According to a report from Heidi Wollaeger, Extension Educator at Michigan State University, growers should be scouting their chrysanthemums for symptoms of tomato spotted wilt virus. So far this year, tomato spotted wilt virus has been detected on five cultivars in chrysanthemum crops in multiple greenhouses in Michigan.
Tomato spotted wilt virus, a tospovirus, is vectored in the greenhouse by western flower thrips (Frankliniella occidentalis). The symptoms of tomato spotted wilt virus are yellow blotching or rings, brown necrotic spotting, and graying and collapsing of stems, often directly under the flower. Growers should look for these symptoms when scouting:
• Blotchy chlorosis in circular patterns and angular necrosis
• Blotchy chlorosis on one branch of mum with circular chlorotic spots on leaves on lower foliage
• Angular necrosis of leaves and plant stunting
• Even yellow, chlorotic blotching on leaves
• Angular marginal necrosis and yellow rings around necrotic region
Symptoms may initially appear on a few leaves or on one branch of the plant, and some cultivars may show symptoms of the virus more readily than others. You should not be able to visually identify the virus, as it has similar symptoms to other viruses and pathogens. You will need to submit samples to a diagnostics services lab or perform in-house testing to verify that the suspected plants do have tomato spotted wilt virus.
For more information, including photos of tomato spotted wilt virus symptoms, recommended products for controlling thrips (managing thrips is essential for preventing the spread of tomato spotted wilt virus because only thrips can spread the virus), and tips on how to keep mums safe for pollinators, check out the complete Michigan State report.