Biological and chemical control capability for Fusarium

In recent times, the market environment for all commodities has been challenging and cereals are no exception. Growers must rightly keep a close eye on all inputs and attention is now focusing on the T3 spray for winter wheat crops. In this regard, when deciding the final T3 spray, growers must have more than Septoria in mind, as Fusarium infection can cause significant losses and problems, especially with mycotoxins.

In order to maximise yield potential, a robust T3 spray is critical but it must be applied at the right time. In recent years, the yield benefits from ear treatments carried out in UCD trials have, on occasion, been significant. However, these yield benefits can also be small as the yield benefit and response depends on the weather and growing conditions at specific times, especially during the flowering period.

In general, it is important to get the spray timing and strategy right for disease control and this is even more important to protect against Fusarium. In addition, the effects of a head spray on quality, and in particular on the specific weight, are important considerations, especially in accessing the market opportunities for good-quality grain.

Consequently, while a lot of the focus on the head spray relates to Septoria control, there can also be significant responses from controlling ear Fusarium (Microdochium nivale). It is also important to remember that quality benefits are at least as important as the yield response – improved visual appearance and specific weights are important in quality wheat and it is what the market demands.

Fusarium head blight

Fusarium head blight (FHB) is an important fungal disease that affects wheat. It can reduce kernel weight, yield and flour extraction rates in many important wheat-growing areas, including Ireland. The extent of the losses depends on the severity of infection, the species of Fusarium involved in the infection, and on the environmental conditions prevailing at time of infection.

From what we know now, and looking into the future, we must develop a biological control system to have long-lasting and durable effects. Integrated control strategies are indispensable for the control of FHB and only the combined effect of planting highly resistant varieties, having reasonable crop rotations and suitable tillage systems, can minimise the damage.

For instance, F. graminearum also causes stalk rot in maize and infects other cereals and grasses. Since it infects all parts of the plant and survives on maize stubble, it is important to minimise the growing of wheat after maize, especially when zero tillage is practised. There is a lot of fundamental research ongoing into the biology and genetics of FHB, including research at UCD by Prof Doohan and myself (see panel right on the recent formation of a new plant science centre at UCD).

New research

It is well known that the host-pathogen interactions involved in FHB disease are very complex. That said, understanding these mechanisms and the gene expressions that give rise to them should provide the means to design new biological strategies to help control the disease.

In 1989, CIMMYT and China initiated a shuttle-breeding and germplasm exchange programme focusing on the integration of the FHB resistance of Chinese wheats into high-yielding CIMMYT germplasm.

As a result, many Chinese derivatives have been included in the CIMMYT international nurseries that are distributed around the world. Improved resistance for FHB worldwide has been based mainly on the deployment of novel genetic material, for example 3BS and 5A Quantitive Trait Loci (QTLs) from Chinese wheat and Frontana from Brazil.

In 2007, the resistance mechanism to FHB in winter wheat caused by F. culmorum was identified in the UK by Gosman and his team. Likewise, new highly FHB-resistant accessions originating mainly from Chinese and Japanese germplasm have been reported so there is cause for some optimism that a real biological approach to solving the FHB may not be too far away.

It is, however, unlikely that a single locus will provide sufficient resistance to FHB to protect the crop against high disease pressure. Therefore finding new sources of resistance and combining sources of resistance from different chromosomes into adapted wheat varieties is critical.

A new FHB resistance gene (Fhb3) was recently reported from the alien species Leymus racemosus, while a major QTL (on the long arm of chromosome 1B) was recently reported in four European winter wheat cultivars which will be valuable in backcross breeding programmes being undertaken currently by the European plant breeding community.

Potential for bio control agents

As well as the genetic resistance element, biological control involves the suppression and reduction of disease-causing organisms by the use of other organisms that occur naturally in the environment and which have been selected using new and modern scientific techniques. This phenomenon has been known for a long time but improved scientific understanding is bringing this concept to a new level where they offer some practical potential.

However, biopesticides tend to be relatively costly to produce, have a more limited storage life than conventional pesticides and are often relevant to a limited range of situations. To work effectively, biocontrol agents should attack only the target organism and have no detrimental effects on other non-target species. In the future, biological control offers the possibility of providing a safe and effective method to reduce disease levels and especially when used as part of an integrated pest management system.

