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B J Nielsen
Danish Institute of Agricultural Sciences, Research Centre Flakkebjerg , DK-4200, Denmark
Danish Agricultural University, Agrovej 10, DK-2630 Tåstrup, Denmark
G C Nielsen
Danish Agricultural Advisory Service, Udkærsvej 15, Skejby,DK-8200 Århus N, Denmark
Danish Plant Directorate, Skovbrynet 20, DK-2800 Lyngby, Denmark
In the short term there is no useful alternative to chemical seed treatment for controlling the important seed-borne diseases in cereals. The only way to secure high quality seed is by discarding seed lots which have more than a specified level of disease. Without any other effective control measure than seed treatment large quantities of cereal seeds would be expected to be discarded. Another reduction model might be a differentiated use of seed treatments in the different certified seed generations. An important condition for the models is a representative seed test which for the moment only is possible in spring cereals. The threshold levels for the different diseases should under these circumstances be re-evaluated. In the long term other measurements may be possible. Investigations have begun on variety resistance to demonstrate effective genetic resistance against leaf stripe and common bunt, but for the moment only little is known on the distribution of resistance in modern Danish varieties. Other possibilities may be microbiological products and non-chemical methods but also here further investigation is required.
Today most cereal crops are treated in Denmark on a routine basis. This routine treatment has maintained serious seed-borne diseases at a very low level for many years.
In Denmark legislation has dictated that the use of pesticides should be reduced by 50 % by January 1st 1997 compared with the average usage during 1981 to 1985. This goal was not achieved and the programme is now under review. Seed treatments were not included in the first programme but this is now under consideration and different strategies are discussed in order to reduce the use of fungicidal seed treatments. The work with strategies for reducing fungicidal seed treatments is part of a bigger review on different scenarios for pesticide reduction in Denmark. This paper summarises the present use of seed treatments in cereals in Denmark and possible alternatives to the chemical control. Different scenarios for reducing the routine use of seed treatments will be discussed .
THE PRESENT USE OF SEED TREATMENTS IN DENMARK
It is estimated that approximately 85% of the winter cereal acreage and 90% of spring cereal acreage are sown with certified seed in Denmark. A large proportion of both certified seed and farm-saved seed (85-90%) is treated against seed-borne diseases with products approved by the Danish Institute of Agricultural Sciences (Nielsen & Scheel, 1997). The total consumption of fungicide and insecticide seed treatments in Denmark is approximately 80-110 tons of active substance (table 1, Anon. 1995,1996,1997) and, in 1994, fungicide and insecticide seed treatments were respectively 11% and 12 % of the total applied. Compared with the total use of pesticides in Denmark, seed treatments only account for 2-3 % of the total use of active ingredients (table 1). The volume for 1996 are lower than previous years, mainly due to a reduction in the use of maneb and thiram. The use of guazatine in wheat has decreased but the use of bitertanol has increased.
The major use for seed treatments is on cereals (72%) with the greatest proportion on this on the winter crop (approx. 48 tons). Insecticide seed treatments are not frequently used on cereals
Table 1. Amount of fungicide and insecticide used as seed treatments for agricultural purposes in Denmark (Anon., 1995, 1996, 1997)
1994 1995 1996
kg active ingredients
carboxin 1410 1960 1280
bitertanol 20541 28932 37087
carbendazim 3095 1190
fuberidazole 1440 1880 2384
guazatine 19140 16547 7200
imazalil 6276 8469 5714
maneb 12378 4758
prochloraz 20 40
Other fungicides 1 30088 28325 20756
Total fungicide seed treatments 101258 92101 74421
All fungicides, in total 892000 1055329 630740
Fungicide seed treatments in % of all fungicides 11 9 12
Insecticides 2 11377 7964 8720
Seed treatments in total 112635 100065 83141
Total use of pesticides 3919000 4809000 3669000
Seed treatments in % of the total 2.9 2.1 2.3
1) hymexazole, metalaxyl, pencycuron, thiram, tolclofos-methyl 2) furathiocarb, mercaptodimethur. These pesticides together with maneb have mainly used been used in potatoes, beets, rape and peas.
