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New Fungicides and Their Role in Disease Control Programmes

B. Dunne, Teagasc, Crops Research Centre, Oak Park


SUMMARY

A number of new strobilurin containing fungicides will be available to cereal producers in 2002. Acanto is available as a straight strobilurin (picoxystrobin). Modem is another straight strobilurin (pyraclostrobin). Straight strobilurins need a non-strobilurin partner in all situations. Opera is a pre-formulated mixture of pyraclostrobin and epoxiconazole and Covershield is a three-way mixture of pyraclostrobin, epoxiconazole and kresoxim-methyl.

They have activity against a wide range of cereal diseases. In particular they give excellent control of net blotch in barley and Septoria tritici in wheat. They also have physiological effects on crop growth, which can result in yield increases even when disease levels are very low.

These products will have an important role in cereal diseases control programmes in the coming year. The new strobilurins, picoxystrobin and pyraclostrobin, have eradicant properties which the earlier strobilurins did not have. While they have this attribute they do not match the eradicant properties of the best triazoles.

Introduction

While fungicides have been used against various diseases of cereals, principally seed-borne diseases, for over one hundred years, it was only with the advent of systemic fungicides in the 1970s that fungicides became widely used on a routine basis in cereal production. In fact, they have become established as an essential input in the growing of cereals and without their use cereal yields would be severely reduced to uneconomic levels in many seasons.

Development of Cereal Fungicides

The first systemic fungicide to be widely used on cereals against foliar disease was ethirimol (Milstem), one of the hydroxypyrimidine group of fungicides and was used to control powdery mildew. This was used both as a seed dressing and as a spray. However, within a few years of use the mildew pathogen developed resistance to ethirimol.

The benzimidazole (MBC generating) fungicides were widely used in the 1970s to control a number of cereal diseases. They controlled eyespot and also had a good effect on foliar diseases like Septoria and Rhynchosporium, which helped to increase green leaf longevity and hence yield. More importantly, they improved the consistency of obtaining good yields from year to year. Resistance to these MBC-generating fungicides developed in the eyespot fungus in the early 1980s and they were of no further use in the control of this disease. However, because of their broad spectrum of activity against many foliar diseases and their low cost, they continued to be used in mixtures with other fungicide products. Eventually resistance to MBC-generating products appeared in Septoria, Rhynchosporium, Fusarium and other foliar pathogens and their use has declined.

Triazoles and Strobilurins

The mid-seventies saw the introduction of the fungicide group DMI or demethylation inhibitors which contain the triazole fungicides. The triazole fungicides have been the mainstay of cereal disease control since then. The first of these to appear was triadimefon (Bayleton) and there have been new triazoles appearing on a regular basis up to the time of the arrival of epoxiconazole (Opus). The triazoles are active against a wide range of cereal foliar diseases and were used alone or in mixtures with non-systemic fungicides and also with the systemic morpholine group of fungicides. Over the twenty-five years since triadimefon was introduced some of the triazoles have disappeared from the marketplace as resistance to them developed and they no longer provided any benefit or advantage in a disease control programme. However, the remaining ones play an important role in the control of cereal disease.

A new group of fungicides the STAR or strobilurin type fungicides, were introduced in 1997 with azoxystrobin (Amistar) being the first one available in Ireland. This was followed by kresoxim-methyl/epoxiconazole (Allegro) and trifloxystrobin (Twist) and famoxadone/flusilazole (Charisma), which is a non-strobilurin but has a strobilurin type action. Strobilurin type fungicides are based on natural anti-fungal compounds, which some forest wood decaying mushrooms secrete to inhibit competitor fungi. They have a novel mode of action to that of the other groups of fungicide products. They are also very safe from an environmental point of view.

Properties of Strobilurins

The strobilurin (QoI) group of fungicides (which includes the strobilurins and famoxadone) are broad-spectrum fungicides active against a wide range of diseases in many crops. They are excellent inhibitors of spore germination and are consequently first class protectant products. They all have the same mode of action, i.e. on entering the fungal cell they affect electron transport in the mitochondria - which are the energy source in cells - depriving the organism of energy and hereby causing cell death.

They largely remain in the waxy layer of plant parts and exert their fungicidal activities there. The various strobilurins do differ in their systemic properties with some of the group being partially systemic and others redistributing themselves around the plant in the wax layer and epidemidermal cells by vapour action.

They also have other desirable non-fungicidal physiological properties in that they can inhibit ethylene biosynthesis and consequently delay senescence and thus enhance yield in this way. They can also improve the assimilation of nitrogen into plants.

As the entire QoI group has the same mode of action then if resistance develops in a pathogen to one strobilurin product there will be cross-resistance to all the other members of the group. This has already occurred in the case of wheat powdery mildew. Consequently, the use of strobilurins in disease control programmes has to be managed to reduce the risk of resistance by other pathogens arising.

The three strobilurin-containing fungicides already available in Ireland have slightly different characteristics. Table 1 shows their activity against a number of wheat diseases.

They have become firmly established in cereal disease programmes in the four years since their introduction. As mentioned earlier, they are excellent protectant products but have little or no eradicant activity and rely on a partner usually from the triazole group to provide eradicant properties.

Table 1: Relative activity of strobilurins against wheat diseases

Strobilurin Eyespot Mildew Septoria nodorum Septoria tritici Yellow rust Brown rust
Azoxystrobin   * ** *** *** ***
Kresoxim-methyl/epoxiconazole * ** *** **** **** ****
Trifloxystrobin   ** ** *** ** **

* = poor

**** = very good

In 2002, two new strobilurin containing fungicides will be available to Irish cereal growers. They are Acanto and Opera.

