Biology and management options for control of Septoria tritici blotch and Net form of net blotch
Author: Hugh Wallwork
GRDC project code: DAS00139
Key words: Septoria tritici blotch, Net form net blotch, disease control, disease management.
Take home messages
- Septoria tritici blotch (STB) is increasing in the south east of SA as a result of intensification of wheat cropping. The pathogen population in the south east may differ from elsewhere in SA.
- Septoria is best controlled with variety resistance, but later sowing and fungicide treatments can also be effective for avoiding crop damage.
- No new virulences in Net form net blotch (NFNB) were detected in 2014 but the disease remains a threat to many barley varieties. The resistance in some varieties appears more durable and should be protected by growers by avoiding the use of the most susceptible varieties.
Septoria tritici blotch
Septoria tritici blotch is an emerging threat to wheat crops in the south east of SA. The fungus survives on straw and develops sexual fruiting bodies called perithecia that produce large numbers of airborne spores that can spread the disease to distant crops. The dispersal of these ascospores is only brief and so crops sown after the dispersal period are much less likely to be affected. Three to four weeks after infection the young wheat leaves will develop black asexual fruiting bodies called pycnidia that produce spores that only spread by rain splash, so from this time onwards the disease will only spread over short distances within crops.
Perithecia and pycnidia look very similar. Both appear as tiny black specks on leaves and straw although the perithecia are smaller, typically 75-100 mm, whereas pycnidia are typically 100-150 mm and easily seen with the naked eye. The size of pycnidia varies with variety, growth stage and seasonal conditions.
The reason that septoria is flaring up in the south east is most likely a result of the intensification of wheat cropping in the region and the growing of some susceptible varieties. The disease is likely to be more severe in seasons where crops are sown early and when rain is more frequent. Also, as the area sown to susceptible varieties increases, there will be higher levels of inoculum being dispersed from stubbles of those crops.
Septoria tritici blotch was a sporadically damaging disease in the Mid- and Lower-North, Yorke Peninsula and Lower Eyre Peninsula during the 1970’s and through to early 1990’s. Since that time, a combination of unfavourable seasons for the disease to develop and avoidance of the more susceptible varieties has kept the disease under good control.
The most effective control measure is to avoid growing the most susceptible wheat varieties. Varieties to avoid in high risk areas are Axe, Dart, Emu Rock, Forrest, Impala, Justica CL Plus and, to a lesser extent, Beaufort, Corack and Grenade CL Plus.
There is evidence that the septoria pathogen population present in the south east of SA differs somewhat from the septoria isolates collected previously elsewhere in SA. This is reflected in the higher levels of disease found in crops of the varieties Revenue and Beaufort. Both varieties have shown good resistance in previous NVT tests at Turretfield. A new NVT septoria disease nursery at Conmurra funded by GRDC should help to identify these differences in future. Resistance ratings for long season varieties will in future be based on observations from the south-east of SA and Western Districts of Victoria.
Net form of net blotch (NFNB)
NFNB has been a serious threat to barley crops in SA for several years now. The fungus survives in barley stubbles from where both sexual and asexual spores are produced. Unlike septoria tritici blotch, the disease disperses widely during the growing season as the spores are readily blown with the wind. The fungal population is also much more diverse than in septoria, making resistance ratings even more variable. For this reason a range of possible resistance reactions are provided in variety guides for SA.
During 2014 most isolates of NFNB collected from crops showed high levels of virulence on Fleet which is rated Susceptible – Very Susceptible. These isolates are not virulent on Maritime although Maritime remains susceptible to other isolates particularly in districts where Maritime is still grown. A number of varieties have shown consistently good levels of resistance to all isolates and these are Buloke, Scope, Flagship, Hindmarsh, La Trobe, Schooner and Skipper. In 2013 one isolate showed virulence on Skipper. Compass has shown good resistance so far in the field and has moderately good resistance to isolates in controlled environment tests.
Several strategies are available for the control of NFNB. Removal of summer volunteer barley and avoiding the sowing of barley into barley stubbles are important strategies that will also help to control several other barley diseases such as leaf rust, spot form net blotch, powdery mildew and scald.
Avoiding the sowing of the most susceptible varieties is the most effective control option but, in the absence of resistance, chemical control strategies will need to be considered. It is a likely that seed treatments will be an effective option from 2016. Fungicide sprays are available and effective provided they are applied early. As with other airborne pathogens, the choices and actions of neighbouring growers could have a significant impact on inoculum levels.
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