THE IMPORTANT SEED-BORNE DISEASES IN DENMARK
Several serious seed-borne diseases would spread rapidly if cereals were grown without efficient methods of control. Ultimately this would lead to significant yield loss and a drastic reduction in seed quality. If no seed treatments were used, common bunt (Tilletia caries) in wheat, leaf stripe (Drechslera graminea) and loose smut (Ustilago nuda) in barley and possibly stem smut of rye (Urocystis occulta) would be expected to cause the greatest problems in Denmark.
Common bunt in wheat is subject to great concern, as infections make the seed unfit for bread and feeding purposes and this could cause the rejection of the entire crop. This disease has been seen more frequently in Denmark since 1989 mainly on farms where untreated seed has been used (Nielsen & Jørgensen, 1994). The situation is complicated by the fact that common bunt can be soil-borne (Nielsen & Jørgensen, 1994; Nielsen & Nielsen, 1994; Borgen & Kristensen, 1997). Leaf stripe in barley is commonly not seen but experiences from the early 1970's, where the use of mercury seed treatment was reduced, showed that the disease can increase rapidly if the control level is reduced. Other seed-borne pathogens like Microdochium nivale, Fusarium spp and Septoria nodorum which are very dependant on the climatic conditions during the growing season can also cause problems but there will not necessarily be an increased infection from one growing season to another. These diseases have not been very common over the past few years (Nielsen & Scheel, 1997) and expected yield losses under Danish conditions are estimated to be 5 10 % under moderate to severe infections.
Table 2. Seed borne diseases in wheat, barley, rye, triticale and oat and recommended threshold levels for seed treatments
|Crop||Diseases||Threshold levels for seed treatment|
|Wheat|| Fusarium spp.1)
Glume blotch of wheat (Septoria nodorum)
Common bunt (Tilletia caries)
| 15 % infected seeds
5 % infected seeds
|Barley|| Fusarium spp.1), Bipolaris sorokiniana
Fusarium spp.1), Bipolaris sorokiniana
Net blotch (Drechslera teres)
Leaf stripe (Drechslera graminea)
Loose smut (Ustilago nuda)
| 15 % infected seeds, winter barley
30 % infected seeds, spring barley
5% infected seeds
5% infected seeds
0,2 % infected seeds
|Rye|| Fusarium spp. 1)
Stem smut of rye (Urocystis occulta)
|15% infected seeds
|Oat||Fusarium spp. 1)||15 % infected seeds|
1) Includes Microdochium nivale (syn. Fusarium nivale) and Fusarium spp.
THRESHOLD LEVELS FOR SEED TREATMENTS
There is no general requirement for seed treatment in the Danish national regulations or certification schemes but the recommendations are based on certain threshold levels (table 2). These thresholds are determined on the basis of experiments and experience, as well as an understanding of the efficacy of current seed treatments (Nielsen & Scheel, 1997, Scheel, 1997). In practice, the threshold recommendations are seldom used and more than 85-90 % of certified seed is treated with approved products on a routine basis.
ALTERNATIVES TO CHEMICAL SEED TREATMENTS
Cereal production with limited use of or in the absence of seed treatments must be based on other equally effective control measures if seed health is to be maintained. Possible control measures are summarised in below.
Potential for resistant varieties
Resistance is, in principle, able to replace the chemical seed treatment but only minor efforts have been made regarding resistance towards seed-borne diseases and knowledge in this field is limited. In the short term the greatest potential is resistance against leaf stripe and common bunt.
Resistance to leaf stripe (D. graminea) is based on a combination of single genes, giving a high degree of resistance, and polygenic based resistance giving different degrees of resistance. Haahr, Jensen and Skou (1989) found race specific resistance in Danish and European barley varieties and discovered that the gene was closely linked to the MlLa-gene giving resistance to powdery mildew (Erysiphe graminis). The resistance has been shown to be effective in field trials (Skou, Nielsen & Haahr, 1994) but the distribution of the resistance gene in the modern Danish varieties is not known. The first results from screening in new Danish varieties show great variation in resistance. In field experiments 4 varieties out of 18 tested were resistant and 5 had moderate resistance (B J Nielsen, unpublished results). The experiment has continued with tests in most of the Danish varieties and some new breeders' lines.
