Phomopsis stem canker is one of the most important yield-limiting diseases in the world. In the 2010’s, Phomopsis stem canker emerged as the most economically important stem disease in the US, particularly in Northern Great Plains. Prevalence of the disease has varied among years and locations, and is generally highest in seasons with frequent rain events. In severely infected fields, shriveled heads and/or lodged plants can result in very high levels of yield loss.
Phomopsis stem canker is caused by fungal species in the genus Diaporthe. Diaporthe (Phomopsis) helianthi and D. gulyae are the most virulent and damaging of the multiple species known to infect sunflower. These fungi survive on infected sunflower plant debris and other live crop and weed hosts and residues. Leaf infection occurs when ascospores from the infected stubble (or live hosts) are either wind-blown or water-splashed onto sunflower leaves during the vegetative growth stages. If inoculum levels are high enough in stubble left on the soil surface (and the environment is favorable), seedlings and young plants may develop basal lesions and die prematurely. More commonly, infection begins at the leaf edge and colonization progresses upwards via the petiole to the vascular tissues of the stem, where a lesion develops. Lower or middle canopy leaves are more likely first infected due to the more favorable microclimate.
Leaf infection occurs during the vegetative stages but symptoms can be easily missed and are not necessarily diagnostic of Phomopsis stem canker. Infections often begin at leaf margins, forming large triangular bronzed to brown lesions. As the disease progresses from the leaf, pale caramel to light/dark brown or brown/black stem lesions originate from a petiole (thus, stem lesions are centered at the nodes) (Figures 1 and 2). Stem lesions are often visible at flowering growth stages, and over time, enlarge to a size often greater than 6 inches. Multiple lesions may occur on a single plant, and lesions may coalesce on the stem (Figure 3). The stem weakens and ‘hollows out’ under the lesions, where it can be easily punctured by light thumb pressure. Small black fruiting bodies (pycnidia) may be seen in or around older lesions as plants mature. Commonly, infected plants wilt, lodge and/or die (Figure 4 and 5).