ProMED
http://www.promedmail.org

Source: SA Grain [summ. Mod.DHA, edited]
https://sagrainmag.co.za/2022/02/08/bls-on-seedlings-cause-for-concern/

Bacterial leaf streak (BLS) in maize is becoming a serious problem in Mpumalanga province. BLS was first reported in South Africa in 1949 and can generally be found in the warm and dry maize production areas such as the North West, Northern Free State and Gauteng provinces during favourable seasons. Early in the 2021/2022 season it was also reported in KwaZulu-Natal. Symptoms of leaf streaks and necrotic lesions, typical of BLS, were already being noticed on young plants under irrigation. The occurrence of the disease on seedlings was surprising, suggesting that BLS might be seed-borne. The lesions reduce the photosynthetic ability of the plant, inevitably resulting in yield losses.

Samples are being collected to confirm if infections are due to BLS and to determine the areas infected. Just before flowering, a complete survey will be done. Isolates will be collected as often as possible, together with information on cultivar, GPS coordinates, damage severity, rainfall and temperature. The disease will be confirmed using PCR, sequences will be compared to determine any variation in different isolates. Further studies will be done to screen various maize genotypes for resistance to the disease and to ascertain whether the pathogen is seed-borne.

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communicated by:
ProMED

[Strains and pathovars of _Xanthomonas vasicola_ (Xv; previously _campestris_) have been reported to cause a range of important crop diseases, including gumming of sugar cane, a bacterial wilt of banana and leaf streak of sorghum. Xv pv. _vasculorum_ (Xvv) is the cause of bacterial leaf streak (BLS) in maize in the Americas. The pathogen causing BLS in South Africa was recently shown to be sufficiently divergent from Xvv to warrant classification as a separate subclade, pv. _zeae_ (Xvz; see link below).

Xvv leaf streak on maize has been associated with inoculum from crop residue, overhead irrigation and warm temperatures. Recent evidence suggests a potential transmission with or on seeds (see link below), but further work is needed to clarify that. A number of graminaceous species, including oats, rice, pasture and weedy grasses, were found to be able to serve as reservoir hosts for infection of maize (see ProMED post 20180121.5574770). It is not clear if maize-infecting strains have emerged naturally, or if intensified agricultural systems may have favoured their evolution.

_Xanthomonas_ pathogens can survive between seasons on volunteer crop plants or plant debris. They can be seed-borne, depending on the host, and can be spread with infected plant and other materials, by water and mechanical means (including human and insect activity). Disease management is difficult and may include phytosanitation (control of reservoir hosts, removal of contaminated materials), cultural measures (optimal plant spacing, fertilisers), bacteriocides (such as copper compounds), as well as use of resistant crop varieties. Genome sequencing of species and pathovars of _Xanthomonas_ is providing information on mechanisms of bacterial virulence.

Maps
South Africa:
https://www.welt-atlas.de/datenbank/karten/karte-2-92.gif and
https://geology.com/world/south-africa-map.gif (with states)

Pictures
Bacterial leaf streak on maize:
https://bugwoodcloud.org/images/1536x1024/5551669.jpg,
http://agrilifecdn.tamu.edu/texasrowcrops/files/2016/10/Leaf-Streak-pic-1.jpg,
https://news.unl.edu/sites/default/files/media/20160826-corndisease-unlt.jpg,
https://cropwatch.unl.edu/images/hero/2016/bls-lesions-tjz-hr.jpg and
http://cropwatch.unl.edu/Plant-Disease-Management/Images/corn-bacterial-leaf-streak-narrow-stripes.jpg

Links
Information on leaf streak on maize (& gumming of sugar cane):
https://cropwatch.unl.edu/bacterial-leaf-streak,
https://cropprotectionnetwork.org/download/2550/ (with pictures),
http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsid=56980 and
https://doi.org/10.1111/j.1439-0434.1991.tb00144.x
Characterisation of Xv strains:
https://doi.org/10.1128/genomeA.00532-18 (Xvz, South Africa),
https://doi.org/10.1094/PDIS-10-16-1426-PDN,
https://doi.org/10.3390/pathogens3010211,
https://bit.ly/3rJ3IlT and
https://doi.org/10.1016/S0723-2020(00)80056-9
Potential seed transmission of BLS:
https://doi.org/10.1094/PHYTO-08-19-0306-R
Xvz taxonomy:
https://www.uniprot.org/taxonomy/2164062
Xv taxonomy & current strains via:
https://www.uniprot.org/taxonomy/56459
Classification of _Xanthomonas_ species:
https://doi.org/10.1099/00207713-45-3-472
- Mod.DHA