In Vitro and In Planta Studies on Temperature Adaptation of Exserohilum turcicum Isolates from Maize in Europe and South America

Abstract

Northern Corn Leaf Blight (NCLB) is a fungal leaf disease in maize caused by Exserohilum turcicum. NCLB occurs worldwide, from tropical to temperate zones raising the question about plasticity of temperature adaptation of local isolates of the pathogen. Seven isolates of E.turcicum originating from South America and seven from Europe were compared for their response to temperature variations in vitro and in vivo between 15 and 30 °C. In vitro, isolates originating from Europe and South America significantly differed in mycelial growth rate at 30 °C and in sporulation at 25 °C and 30 °C. Aggressiveness of E. turcicum isolates was evaluated on three susceptible maize cultivars (maize lines B37, Sus1 and the German hybrid Niklas) under different day/night temperature regimes (15/10 °C, 20/15 °C, 25/20 °C, or 30/25 °C) with a photoperiod of 14 h. Aggressiveness, recorded as area under the disease progress curve (AUDPC), of South American isolates was higher than for European isolates at 15 °C, 20 °C and 25 °C, and for sporulation in vivo in all temperatures. In general, aggressiveness components were most influenced by temperature. Therefore, multivariate analysis was performed with aggressiveness component data at 30 °C, which expressed the highest number of variables with significant differences between isolate origins. According to their aggressiveness, European and South American isolates can be grouped separately, demonstrating that South American isolates are better adapted to higher temperatures and display a higher level of aggressiveness under similar conditions than European isolates from a cool climate. It is concluded that plasticity of temperature adaptation in E.turcicum populations is relatively large and allowed E. turcicum to follow the recent expansion of maize cultivation into cool climate zones in Europe. However, our data suggest that adaptation to higher temperature is likely to increase aggressiveness of NCLB on maize in cooler climate zones when experiencing further climate warming. This plasticity in adaptation to environmental conditions of E.turcicum may also hamper the success of breeding programs as it may decrease the durability of resistance.

Global warming and increasing maize cultivation demand comprehensive efforts in disease and insect resistance breeding in north‐western Europe

“…A comparison of isolates of S . turcica from South America and Europe suggested that aggressiveness of NCLB on maize in cooler climate zones will increase with rising temperatures (Navarro et al, 2021). In addition, recent climate change, namely warmer weather conditions throughout the year (e.g., mild and moist winters supporting inoculum survival and reproduction on maize residues) may also have favoured leaf diseases in maize (Miedaner, 2018).…”

“…For example, NCLB caused by S . turcica is at present the most troublesome leaf disease in European maize cultivation (Navarro et al, 2021), provided there is sufficient humidity for infection (Galiano‐Carneiro & Miedaner, 2017) during the growing season. There are qualitative and quantitative resistance sources; however, some of the qualitative, race‐specific Ht genes are already ineffective due to virulent pathogen populations.…”

Distribution, Etiology, Molecular Genetics and Management Perspectives of Northern Corn Leaf Blight of Maize (Zea mays L.)

“…The disease is widely distributed and economically the most important foliar disease of maize [2,3]. The disease has a worldwide distribution predominantly in areas with 75%-90% relative humidity and 22°C-25°C temperature during the cropping season [4,5]. NCLB causes enormous damage to the maize crops, and grain yield losses range from 24% to 91% [6,7], depending on the growth stage of the crop at which infection occurs, the severity of the outbreak, the resistance of the host plant and the virulence of the pathogen.…”

Comparative mitochondrial genome analyses reveal conserved gene arrangement but massive expansion/contraction in two closely related Exserohilum pathogens

“…As the type species of Exserohilum , E. turcicum , a filamentous hemibiotrophic fungus, is notorious for causing northern corn leaf blight (NCLB) [3] , [5] . Of late years, this disease has become increasingly problematic worldwide with the extensive planting of susceptible varieties of maize in many countries [6] , [7] . The most common symptom of NCLB is necrotic green-gray lesions on leaves with a length range from 2 to 14 cm, which could lead to 10–40% grain yield losses in severe cases [8] .…”