Source: CountryGuide [summ. Mod.DHA, edited]
Tar spot continues to spread in Ontario; 2021 saw a "perfect storm" of conditions for the disease, allowing it to spread to 14 more counties, thus expanding the infected region from 5 counties in 2020 to 19 in 2021.
Tar spot was identified in the United States in 2015 [ProMED post 20150920.3658925]. By 2018, it was beginning to make news in Canada, and every year since has brought more evidence of its devastating potential. It was surprising how quickly it developed in 2021, and a major concern is in the overwintering of spores. The call is for a broader understanding of the disease.
Tar spot must be monitored closely as it has the potential to severely affect yields. The weather will drive a lot of the disease, but build-up in geography and inoculum makes it inevitable. The biggest difference from 2020 was growers reporting that every field had some disease.
Available tools need to be adjusted to maximize control. It is important to take an integrated management approach including suitable crop hybrids, scouting, fungicides, and forecasting tools. Effectiveness of longer crop rotations is uncertain.
[byline: Ralph Pearce]
[Tar spot of maize has been known to lead to serious yield losses of up to 75% in Central and South America. It is considered to be a disease complex involving the synergistic association of at least 3 fungal species: _Phyllachora maydis_, _Microdochium maydis_ (previously _Monographella maydis_), and _Coniothyrium phyllachorae_.
Of these, _P. maydis_ is usually the 1st to cause leaf lesions. While _M. maydis_ is a common benign saprophyte on leaf surfaces, it becomes highly virulent only in association with _P. maydis_ and forms necrotic rings around the _P. maydis_ lesions. _C. phyllachorae_ may be a hyperparasite of the other 2, but its role is not fully understood yet. Leaf lesions may coalesce, causing blight and complete burning of the foliage. In addition, characteristic black shiny spots ("tar spots") are produced both within lesions and on other leaf areas. Affected ears have fewer kernels, which may germinate prematurely on the cob. Weakening of stems may lead to increased lodging. The disease reduces photosynthetic potential and therefore plant vigour.
_P. maydis_ is an obligate parasite; its spores are spread by wind and with infected plant material. It produces a potent toxin killing plant tissue. The disease is favoured by cool, humid conditions. Tar spot management may include fungicide treatments and use of maize varieties with tolerance or low sensitivity to the disease. However, resistance breeding is difficult because of the involvement of multiple pathogens. So far, little is known about the genetics of tar spot resistance.
Initially, only _P. maydis_ had been detected in North America with a reported limited effect on yield (ProMED post 20171015.5382480). Increase in disease severity in the region since then may indicate either the additional presence (naturally or by introduction) of _M. maydis_ or appearance of more virulent fungal strains. Both scenarios could potentially lead to more severe yield losses and an increased risk to maize production.
Canada (with provinces):
Tar spot on maize leaves:
Tar spot symptoms on maize ears:
Information on tar spot complex of maize:
https://www.youtube.com/watch?v=ErB9pdiXPp4 and via
Tar spot information & resources via:
_Phyllachora maydis_ taxonomy:
_Microdochium maydis_ taxonomy and synonyms:
_Coniothyrium phyllachorae_ taxonomy: