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News archive 1997-2008

First report of blast disease on wheat in Africa (Zambia)

A ProMED-mail post <https://url.emailprotection.link/?bczGkw5NaxKViInVN3o8WI7SkfPjCplYcnrvkSnR7vDdzBseB4Z4JgOzMtf4NyhKwZ-87t_ekCqY4X1TR9IACFg~~>

ProMED-mail is a program of the International Society for Infectious Diseases <https://url.emailprotection.link/?bJcYJd1UM9bbZyueyHGqCfuWBsCz6JLr6QpMnBNG04Pz2V8akThtcwE1bjLVZbAR2-PZoubzhx-ALOhYt6Kir6w~~>

Date: Mon 21 Sep 2020
Source: PLoS ONE [edited]
<https://url.emailprotection.link/?bilf61H_-yu8-qOBy_JcnNvo01LYw4KoQ-76s__xVWswe1CutjvaBRauDihZij3-k-R0wjN28-YKf9ReSiYf28qftPD-y95PyO6KAdVSeUp0aEf9pePAX2nP-AQPRxJOs>

[ref: B Tembo, Mulenga RM, Sichilima S, et al. Detection and characterization of fungus (_Magnaporthe oryzae_ pathotype _Triticum_) causing wheat blast disease on rain-fed grown wheat (_Triticum aestivum_ L.) in Zambia. PLoS ONE 2020; 15(9): e0238724; DOI: 10.1371/journal.pone.0238724]

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In Zambia, wheat blast symptoms were observed for the first time on wheat (_Triticum aestivum_ L.) grown in Muchinga Province during the

2017-18 rainy season. Infected plants showed typical symptoms in a short span of time. Incidence of blast symptoms on nearly all wheat heads was high and ranged from 50% to 100%.

A study was conducted to isolate and identify the causal pathogen using classical and molecular methods. Morphobiometry revealed conidia with characteristic spores associated with _M. oryzae_ (MoT).

Preliminary PCR screening of 6 isolates from infected samples was conducted with diagnostic primers. Subsequent analysis of 2 isolates was performed. Both experiments confirmed that MoT is the causal agent of wheat blast in Zambia. Further, pathogenicity tests performed with pure cultures from 2 samples produced typical blast symptoms on all 6 inoculated wheat genotypes.

Results of this study indicate that MoT is causing wheat blast in rain-fed wheat grown in Zambia, thus making it the first report of MoT in Zambia and Africa. This inter-continental movement of the pathogen has serious implication for wheat production and trade that needs to be urgently addressed.

However, full genome characterization of the Zambian isolates is needed so as to understand the phylogenetic lineage and speciation of the fungus. It is also important to note that the pathogenicity tests in this study were limited to wheat samples that were observed with blast symptoms in the field. The study should be extended to other Poaceae so as to infer host range and pathotype differentiation. In the meantime, policy makers, plant quarantine and phytosanitary services, plant pathologists, wheat breeders and all stakeholders in the wheat value chain should collaborate in limiting further spread of the disease. Resources should be allocated to regular monitoring and awareness among wheat farmers.

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

[Both wheat and rice blast are caused by the fungus _Pyricularia oryzae_ (synonym _Magnaporthe oryzae_). Although the pathogens are currently classified as the same species, the wheat blast pathogen is a distinct population (referred as _P. oryzae_ Triticum population) and does not cause disease in rice. Over 50 species of grasses and sedges can be affected by related fungal strains, each of which generally appear to affect only a limited number of host species.

Further work is needed regarding genotypic differentiation related to host range, including differences between the wheat and rice pathovars. Rice blast is one of the most destructive diseases of rice worldwide. Wheat blast is now considered an emerging disease and a threat to global food security.

Blast symptoms on wheat (and barley) may be confused with fusarium head blight (see previous ProMED-mail posts in the archives and link

below) and include necrotic leaf spots, bleaching of ears, shriveled kernels and no seed production at all for severe infections. Yield losses seem to average 40 to 50 per cent, but cases of 100 per cent losses have also been reported; wheat production in some affected areas has ceased.

Humid and warm conditions favour disease development, but the life cycle of the fungus is still unknown. Spread of the rice pathogen occurs with infected plant material (including seed), mechanical means (including human and insect activity), water and wind. It is likely that the wheat pathogen is spread in similar ways. Disease management of rice blast may include fungicides and cultural practices but relies mainly on resistant varieties. Available wheat cultivars lack resistance to wheat blast and only limited tolerance can be found.

Resistance breeding is difficult because the fungus is highly variable, and this favours the emergence of new strains with increased virulence.

