Protecting WA crops

Economical management of Spot form net blotch in low rainfall environments: using less susceptible varieties makes sense

Spot form net blotch spots elongate and join together. Severely affected leaves turn yellow and die back from the tips. Image: DPIRD.

At a glance:

• Spot form net blotch (SFNB) is a major foliar disease of barley across Western Australia, promoted by the dominance of varieties with an adult plant rating of Susceptible or worse (94 per cent of crop area sown to barley in 2021).
• This research found that fungicide management of SFNB in low rainfall zones does not always provide yield improvements in Maximus or Spartacus. However, grain quality increases in less plump varieties such as Spartacus are more likely and may result in an economic return.
• Growing varieties with even modest improvements in genetic resistance, such as Maximus, can reduce both inoculum carryover and in-crop disease severity of stubble borne diseases like SFNB. This may also reduce the need for in-season fungicide application.

Spot form net blotch (SFNB) of barley has emerged as a major foliar disease across the agricultural area of Western Australia, including the low rainfall zones where continuous barley is a significant component of cropping programs. The disease is stubble borne so continuously cropping barley poses the greatest risk of disease, especially given that most varieties are susceptible. Spores are blown from infected barley stubble and regrowth barley to infect early sown crops.

Symptoms of SFNB include dark brown oval spots with yellow edges on the older leaves. As the spots age, they elongate and join to make a blotch. In the case of severe infection, leaves can turn yellow and die back from the tips.

Yield and quality losses are likely to be greatest when crops are exposed to early infection, and seasonal conditions favour ongoing development of disease through the crop canopy. Spring rainfall, particularly rainfall and number of rainy days in September, determines the likelihood of a yield response to fungicide application (Hills, 2016). 

Analysis of SFNB management from 26 DPIRD trials in low rainfall environments over the last 20 years, conducted by Andrea Hills (DPIRD, Esperance), concluded that whilst multiple fungicide treatments significantly reduced SFNB disease, yield responses have been unreliable.  

In the 2021 and 2022 seasons, a Department of Primary Industries and Regional Development (DPIRD) research project, with co-investment from GRDC, investigated management options for SFNB, including varietal resistance and foliar fungicide applications in two trials at Nungarin and single trials at Muntadgin and Merredin.

DPIRD research scientist Jason Bradley compared the widely grown variety Spartacus CL to its most likely replacement Maximus CL. While Maximus and Spartacus are both susceptible at seedling through to tillering to SFNB, a degree of adult plant resistance in Maximus significantly reduces the amount of diseased leaf area in the upper canopy. Mr Bradley found that in the Merredin trial, there was less carryover of SFNB disease in the stubble from Maximus compared to Spartacus, with Spartacus generating higher disease pressure in seedlings in the following year.

Figure 1. Higher disease carryover with stubble from susceptible variety Spartacus compared to the moderately susceptible variety Maximus. Image: Jason Bradley, DPIRD.

In fungicide trials, Mr Bradley found that SFNB was reduced significantly by foliar fungicide applications compared to the untreated control, but there was no consistent impact on yield. There was yield gain from two foliar fungicide treatments compared to the untreated control in only one of the four trials. There was however an improvement in grain quality, with improved grain weights and higher grain retention in all trials, particularly in the more susceptible and less plump variety Spartacus. In Nungarin, during 2022, the retentions in Maximus were 3.2 percent higher compared to the control, while in Spartacus, the retentions were 7.1 percent higher.

Mr Bradley's conclusions from these trials were that in low rainfall areas, where the economic response to fungicide management of SFNB is uncertain, cultivating varieties with enhanced genetic resistance reduces the necessity for and uncertainty related to fungicide intervention. Furthermore, additional research is being conducted on conditions required for SFNB spore production and release. DPIRD staff are investigating whether weather forecasts and disease thresholds can be used in management decisions. 

For more information refer to Mr Bradley’s Research Updates abstract Economical management of spot form net blotch in low rainfall environments: using less susceptible varieties makes sense.

For more information on Spot form net blotch refer to DPIRD’s Diagnosing spot type net blotch webpage.