Robigalia Roundup LXXVIII: Fields of Remembrance

Exserohilum turcicum was first described in Italy by Passerini in 1876, and in the nearly 150 years since, it has become one of the most consequential foliar pathogens of maize on earth. Between 2016 and 2022 alone, northern corn leaf blight (NCLB) was responsible for an estimated 11.8 billion kilograms of corn grain lost across the United States and Ontario, Canada.

Exserohilum turcicum (teleomorph: Setosphaeria turcica) is a hemibiotrophic ascomycete, biotrophic for roughly the first eight days of infection before switching to a necrotrophic lifestyle. It produces characteristic elongated conidia with a protruding hilum, and overwinters as mycelium and conidia in infected corn residue on the soil surface.

Infection requires a sustained period of leaf wetness, typically six hours or more, while temperatures sit between 18 and 27°C. Under those conditions, new lesions can form within one to two weeks and fresh conidia are rapidly dispersed by wind and rain, enabling cycles of secondary infection through the canopy.

The disease affects maize (Zea mays) as its primary host, though sorghum is also susceptible. On susceptible hybrids, the first visible symptoms are small, water-soaked, grey-green spots that elongate parallel to the leaf veins into the disease's diagnostic feature: cigar-shaped, tan to grey lesions typically between 2.5 and 15 centimetres long. As lesions mature, dark areas of conidial sporulation develop within them, giving the lesions an almost dusty appearance.

Infection progresses from the lower canopy upward, and when it takes hold before silking, severe blighting can leave a plant looking as though it has been killed by frost well before season's end. On hybrids carrying race-specific Ht genes, symptoms are attenuated to smaller, chlorotic lesions with reduced sporulation, though these responses are not durable against all races.

Exserohilum turcicum is globally distributed across temperate, tropical, and subtropical maize-growing regions, with disease pressure particularly high across sub-Saharan Africa, South Asia, Latin America, and the U.S. Corn Belt. In Ethiopia, yield losses of 30–70% have been recorded under favourable conditions. In heavily infected fields, NCLB also predisposes plants to secondary stalk rot, compounding the final yield penalty beyond what foliar damage alone would suggest.

Management relies primarily on genetic resistance, and breeding programmes have identified multiple dominant Ht genes (Ht1, Ht2, Ht3, Htn1) that reduce lesion size and sporulation. However, the pathogen's genetic diversity means that individual Ht genes have limited durability; races virulent on one gene selection are routinely found alongside it, which has pushed breeders toward pyramiding multiple resistance sources.

Polygenic quantitative resistance tends to be more durable than single-gene approaches. Fungicide applications between VT and R3 offer additional suppression when conditions favour severe disease, though fungicide sensitivity monitoring is increasingly important as populations with reduced sensitivity to demethylation-inhibitor chemistry have begun appearing.

This week's Plant Pathologist of the Week, Vitor Augusto Silva de Moura, is characterising the genetic and pathogenic diversity of E. turcicum populations in Florida, where the pathogen is an active target of research alongside tar spot. Keep reading to learn more about his work.

This week, we meet Vitor Augusto Silva de Moura, a PhD candidate in Plant Pathology at the University of Florida, based at the Everglades Research and Education Center in Belle Glade, where he works under the mentorship of Professor Katia Xavier.

Vitor traces his interest in plant pathology to field days he attended as a teenager in Brazil, where growers gathered with questions about nematode damage in their row crops. Watching a pest so invisible cause such visible economic harm left an impression.

When he enrolled in Agronomy at the Federal University of Viçosa (UFV), specialising in the biological control of plant-parasitic nematodes felt like a direct answer to what he had witnessed. That undergraduate training sharpened his instinct for applied research and eventually pointed him toward a PhD in Florida.

His doctoral work sits squarely in the Everglades Agricultural Area, one of the most productive sweet corn regions in the United States and one increasingly threatened by two fungal diseases: northern corn leaf blight, caused by Exserohilum turcicum, and tar spot, caused by Phyllachora maydis. Both can cause serious losses if management decisions are made too late.

Vitor approaches these diseases across multiple scales, combining field trials, greenhouse studies, and laboratory analyses to understand how they develop, how they can be controlled, and where sources of resistance might be found in sweet corn germplasm.

His published work on the survival of P. maydis ascospores across different Florida soil types and environmental conditions is already contributing foundational knowledge to a disease that, until recently, was poorly understood in subtropical settings.

Within his first three years, two papers have been accepted, spanning phylogenetics, genome-wide association studies, survival analyses, field fungicide efficacy trials, and in vitro sensitivity testing. His research reflects his commitment to keeping his work connected to the agricultural systems and the growers it is meant to serve.

You can connect with Vitor on LinkedIn to follow his work on sweet corn pathology in Florida.

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See you next Monday!

P.S. Why Robigalia? The name originates from the Ancient Roman festival dedicated to crop protection. You can read all about the history here: