Net closes in on barley blotch

One of the most intriguing things about barley net blotch is that it is not a disease at all – but diseases. It comes in two key forms and this has implications for management. This article explores new research findings that shed light on a disease that can botch both yield and quality.

No matter where you are in the UK, you will find net blotch symptoms in some local barley fields. This is particularly true for the more problematic winter barley crops. The good news is that an integrated approach to management can prevent pathogen populations reaching economically damaging levels.

Thankfully, crop management does not require an understanding of the net blotch form present. Our new report confirms this. However, the presence and abundance of each form does already have implications for variety and fungicide performance trials. With the net blotch population evolving, it could eventually affect field management strategies, too. Consequently, it is worth keeping an eye on.

During 2018, researchers grew net blotch isolates on agar-filled petri dishes. One plate stood out. It contained an unusual and unexpected morphology. Symptoms in infected leaf fragments also revealed a clear-cut difference (Figure 1). Following species-specific molecular tests (refined by the project), it was established that this isolate represented the spot form of net blotch, caused by Pyrenophora teres f. maculata (Ptm). Not the net form of net blotch, caused by P. teres f. teres (Ptt)), more typically observed in UK fields.

Figure 1. Net blotch symptoms on leaf fragments in petri dishes

Left: The less common spot form (from a Yorkshire field in 2018), which shows as dark-brown circular-to-elliptical lesions. Middle and right: The more common net form, which shows as dark-brown narrow-netted lesions. The middle example shows rarer stripe-like symptoms, whereas the right-hand image shows typical lesions.

NIAB’s Bart Fraaije, who led the project, turned to Rothamsted Research’s incredible archive of spring barley grain samples – which stretch back to 1852 – to retrospectively detect the two forms of net blotch over time. The analysis identified the relatively common Ptt form 21 times (all since 1890), with Ptm detected just four times (1982, 1995, 2001 and 2012). Both species were only once simultaneously present (2012). Although less common, the presence of Ptm in multiple years showed that the 2018 sample was not a one-off.

The project also examined whether the two net blotch fungi have different levels of fungicide sensitivity. Reduced sensitivity to quinone outside inhibitors (QoI), azoles (DMI) and succinate dehydrogenase inhibitors (SDHI), has been detected in UK net blotch populations in recent years. However, it is important to stress that disease control has remained sufficiently reliable to date. However, several SDH mutations in Europe are associated with a loss of field efficacy, so it is essential to monitor the situation for both net blotch forms.

Fungicide sensitivity assays (conducted on mycelium plugs) showed that the Ptm population sampled in Yorkshire was more sensitive to azole and SDHI fungicides, relative to the Ptt populations sampled in four other UK regions. The research team also identified known mutations associated with fungicide insensitivities in the samples (see full report).

The most SDHI-insensitive Ptt strain showed partial growth at 100 ppm (fluxapyroxad), whereas the most SDHI-insensitive Ptm strain only tolerated 1 ppm. Additionally, the majority of Ptt strains showed some partial growth at 100 ppm of epoxiconazole (the highest concentration tested). Although the majority of Ptm strains were able to grow at 1.0 ppm epoxiconazole, few showed partial growth at 10 or 100 ppm.

This research has revealed the importance of species-specific diagnosis. The development and use of rapid in-field tests for difficult-to-visually-diagnose diseases, such as net blotch forms and ramularia, could provide a power boost for trials.

For net blotch assessments in variety trials, it is important to understand which species is present, how much is present and, indeed, if there are mixed infections. This is because each net blotch R-gene is likely to deliver varying levels of resistance to each form. Rapid tests will also provide a valuable training tool, helping trial operators fine-tune their disease recognition skills.

When it comes to fungicide sensitivity, monitoring of efficacy is a core part of the AHDB research programme. Our annual fungicide performance update each winter provides a detailed overview of the latest information on efficacy shifts, along with expert opinion on the potential drivers of change.

The full report includes detailed recommendations for research.