These products are second-generation strobilurins in that they have properties and advantages that the first group of strobilurin released on the market do not possess. The most striking feature of the second-generation strobilurins, which is not present in the earlier strobilurins, is that they possess curative activity against plant diseases. Both products are rainfast a short time after application and remain active for a considerable time.

Acanto

This product is from the Syngenta agrochemical company. The chemical active ingredient is picoxystrobin. Acanto is a truly systemic fungicide in that it is systemic in the xylem. It also has vapour activity as have a number of other fungicides such as morpholines and trifloxystrobin. The vapour activity of fungicides allows redistribution of the chemical around the plant. As mentioned previously, Acanto has curative activity. This curative activity is not as good as that of the best triazoles on most diseases but against net blotch, in particular, it has superior curative activity to that of the triazoles.

Acanto is a broad-spectrum fungicide active against a wide range of both wheat and barley diseases.

Opera

Opera is manufactured and sold by BASF. It is also a broad-spectrum product. The active ingredients in the product are the strobilurin pyraclostrobin and the triazole epoxiconazole. The strobilurin component of Opera, pyraclostrobin, is not systemic but the epoxiconazole component is. The product remains primarily on the leaf surface bound to the waxy layer but as it has strong translaminar activity it penetrates the leaf where it can eliminate fungal mycelium already present, which is how it exerts its eradicant or kickback action.

Table 2: Disease spectrum of Acanto and Opera

Wheat Barley

Septoria tritici

Septoria nodorum

Brown Rust

Yellow Rust

Tan Spot

Fusarium nivale

Net Blotch

Rhynchosporium

Brown Rust

Ramularia

Powdery Mildew

Acanto has been included in trials at the Crops Research Centre, Oak Park since 1999 and Opera since 2000.

Table 3 shows results obtained from winter barley trials over three years.

Table 3: Winter barley fungicide trials at the Crops Research Centre, Oak Park

1999 2000 2001
Product % scald t/ha 15% m.c. Product % scald t/ha 15% m.c. Product % scald t/ha 15% m.c.
Acanto 4.3 7.8 Acanto 6.6 10.7 Acanto +
Menara
3.7 9.2
Amistar Pro 5.2 7.7 Amistar Pro 4.5 10.6 Twist
+ Menara
3.0 9.1
Allegro 3.6 7.8 Acanto + Stereo 5.4 10.8 Amistar +
Menara
4.6 9.5
Control 12.0 6.6 Allegro 4.2 10.8 Allegro 3.4 8.9
      Control 36.2 9.4 Opera 3.0 9.0
            Control 18.4 8.6
LSD 2.0 0.29   3.0 0.2   2.1 0.3

Rhynchosporium was the main disease occurring in these trials over the three years and levels were generally low. Yield responses were correspondingly low but the fungicides did give significant yield responses.

Acanto is particularly effective against net blotch and in two years trials on spring barley has given excellent control of net blotch and substantial yield increases. Opera has also good activity against the disease (Table 4).

Table 4: Spring barley fungicide trials

2000 2001
Product % Net blotch t/ha 15% Product % Net blotch t/ha 15%
Acanto 3.7 8.7 Acanto 3.0 8.1
Opera 5.4 8.3 Opera 4.0 8.1
Amistar Pro 5.7 8.3 Amistar Pro 3.4 8.4
Allegro 11.4 7.8 Twist 10.3 7.8
Twist 8.0 8.1 Control 67.0 3.7
Control 26.3 6.1      
LSD 3.4 0.6   1.0 0.9

Both Acanto and Opera have an effect in reducing the levels of the necrotic spotting that has been a problem on spring barley for the past few years Table 5.

Table 5: Effect of fungicides on necrotic spotting of barley

Fungicide % spotting 2nd. leaf Yield @ 15% (t/ha)
Allegro 1.35 8.0
Acanto 0.97 7.9
Opera 0.76 8.1
Control 1.67 7.5
LSD N.S. 0.26

N.S. = not significant

The eradicant effect of Opera is illustrated in Table 6. This shows results from a winter wheat trial carried out in 2000 in which there was very high pressure from Septoria tritici.

A single spray of a number of products was applied at Growth Stage (G.S.) 37, at which stage there was a high level of Septoria infection in the crop, and the reduction in Septoria levels in the Opera treated plots is an indication of its eradicative properties.

Table 6: Winter wheat trial 2000. Single spray at G.S.37

Fungicide Rate
l/ha
% Septoria
Flag leaf
% Septoria
2nd. leaf
% Septoria
3rd. leaf
Yield @ 15%
(t/ha)
Amistar + Opus 1.0 + 0.5 78 78 99 9.6
Allegro 1.0 80 78 98 9.5
Opera 1.5 59 43 90 10.7
Untreated   99 100 100 6.2
LSD   7.4 11 14 0.5

CONCLUSIONS

Two new fungicides, picoxystrobin and pyraclostrobin, have a broad disease control spectrum and eradicant and persistent action. These fungicides are ideally suited for use in disease control programmes in Ireland in 2002. There is obviously potential for more flexibility in their application timing than the existing fungicide products but for best effect they should be used at the current standard recommended timings. These timings are G.S. 31/32, 37/39 and 55/59 in wheat, G.S. 32 and 39/45 in winter barley and G.S. 31/32 and 39 in spring barley. They offer higher standards of control of net blotch and Septoria tritici than the existing products.