By Tim Murray

There is a light at the end of the Cephalosporium stripe and eyespot tunnel—and it’s not a train!

Cephalosporium and eyespot, which was formerly referred to as strawbreaker foot rot, have been through several tunnels already and come out better for it in terms of our understanding of the diseases as well as reducing the damage they cause.

Before exploring the work yet to be done, it’s important to look at past challenges to determine our progress. I proposed two projects at my first Wheat Review in 1984, one was on what is now called eyespot, and another on Cephalosporium stripe (Figures 1 & 2) But I wasn’t the first to work on them. Bill Bruehl had a project that focused on identifying sources of eyespot resistance, its disease biology and using fungicides for control.  Meanwhile, Bob Allan and Clarence Peterson, both with USDA, were developing resistant varieties as part of their breeding programs.

Bruehl identified Cephalosporium Stripe in Washington in 1955 and worked-out much of the disease biology over the next three decades. One of his most important accomplishments with the pair of diseases was the development of methods for creating epidemics in research nurseries—even today the bread and butter of field research and breeding for disease resistance.

I proposed my first eyespot project when up to 1 million acres of winter wheat were sprayed each year with one of three fungicides to control the disease. The biggest question at the time was: Which fungicide is best? The objectives of that first proposal included improving disease resistance and methods of screening for resistance, as well as looking for new fungicides and fungicide resistance in the pathogen. Since then, my main focus has since been on improving disease resistance because it’s both biologically and economically effective, and fits well in all management systems.

An early accomplishment of our research was developing a new seedling test for eyespot resistance. By using a growth chamber, it dramatically reduced the time it took to determine resistance (two months) than in the field test (12 months). (Figure 3). We used that method to identify potential new sources of resistance in wild relatives of wheat and in new varieties and to conduct genetic studies on resistance. In the process we named two new genes.

In 1988, Bob Allan released the first two eyespot-resistant varieties, Hyak and Madsen. Since then several resistant varieties have been released and currently there are 12 listed in the Washington State Crop Improvement’s Seed Buyers Guide that are rated equal or better than Madsen.

Madsen became a dominant variety being grown on very large acreages for over 20 years. The adoption of eyespot-resistant varieties led to a substantial reduction in the acreage sprayed for eyespot. And the timing couldn’t have been better because of documented and widespread resistance of the eyespot pathogens in the 1990s to the fungicides then being used and which all belonged to the same chemical class (Figure 4).

We conducted field trials that compared the effectiveness of experimental products and the new fungicides coming to the market. Today, farmers have a choice of eight fungicide treatments for eyespot with active ingredients from multiple classes.

In 1984, Stephens wheat was widely grown. Although it had excellent yield potential, quality, and wide adaptability, its Achilles heel was Cephalosporium stripe; consequently, the disease became widespread and caused substantial damage.

Cephalosporium stripe was very severe in the 1983-84 season and there were significant yield losses in Stephens. As a result, the Cephalosporium Stripe project began in 1985 with an emphasis on identifying sources of disease resistance, understanding the role of tillage, and the potential for chemical control. We later added research on soil pH and demonstrated that liming could be beneficial against the disease in acidic soils.

From that early work, we came to understand that highly effective resistance to Cephalosporium Stripe didn’t exist in wheat, but that some hybrid crosses with wheatgrass were very resistant (Figure 5). That led to the perennial wheat project and development of several resistant germplasm lines that are being used by PNW breeding programs today.

Why do we continue to work on these diseases? What remains to be accomplished?  The short answer is that we still experience losses from both eyespot and Cephalosporium stripe—very significant losses some years and locations. For example, we knew when Madsen was released that yield could be reduced about 10 percent under heavy disease pressure. We also knew that resistance in Madsen is conferred largely by a single gene known as Pch1. All of the eyespot-resistant varieties available in the PNW today contain that same gene as the main source of resistance (Table 1).

Although Pch1 has been stable, it’s scary from a biological perspective to rely on a single gene. That’s why we began looking for other sources of resistance. We now test advanced breeding lines and new varieties for resistance every year, but it has been difficult for them to match the effectiveness of Madsen’s resistance. That tells us that minor genes are also important; unfortunately, we know very little about them.

