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Western Australia - New hybrid strain of net blotch on barley with high resistance to some Group 3 DMI fungicides

A ProMED-mail post <http://www.promedmail.org>
ProMED-mail is a program of the International Society for Infectious Diseases <http://www.isid.org>

Date: Tue 18 Feb 2020

Source: Farm Online [edited]

<https://www.farmonline.com.au/story/6636217/hybrid-resistant-super-disease-threatens-barley/>

Authorities have confirmed the discovery of a new hybrid strain of net blotch in southern Western Australia. [It] is a hybrid of both the spot and net forms (SFNB and NFNB) and is highly resistant to some Group 3 (azole or demethylase inhibitor) [DMI] fungicides.

The alarming discovery was made by the Centre for Crop and Disease Management (CCDM). CCDM's Fran Lopez-Ruiz said researchers had known for a while a hybrid of SFNB and NFNB existed but they had just discovered the multiple genetic mutations within the strain.

Genetically identical clones [of the hybrid] notable for the virulence of their fungicide resistance [were] detected in locations up to 350km [217 mi] apart, which means they are reproducing asexually.

CCDM's Wesley Mair [said], "From genetic analysis we saw that this [strain] carried known mutations for fungicide resistance in NFNB, and known mutations for fungicide resistance in SFNB. After further tests we are now convinced we have a highly resistant hybrid."

Dr Lopez-Perez said farmers needed to mix their fungicide rotations up. "However, the use of SDHI fungicides should be monitored carefully given the recent discovery of SDHI resistance in NFNB in South Australia [ProMED-mail post http://promedmail.org/post/20191219.6849906]."

[Byline: Gregor Heard]

Communicated by: ProMED-mail <promed@promedmail.org>

[Net blotch of barley is caused by the fungus _Pyrenophora teres_ and affects only domestic and wild barleys (_Hordeum_ spp.). The disease causes necrotic areas on leaves that reduce photosynthetic capacity and hence plant vigour, leading to yield losses of up to 20%. There are 2 types of the fungus, the net type caused by _P. t._ f. _teres_(Ptt) and the spot type caused by _P. t._ f. _maculata_ (Ptm). The fungus survives between seasons on barley residues, volunteer barley plants, some grasses, and seed. Disease management includes cultural practices to remove pathogen reservoirs, fungicide applications, use of clean seed and use of resistant cultivars.

Both sexual and asexual reproduction of the fungus may occur, at varying ratios depending on environmental conditions. There appear to be no clear morphological or life cycle differences between the forms.

However, phylogenetic studies have shown that Ptt and Ptm isolates are separated into two genetically divergent groups. Furthermore, host resistance genes against SFNB were found to be genetically distinct from those effective against NFNB. The mechanism for spontaneous formation of a hybrid between Ptt and Ptm in the field, as described above, remains to be investigated.

Highly aggressive new strains of particularly the net form are emerging worldwide. Integrated disease management, including varying crops or crop cultivars in time and space, as well as rotating or mixing chemical classes of fungicides, as mentioned above, is vital to extend the useful life of host resistances and agrochemical compounds.

In Australia, net blotch is considered an emerging threat to southern barley-producing areas (for example, see ProMED-mail post http://promedmail.org/post/20100323.0921). The SDHI resistant net form strain referred to above and reported recently in South Australia (ProMED-mail post http://promedmail.org/post/20191219.6849906) also showed resistance to some DMI fungicides. It is still being investigated, but available data suggest that its resistance mechanisms are different to earlier DMI net form resistances in WA [ProMED-mail post http://promedmail.org/post/20180727.5931094].

Maps

Australia (with states):

<https://www.interkart.de/media/catalog/product/p/o/pod10th103_103_australia_physical.jpg>

Pictures

Net blotches on barley:

<http://agriculture.vic.gov.au/__data/assets/image/0005/227984/barley_2.jpg>

(net form),

<https://www.agric.wa.gov.au/sites/gateway/files/styles/original/public/Spot_type_net_blotch_mild_lesions.jpg?itok=ia7M9XDs>

(spot form),

<http://www.inra.fr/hyp3/images/6034311.jpg>,

<https://www.agric.wa.gov.au/sites/gateway/files/Barley%20NTNB%201_0.JPG>

and