In barley, most varieties exhibit passive resistance to loose smut because of closed blooming and the need for treatment can be focused on the most susceptible varieties. The possibility of improving varieties, for example by incorporating specific resistance, are considered poor within the near future.
In wheat at least 15 specific resistance genes (Bt) against common bunt (T. caries) are known from the literature, but also partial resistance has been described (Gaudet et al., 1993). A number of Danish winter and spring wheat varieties were tested and the results showed great variation in resistance. A number of varieties had full resistance including the Swedish varieties Tjelvar and Stava and a group of varieties had some resistance (B. J. Nielsen, unpublished results). Tjelvar and Stava have been marketed in Sweden and are resistant towards seed as well as soil-borne bunt (Jönsson and Svensson, 1990). However, since the resistance is based upon specific resistance genes, there is a risk that new virulent races could propagate. The bunt fungus varies a lot and the efforts to incorporate race specific resistance genes have not been very successful in the United States due to occurrence of new virulent races (Hoffmann and Metzger, 1976). Tests so far in triticale seem to show very low susceptibility to seed-borne pathogens. However, there is a continuous need to test these varieties.
Potential for biological control and other non-chemical methods
In recent years, there has been a development in biological control of plant diseases and several products are also suitable for seed treatment. However, the products are not fully developed and they require further testing for efficacy and their practical application. Other alternative control methods are available including hot water, hot air, irradiation of seed or brush treatment for diseases like bunt where there is surface infection. Organic products like acetic acid, mustard and milk products have shown some effects against common bunt in wheat (Borgen, Kristensen & Kølster, 1995; Borgen 1997). These alternative methods are for the moment not usable in practice but must be considered in future together with other control methods and possible integrated with a differentiated use of chemical seed treatment.
OPTIONS FOR reducing CHEMICAL seed treatment
Different scenarios for reducing the reliance on chemical treatment are discussed together with an evaluation of the consequences for Danish agriculture. In table 3 an overview is shown of the various stages of seed production and where decisions could be made about the need to treat or discard seed with infections above defined threshold levels.
Table 3. Different scenarios for managing seed health
PB seed B seed C1 seed C2 seed
(0.6) (4.9) (26.8) (229.5) 1
Current Situation + + + +
Reduced Chemical Scenario + + + +/- chem
Discard Scenario + + +/- disc +/- disc
Complete Reduction +/- disc +/- disc +/- disc +/- disc
where:-+ Routine, standard seed treatment
+/- chem Use of chemical seed treatment based on seed testing
+/- disc Seed lots discarded based on seed testing or use of alternative methods if available
PB Seed Pre basic seed used for producing basic seed
B seed Basic seed used for producing certified C1 seed
C1 seed Seed used for producing certified C2 seed
C2 seed Certified seed sold for ware production. C2 seed is not produced in rye
1) Amount of Danish certified seed 1996/97 in '000 t are indicated in brackets. Amount of farm saved seed is estimated to be 40000 t
Possible future strategies for seed health and seed treatment use have also been discussed elsewhere e.g. in UK where a limited 'treatment according to need' approach to the use of fungicide seed treatment to spring cereals were regarded as feasible in the short term with existing seed testing techniques (Paveley et al., 1996). This approach is very close to the 'reduced chemical scenario' stated in table 3. Long-term change towards a wider strategy of treatment according to need in UK and possible treatment based on compulsory testing of all seed was also discussed. The Danish discussion is more radical with focus upon a maximum in reduction in fungicide use eventually a complete reduction.
