Phomopsis Stem Canker
Phomopsis Stem Canker
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 of 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 to 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 (pycnida) may be seen in or around older lesions as plants mature. Commonly, infected plants wilt, lodge and/or die (Figure 4 and 5). 
 
fig 1 phomopsis
Figure 1. Developing Phomopsis stem canker lesion
fig 2. phomopsis
Figure 2. Developing Phomopsis stem canker lesion
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Figure 3. Large Phomopsis stem canker lesions coalescing
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Figure 4. Lodged sunflower at a Phomopsis stem canker lesion
fig 5. phomopsis in field
Figure 5. Severe wilting and premature plant death caused by Phomopsis stem canker

Lesion color and stem integrity are helpful when differentiating Phomopsis stem canker from Phoma black stem lesions or Sclerotinia stem rot. Phomopsis stem canker lesions are generally brown, while Phoma black stem lesions are distinctly black (Figure 6), and Sclerotinia stem rot lesions are cream-colored and resemble dried bone.  When pressure is applied to lesions of Phomopsis stem canker (such as by pressing infected stems between thumb and forefinger) the stem often feels hollow, whereas Phoma black stem lesions are usually superficial and do not weaken the stem. Mature lesions of Sclerotinia stem rot are also weaken the stem, but the lesion can commonly be shredded; whereas Phomopsis stem canker lesions are structurally intact but with a hollow pith.
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Figure 6. Brown Phomopsis stem canker lesion (left) and black phoma black stem lesion (right)

Even when using every management tool available, Phomopsis stem canker may not be adequately controlled in favorable environments for disease development. Crop rotation, elimination of residue, diligent control of volunteer and wild sunflowers and strategically avoiding planting near a previously infected crop are important. A four-year crop rotation with non-hosts (e.g. wheat and corn) may lower the pathogen survival, but research data is lacking. Selection of a less susceptible (or partially resistant) hybrid is critical in areas prone to Phomopsis stem canker. Foliar application of an efficacious fungicide at early reproductive growth stages (R1 growth stage) may help mitigate the disease, but will only provide partial control of the disease. Research efforts to optimize fungicide timing, selection and economics is ongoing, and consultation of the most recent recommendations is warranted before making an application.
Images
Figure 1. Developing Phomopsis stem canker lesion (Sam Markell, NDSU).
Figure 2. Developing Phomopsis stem canker lesion (Sam Markell, NDSU).
Figure 3. Large Phomopsis stem canker lesions coalescing (Tom Gulya, USDA).
Figure 4. Lodged sunflower at a Phomopsis stem canker lesion (Sam Markell, NDSU).
Figure 5. Severe wilting and premature plant death caused by Phomopsis stem canker (Sam Markell, NDSU).
Figure 6. Brown Phomopsis stem canker lesion (left) and black phoma black stem lesion (right) (Sam Markell, NDSU).
Additional Resources
Recent NSA Research Forum Papers:

Evaluation of fungicides for their efficacy against Phomopsis stem canker of sunflower using remote sensing technology.

Physiological and molecular mechanisms of resistance to Sclerotinia and Phomopsis.

Evaluation of fungicides for their efficacy against Phomopsis stem canker of sunflower.

Isolation and pathogenicity of Phomopsis from symptomless sunflower.

Identification of Phomopsis gulyae on weed hosts.
Other NSA Resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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