Most of the new resistance genes we’ve found occur in wild relatives of wheat, just as Pch1 did (Figure 6) before it was transferred to cultivated wheat. Transferring genes from wild relatives (known as introgression) is a long-term process, even with new molecular tools. We have begun making crosses between adapted varieties and wild relatives, but the process will not be completed before my career ends and someone else will likely finish the process. Genetic studies are also needed to identify the minor genes that contribute to the effectiveness of resistance.

The situation for Cephalosporium stripe-resistant varieties is different because we know less about the genes involved and they’re not as effective as those for eyespot (Table 2). In addition, their effectiveness varies from year-to-year because of environmental conditions we don’t understand. Soil acidity is one of the environmental factors we understand best. For instance, we know that raising pH above 5.7 results in much less disease. However, liming is a costly practice in our region, so new research is needed to identify more cost-effective methods of managing soil pH.

Developing a better understanding of what happens in soil that allows these and other soilborne diseases to become problems is needed. We also need to continue searching for effective sources of resistance for Cephalosporium stripe and transfer them into PNW-adapted varieties.

We’ve come a long way in our understanding of eyespot and Cephalosporium stripe and we’ve made headway in reducing losses, but there is more to do. Breeding new varieties to improve resistance and yield potential is an ongoing process that will continue as long as we’re growing wheat. Fungicide resistance remains a concern. Mother Nature is resilient and we should expect changes to occur when we introduce new varieties and control practices.

Table 1.  Field ratings for eyespot resistance of PNW-adapted varieties compared with Madsen.

Variety Pch1 # years tested Disease Index Normalized Disease Index Disease Rating
VPM-1* + 3.0 21.4 28.9 3.1
Chukar + 4.0 28.6 44.5 3.9
Madsen + 8.0 40.0 50.0 4.5
ORCF-102 + 4.0 33.5 51.8 4.5
Puma + 3.0 48.3 55.0 5.0
Tubbs ’06 + 5.0 43.9 55.4 5.0
Finch + 8.0 47.5 60.0 5.4
Brundage 96 3.0 54.3 75.6 6.9
ORCF-101 5.0 58.9 86.2 7.7
Xerpha 5.0 62.1 91.6 8.1
Bruehl 4.0 63.2 93.4 8.2
Eltan 7.0 70.5 94.7 8.4

Experiments are conducted in inoculated field plots with Madsen and Eltan as the long-term resistant and susceptible controls, respectively. Pch1: + = present; – = absent. Disease index ranges from 0 to 100, where 0 = all healthy stems and 100 = all severely diseased stems. The normalized disease index takes year-to-year variation into account. Disease rating ranges from 0 to 9, where 0 = most resistant and 9 = most susceptible. *VPM-1 is the source of eyespot resistance in Madsen and all other eyespot-resistant varieties.

Table 2. Field ratings for Cephalosporium stripe resistance of PNW-adapted varieties compared with Eltan.

Variety # years tested Disease index Normalized Disease Index Disease Rating
Eltan 10 37.3 47.6 4.3
Finch 6 36.0 49.7 4.5
VPM-1 2 32.0 51.7 4.7
Xerpha 7 42.8 55.7 5.1
Tubbs ’06 4 34.6 59.1 5.4
Bruehl 6 43.6 60.4 5.5
Chukar 4 42.2 69.4 6.3
Puma 2 66.5 72.8 6.6
Madsen 9 61.2 76.7 7.0
Brundage 96 6 64.2 78.2 7.1
ORCF-102 4 49.8 79.7 7.2
ORCF-101 4 51.6 82.6 7.5
Stephens 10 73.8 97.1 8.8

Experiments are conducted in inoculated field plots with Eltan and Stephens as the long-term tolerant and susceptible controls, respectively. Disease index ranges from 0 to 100, where 0 = all healthy stems and 100 = all severely diseased stems. The normalized disease index takes year-to-year variation into account. Disease rating ranges from 0 to 9, where 0 = most resistant and 9 = most susceptible.