Reduced chemical model
In this scenario the breeding generations including certified seed C1 are treated with chemical seed treatments. This would reduce the possibilities for transmitting serious seed-borne diseases to the C2 generation. Pathogens like Microdochium nivale, Fusarium spp., Drechslera teres and Septoria nodorum could infect during the growing season. The big seed generation C2 (80 % of the seeds) is analysed for seed infestations and is only treated if the level of pathogens is above defined threshold levels (table 2). A more selective use of chemical seed treatments or alternative methods, if available, would be possible depending on target pathogens. A crucial condition for this model is a fast, effective and representative seed analysis in the period from harvest to sowing. This approach is only feasible with existing techniques in spring cereals where the expected reduction in chemicals would be between 30 and 70 %. This scenario is the most realistic even in the nearest future. If suitable analytical methods were available selective use of seed treatments would, however, also be feasible in winter cereals.
Reduced chemical model with discard of infested seed
The underlying principal in this scenario is the same as that above, but use of chemicals is only allowed during the production of basic seed (table 3). This ensures that seed with high seed health is produced. The seeds used for C1 or C2 are analysed and the seed lots are discarded if seed infestations are above threshold levels. In this case, however, the threshold levels have to be re-evaluated because they are defined for a system where chemicals are available. The threshold levels in C1 seed should be made as low as possible, especially for T. caries, U. nuda and D. graminea (zero tolerance). For C2 seed, the threshold levels could be as stated in table 2 because it is assumed that there will be no further multiplication of seeds and that the farmers buy new certified seed. The basic problem in this model is the same as stated above. The system must be based on seed analyses which for the moment is very difficult to achieve in winter cereals. The use of chemicals would be reduced by approximately 80 % but very large quantities of seeds could be expected to be discarded. If other non-chemical methods were available (e.g. biological control, alternative techniques ), they could be integrated to reduce the quantity of seed that would otherwise be discarded. This scenario is close to the principles regarding seed health in organic farming.
If the use of seed treatments for seed-borne disease control were ceased immediately in all seed generations, then the possibilities for producing cereals would be very limited, especially in the short term. The only realistic possibility, in the long term, would be seed analysis followed by rejection of the infested seed together with the full integration of alternative control measures. Resistance and biological control methods would play an important part but our knowledge today is too limited for us to tell if these methods could completely replace the use of chemical seed treatments
The development of a quick and reliable seed test will be important if there is to be any shift away from the current situation and if chemical seed treatments are going to be reduced, particularly for the 'reduced' and 'complete reduction' models, where lack of certified seeds is likely to be a problem.
There are some important problems with seed analysis, which must be solved before a threshold-based system can be used effectively. There is need for new, quick and reliably methods especially in winter cereals where there is a short time from harvest to sowing. An enlargement in the range of analyses would demand a fundamental and substantial expansion of the analytical capacity.
Another problem is specificity of the existing diagnostic tests. For example, leaf stripe (D. graminea) and net blotch (D. teres) of barley can not be separated from each other by a normal analysis. The threshold for net blotch of barley is different than that of leaf stripe and recommendations for seed treatment in barley could in many cases be triggered by the presence of seed-borne net blotch, which frequently occurs in barley. It is anticipated that a PCR-based seed health test that could detect and differentiate Drechslera spp. patogenic on barley will become available to agriculture in the future (Stevens et al., 1997).
The third and probably the most difficult problem to solve is the sampling technique. In Denmark, the basis for one seed analysis is 25 tonnes of seed, which strongly underlines the necessity to make any sample representative. There can be significant variation within the field crop and a seed batch may often be taken from several different fields. For the majority of stored seed, a representative sample can not be taken with today's sampling equipment and there is a need to develop new techniques.
Seed availability and seed quality
In the 'discard' and 'complete reduction' models there will be the potential for the rejection of large quantities of seeds including the valuable early seed generations and it may be necessary to increase the breeding area considerably if this approach was adopted. Furthermore, a move towards genetic resistance to seed-borne disease could mean that the choice of varieties is restricted and it may not always be possible to use the varieties with the highest yield potential. A reduction in the systematic seed treatment could entail uncontrolled spreading and propagation of a number of seed borne diseases and perhaps also of new, so far, rare diseases.
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