A negative association between leaf sugar content and the development of northern leaf blight lesions in sweet corn

In field-grown sweet corn plants (cv. Jubilee), severity of the northern leaf blight caused by Exserohilum turcicum increased and leaf sugar content decreased with increase in leaf age. Rate of lesion enlargement in these plants was also 1·6 to 3·9 times faster during the night than during the day. Growth chamber studies, using plants at the six-leaf stage, showed that lesions enlarged at a faster rate in plants incubated in short photoperiods under low light intensities than in plants incubated in long photoperiods under high light intensities. Leaf sugar content of plants incubated in long photoperiods and/or high light intensities was higher than in plants incubated in short photoperiods and/or low light intensities. External application of DCMU to intact infected corn leaves inhibited ¹⁴CO₂ assimilation, reduced leaf sugar content in leaves and enhanced lesion enlargement, while external application of sucrose to intact infected plants suppressed lesion enlargement. We conclude that E. turcicum is a "low sugar fungus" and lesion development in the host leaf tissue is negatively associated with photoperiod, light intensity and leaf sugar content.