Symptoms appear as light green patches on upper surfaces of older leaves. Underneath the leaves in these areas, a purplish or olive-green patch of mold growth is visible. Infected leaves turn yellow and drop off the plant.
Fungus is spread is by wind currents. High humidity and warm temperatures encourage mold growth. The problem is especially severe in greenhouses, where adequate ventilation and air movement reduce disease severity by lowering moisture at the leaf surface. Fungicides are effective controls.