Wheat blast was identified in 1985 in southern Brazil and spread in the Americas. Initially, it was thought to be caused by a fungal strain which crossed from rice to wheat, but is now considered more likely to have originated from local wild grasses. In Asia, the disease was found for the 1st time in 2016 in Bangladesh (ProMED-mail post https://url.emailprotection.link/?bBRANpuBS79XWr6_GuT7B4w8g7jhnNap0fHOTak7cnSDn9Jpj8xeilTCQYVzhS_82TUPt08NG296_hRjq1H69m76i8VBLSXEdMMz92fRhvuMmtm2zQ-TE8k2zUTScqKBr) and subsequently reported in India (ProMED-mail post https://url.emailprotection.link/?bNHxmMPn11ovnwvoem1undw5MbMqmFw5TRdEXOR6awudBSvM6YDCc6G02xC63ReSANCSHILP6_UrsR9WYWLXUC9FVHYNT5SDj5llX-vV-VrAl5tt72KUlh66775fa-sLj). Wheat blast strains in Bangladesh were found to be highly similar to strains from Brazil (ProMED-mail post https://url.emailprotection.link/?bPA4f1KLdGwFQ3ssBrjMiINmOD_TnjG0U56gkFXAT3m7LCoUzNzBGl4xXfCSt9wXwGTaY13B49mYi2Ehaaxr4s6KzfeF0KGsyXeYVU0hjoXAVEf0SsPP2JcwQfQImXtPb).

Similarly, rice blast strains in northern India were shown also to be closely related to strains from the main origin of seed and food grain (ProMED-mail post https://url.emailprotection.link/?bPvBJE7hOPxetBusVyIBpMLjn88edZAUSV1w-DgWGwG_0dYRGh7FymeRQx13tMKYQo0JH8-wEiY1JOQP9qC_S_Y-hqDM9oF0ARRTRxzo7oC8GvNhBJFmUuJbPnnswFLsS). Based on these findings, the disease in Asia is now considered to be due to introduction of the fungus with grain. This would suggest a similar way of introduction of the disease to Africa and stress again the importance of strict quarantine measures and biosecurity protocols for the international movement of any kind of plant material.

Maps

Zambia, with provinces:

<https://url.emailprotection.link/?bL3Did64DERbxh_ixn_UK11NDU0yXqZEiKF5_i9199lfQmNFR59gzk3xRmQZWPFIQ-Rg0AIeqK0HKdttsJqyAngjkDII4T8GuFmkjauAY4g1nuzlBlU6gqQ6omxGwkEsw>

Africa, overview:

<https://url.emailprotection.link/?b78cD7aI7D_Lk272KNMRJ5XSGOtgd1TcTOlsJNLT8RFAwEgNhXzybsalgxBnhWzfmRdtTFWGFNOU8z4tUFb4qqzqZO2UqO7FtBUVU03R4gpX2S8efhyemuPpuuUqe3pO3>

Pictures

Blast symptoms on wheat:

<https://url.emailprotection.link/?brNx9m1Exay0Xbso6m0k3uimXTPUz43AjmZdPlnNmfnt5OciXRxa5qdryu_kIEcihni-P9xrZbaBNfR28cpfpX0pDy88L8puSpfBdwuKKukfKHBAbM8ckjjtSUc4HfcP6rsNLenc1HGTAF1z_JXOdPoVwBDJ8fgQz6tBlOdqSWz8~>,

<https://url.emailprotection.link/?bKw3i57nIX-Nv3mzTO_TMA6_5cHJ-EY92AhhsGRH-cTIloa_9ZYemXOb5aHGIxJE3t2z-RrDZbBJFyoIsEqwYOQ~~>,

<https://url.emailprotection.link/?bIAi5Ztpc29ryb4napLh2HIW-ZoiyusioiRwpf5yJWtfTLHCggcueMq-6cno-1ZDEyWTlVB4S6RQFkteAAy5ph7R-OPNZR4pX9FvHrh6gfvVc6ZvqM0LjcbCtg3YF-x5f>

<https://url.emailprotection.link/?bftBiSf6KNpvB_8Vm4EUJ9DgZtqitGyOTDN5UV6BCa0t8EAo7uWyq4sxy2kHwchj2hngX2mkZAae_tQN5_CZ8ljRixPEisQyeJWFTY_e-J8wWzDoOwTrNeDWylfj_JftDEHYjayUrbnn7avkFRhNzFFkkcOgOfCUhIumAxEWWy3YxXy9o4xfixa7clcJeVH45VrgvZpbh48WR9GAdWhlESy3-zkWUaw2nj2RvrdbMQtA6HVpQKRlg3XCLfV4DOsxv>

and