Verticillium Wilt
Verticillium Wilt / Leaf Mottle
Verticillium wilt (Verticillium dahliae, V. albo-atrum) is found sporadically in the U.S. Great Plains but is economically damaging when it occurs. The common name is somewhat misleading, since the most commonly seen symptom is interveinal chlorosis, which is termed leaf mottling. Actual wilt only occurs in the most severe cases. The fungus is soil-borne, survives as microsclerotia for many year, and has a wide host range, which includes potato and several broadleaf weeds.  Weed hosts commonly found in the U.S. sunflower growing region include Abutilon theophrasti, (velvetleafAmaranthus spp.(amaranth, pigweed), Ambrosia spp.,(ragweedChenopodium album (Lambsquarter),  Cirsium arvense, (Canada thistle)  Datura spp. (Jimsonweed), Malva spp.(Mallow) Polygonum persicaria, (smartweed), Senecio vulgaris( groundsel), Solanum nigrum, (Black nightshade), Sonchus oleraceus (Sowthistle) and Xanthium spp (cocklebur). Verticillium also may be seed-borne, and overwinters in infected plant residues.

Infection begins when germinating microsclerotia infect growing sunflower roots. The disease progresses inside the stem, initially producing a brown ring in the vascular tissue (Figure 1). The outside of the pith will become colonized with gray-black powdery microsclerotia, which eventually detaches from the stem tissue, leaving a hollowed stem with limited integrity, prone to lodging. (Figure 2).
Verticillium Wilt
Figure 1. Verticillium wilt – healthy (L) and developing infection (R).
 
 
verticillium wilt
Figure 2. Verticillium wilt - compressed and blackened with pith.
 
In severely infected plants, the exterior of the stem may be coated with dusty grey-black microsclerotia that can be scraped off easily with a fingernail. Although a root/stem disease, symptoms are first noticed in leaves. Interveinal chlorosis (yellowing) and necrosis (browning) occur earliest (and most severely) on the lower leaves and appear to progress upward on the plant (Figures 3, 4, 5). The fungus produces a toxin, which causes the leaf mottling (chlorosis between veins) and the fungal mycelium plugs the water-conducting (phloem) tissues, which leads to the eventual wilting.

Verticillium wilt commonly occurs in small clusters of sunflower plants, and yield loss occurs as plants wilt and die in reproductive growth stages. Crop rotation ( > 5 yrs) with grain nonhosts (for example, wheat and corn), avoiding fields with a history of the disease and selection of a resistant sunflower hybrid (if available) are important management tools for Verticillium wilt.  The fungus is known to have at least two races that overcome single resistance genes, so planting a “resistant” variety is effective only if the race and the genetics of the resistant hybrid are known. 
verticillium wilt
Figure 3. Beginning stages of Verticillium wilt, with only lower leaves showing symptoms.
 
verticillium wilt
Figure 4. Verticillium leaf mottle symptoms.
 
verticillium wilt
Figure 5.   Row of Verticillium-susceptible plants, showing leaf symptoms on all leaves, compared to resistant hybrid in adjacent row.
Images
Figure 1. Verticillium wilt – healthy (L) and developing infection (R). (Sam Markell, NDSU)
Figure 2. Verticillium wilt - compressed and blackened with pith. (Sam Markell, NDSU).
Figure 3. Beginning stages of Verticillium wilt, with only lower leaves showing symptoms. (Bob Harveson, University of Nebraska)
Figure 4. Verticillium leaf mottle symptoms. (Sam Markell, NDSU)
Figure 5. Row of Verticillium-susceptible plants, showing leaf symptoms on all leaves, compared to resistant hybrid in adjacent row. (Tom Gulya, USDA-ARS).
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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