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International Society for Infectious Diseases
Date: Fri 20 Mar 2020
Source: Multidisciplinary Digital Publishing Institute (MDPI) [edited]
Ref: X Sun et al. (2020): Etiology and Symptoms of Maize Leaf Spot Caused by _Bipolaris_ spp. in Sichuan, China. Pathogens 9, 229; DOI: 10.3390/pathogens9030229
Many species of the genus _Bipolaris_ are important plant pathogens and often cause leaf spot, root rot, and seedling blight in an extremely wide range of hosts around the world. In recent years, maize leaf spot caused by _Bipolaris_ species has frequently occurred with complex symptoms and is becoming increasingly serious in Sichuan Province of China.
To investigate the population diversity of _Bipolaris_ spp. and their corresponding symptoms in maize, 747 samples of maize leaf spot were collected from 132 sampling sites in 19 administrative districts of Sichuan Province from 2011 to 2018. Based on morphological characteristics, pathogenicity testing, and phylogenetic analysis of the rDNA internal transcribed spacer (ITS) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes, a total of 1186 _Bipolaris_ isolates were identified as _B. maydis_, _B. zeicola_, _B. cynodontis_, _B. oryzae_, _B. setariae_, and _B. saccharicola_.
_B. maydis_ and _B. zeicola_ were the dominant pathogenic species, accounting for 57.34% and 42.07% of the isolates, respectively. We found that _B. zeicola_ isolates were mainly distributed in high altitude and cool mountainous areas, while _B. maydis_ was more widely distributed in Sichuan Province. The typical symptoms caused by the _Bipolaris_ species were clearly distinct in maize. The typical symptoms caused by _B. maydis_ were elongated strip lesions, or fusiform, elliptical lesions, and those caused by _B. zeicola_ were narrow linear lesions. _B. saccharicola_ was 1st reported on maize and caused subrotund lesions.
This study provides useful information for disease diagnosis and management for _Bipolaris_ leaf spot in maize.
[The fungus _Bipolaris maydis_ causes southern leaf blight (SLB) of maize. Of the 2 races of the pathogen, race O normally attacks only leaves, while race T attacks all above plant parts, including leaves, husks, stalks, and ears. Symptoms include necrotic spots on leaves and any other affected parts; size and shape of spots may vary with pathogen race and infected crop cultivar. Husk infections extend into the kernels as mould; seedlings from infected kernels are often blighted. Yield losses of over 30% have been reported and are more severe the earlier in the plant's growth cycle infection occurs. Disease development is favoured by warm temperatures.
Related species can affect other crops and cause, for example, target spot of sorghum (_B. sorghicola_, currently synonym of _B. cookei_), coconut leaf blight (_B. incurvata_), eye spot of sugar cane (_B. sacchari_), spot blotch of wheat (_B. sorokiniana_), and brown spot of rice (_B. oryzae_).
The fungi can be transmitted with seeds and other plant parts. Spread also occurs with wind and rain splash and on crop debris. The fungi survive on seeds or crop residue at least until the next cropping season; some can remain viable on these for many years. Volunteer crop plants or grassy weeds may serve as pathogen reservoirs. Disease management may include fungicide sprays, seed treatments (hot water, fungicides), and crop rotation with non-hosts, but mainly relies on the use of clean seed and resistant crop varieties.
It is noteworthy that the report above also includes a new host record, i.e., the 1st report of _B. saccharicola_, known as a pathogen of sugar cane, infecting maize.
SLB on maize:
https://crop-protection-network.s3.amazonaws.com/articles/Corn-SCLB-Ear-symptoms-signs-Grau.jpg (ear) and
https://www.apsnet.org/edcenter/disimpactmngmnt/labexercises/PlantBiotechnology/Article%20Images/fig33.jpg (affected field)
SLB susceptible vs. resistant maize cultivars: