008; www.lr.dk/regnet ). The septoria attacks vary annually but no specific trend in disease risk has been observed. The septoria attacks may seem severe, but it must be noted that registrations are made using percentage observations with > 25% attacked plants. Many plants are often attacked by Septoria, but the percentage coverage varies consid-erably over the years, even though there are 100% attacked plants.

1998 2000 2002 2004 2006 2008 2010

Percent observation with >25% attacked plants

0 20 40 60 80 100

Mildew Septoria Brow rust Yellow rust

Figure 6.8 Data from the plant production advisers’ monitoring network in 1999-2008

(www.lr.dk/regnet) in which registrations are made in untreated plots in the wheat varieties most commonly grown. The threshold for including data is >25% of plants attacked.

1980 1985 1990 1995 2000 2005 2010

Percent attack at GS71-75

0 20 40 60 80

Mildew Septoria Yellow rust

Figure 6.9 Data from the registration trials (DJF) in 1983-2007, in which assessments are made in untreated plots in commonly used wheat varieties. The data indicate a general increase in Septoria (from the early part of the 1980s and to the end of the 1990s) and a stable or reduced level of mildew and yellow rust.

The third source of information used to visualize any changes in disease pressure originates from DJF’s registration trials, which have data back to 1982. Data in Figure 6.9 show an in-crease in Septoria leaf blotch from the early part of the 1980s. Before this period this disease was not recognized as a major problem. Stagonospora nodorum dominated up to 1982 when a major shift occurred to Septoria leaf blotch (Septorio tritici) in Denmark as well as in many other European countries. Since the end of the 1990s no sign of a further increase in the Sep-toria problem has been observed (Figures 6.7-6.9).

6.4.2 Conclusion: Development of disease pressure during the last 30 years

It can be concluded that the plant breeders have been capable of developing resistant varieties with competitive yields faster than the pathogen populations have changed during the last 10-year period. Therefore nothing indicates that the disease pressure has increased from 1999-2008.

The impact from development of significant attacks of Septoria leaf blotch since the early 1980s compared with the attack of S. nodorum prior to this period cannot be quantified. As the fungicides during the same period have become more effective, the disease is not believed to have had a major impact on the total yield level. With respect to other diseases no major changes have been found, which could have caused the stagnating yields.