By Kaeley Kindrachuk, TechAg, Crops Extension Specialist, Outlook
Verticillium stripe (Verticillium longisporum) is relatively new to Saskatchewan canola growers. First identified in Manitoba in 2014, the Canadian Food Inspection Agency (CFIA) surveyed fields across Canada in 2015 to gain an understanding of where this disease was. Verticillium stripe was identified in six provinces including Saskatchewan.
Verticillium stripe is a soil-borne fungi causing vascular disease in brassica hosts. This disease is monocyclic disease, which means it only has one lifecycle per season. Verticillium stripe has a very similar lifecycle to clubroot, another soil-borne disease. Verticillium stripe starts as microsclerotia, which are fungal resting bodies, that overwinter in the soil. They seek out root hairs of brassica plants and enter into the vascular tissue of the root or through open wounds on the plant. From there, the microsclerotia send out fungal hyphae that grow into the root tissue. Once in the root tissue, the disease will move up the vascular system of the plant providing the opportunity for the microsclerotia to start forming on the dying tissue of the infected plant. The roots will turn darker grey and stems, as they start to collapse, will begin to turn grey or black, resulting in stem striping. After harvest, the microsclerotia are released back into the soil from infected stubble and debris.
There is still a lot of research to be done on Verticillium stripe but there are a few trends we are seeing at this point. Researchers have seen slow development of this disease in canola during periods of lower soil temperatures (below 12 C). As soil temperature increases, we see increased colonization of plants with Verticillium stripe. This disease also thrives in hot, dry conditions with air temperatures over 15 C. Optimal temperatures for growth of this disease are 23 C and greater.
Over the past few years, agronomists have seen varying symptoms on different parts of the plant. However, symptoms on the whole plant include stunting, early senescence, dieback or entire plant death. Symptoms on leaves may include chlorosis, necrotic areas, abnormal colours, abnormal leaf fall and wilting. Symptoms on the stems may present as stunting or resetting, dieback, internal discoloration and blackening as microsclerotia appear. A telltale characteristic of Verticillium stripe is stem striping, where it looks like only half of the stem is turning color. Later in the season, this striping will start to shred (Figure 1) where the outer layer of stem can easily be peeled revealing the microsclerotia underneath (Figure 2). Typically, Verticillium stripe can be identified around the same time that you would be scouting for other canola diseases near harvest. If you clip the plant at the base of the stem, you may see a grey discoloration (Figure 3) in the cross section. Many of these symptoms can be characteristic of other diseases as well but this list helps to narrow down the possibilities.
At this time, there aren’t any fungicide or seed treatment controls. The best management option is to control brassica weeds and volunteer canola, increasing crop rotation, and biosecurity practices including equipment sanitation and developing an on-farm biosecurity plan.
For more information, contact your local regional crops extension specialist or call the Agriculture Knowledge Centre at 1-866-457-2377.