When developing robust bio control methods, several factors will influence their efficacy. These include the type of bio control agent, the method of application and the longevity and ability of the organisms to survive in the open environment. This is a topic receiving a lot of attention internationally and at UCD we have ongoing research, supported by the DAFM Stimulus research programme, where we are examining the effects of beneficial microorganisms and other biological products, which may provide some control of FHB on a field scale in an Irish context.

In the past, there have been studies reported in the literature where plants treated with bio control agents experienced less than 10% FHB severity compared to controls which had over 80% FHB severity. Consequently, this is an area that we are going to hear more about in the future.

Resistance overview

Resistance is generally characterized as type 1 (resistance to initial infection) and type 2 (spread of the fungus within the spike/ear from an infected spikelet to another via the rachis) but other mechanisms of resistance have been proposed in relation to the effect on the grain mycotoxin deoxynivalenol, better known as DON.

Breeders and pathologists rely on visual scoring of disease symptoms and the analysis of mycotoxins is also commonly applied to assess the tolerance of new lines. But these approaches are indirect methods and do not determine the accumulated fungal biomass present in ears.

New scientific tools (Quantitative PCR) are now being used to assess fungal biomass based on the abundance of organism-specific genetic material (DNA) and this could offer new possibilities to determine Fusarium resistance in wheat. The choice of semi-dwarfing genes used in current plant-breeding programmes may also be a significant consideration where resistance to FHB is an important breeding objective.

Product choice

Products based on prothioconazole, such as in Helix, Aviator, Boogie, Prosaro, Coyote, FarmCo Bixapro, or other combinations of metconazole and epoxiconazole, as in Gleam, or difenconazole plus tebuconazole as in Magnello, have given the best reduction of Fusarium and, consequently, mycotoxins.

Use prothioconazole-based mixtures for high-risk crops (eg, crops after maize) or where heavy rain (>40mm) falls during flowering. Use Gleam for lower risk crops.

Application timing

Microdochium is best controlled by fungicide application during late ear emergence (GS55-59), while control of true Fusarium is best targeted by application closer to flowering (GS61). So timing of treatments tends to be a compromise. Generally, one should target treatment at the beginning of flowering (GS61).

Any ear treatment must be applied by mid-flowering (GS65) to have any real chance of being effective against Fusarium – ideally within the three days before and two days after infection but this is not always possible. However, it is better to be slightly early (best as a protectant) than slightly late (curative), as fungicides have more protectant than curative activity. Helix, Gleam and Magnello give a little more timing flexibility.

Conclusions

Despite the importance of the disease, particularly during epidemic years, control methods for Fusarium are limited. Much effort has gone into breeding resistant wheat varieties and into improving our understanding of the possible mechanisms and the genetic basis of resistance, with only moderate success to date. However, with all the recent advances in our understanding of the biological mechanisms involved, there is optimism that real advances are being made.

Despite this, we still do not have a variety which shows good resistance to Fusarium. This means that due diligence relies heavily on integrated control measures, including a well-timed T3 fungicide. Particular care should be taken in wheat crops following maize (in particular grain maize and min till establishment).

In order to keep infection levels to a minimum it is important to:

University College Dublin has recently set up a new Plant Science Centre. This centre brings together a broad range of scientists from within the University in order to focus on the implementation of sustainable crop production systems. This includes the development of plants which can contribute to combating a number of serious economic diseases, including Septoria and Fusarium.

The members of the UCD team are drawn predominantly from the UCD School of Agriculture and Food Science and the UCD School of Biology and Environmental Science. Much of the research is focused around a cereals disease research programme, which aims to help farmers control diseases of Irish and International importance, including Septoria tritici of wheat, Fusarium head blight disease of barley and wheat, net blotch in barley, take-all of wheat and barley and mildew in all cereals.

This new centre is already identifying new plant genes, including orphan genes, receptors and special factors, which enhance disease resistance and facilitate the development of markers for novel genes that aid breeders in their genetic improvement programmes. The team has close links to a number of European plant breeding companies.

See the following link for more information: www.ucdplantscience.com.

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