From Jan Byrne, MSU Diagnostic Services, Department of Plant, Soil and Microbial Sciences, and Tom Dudek, Michigan State University Extension
Thrips can be a major pest, during the summer months, to fall garden mums. Michigan State University Extension advises growers to be aware that thrips are the vector for tospoviruses like tomato spotted wilt virus (TSWV), impatiens necrotic spot virus and iris yellow spot virus.
TSWV has one of the largest host ranges of any plant virus, affecting both dicots and monocots. It infects more than 800 plant species with more than 80 plant families. In the past few weeks, TSWV was found in a crop of greenhouse-grown fall mums.
Western flower thrips (Frankliniella occidentalis), onion thrips (Thrips tobaci) and tobacco thrips (Frankliniella fusca) are the three primary species to vector tospoviruses in Michigan. The western flower thrips are the most common pest found in greenhouses during spring production of bedding plants. However, with the heat of the summer, greenhouses utilize passive and active venting that allows the entry of the other thrips species.
Symptoms of TSWV on mums can be easily confused with those caused by fungal pathogens. Growers need to be aware of the symptoms in order to scout effectively. Symptoms can develop on the leaves, stems and flower buds of mums. The leaf tissue may have yellow rings or ring patterns that become necrotic. The stems of infected plants show a grayish canker (Photo 1), causing the stem to collapse and fall over (Photo 2).
Photo 1 (left); photo 2 (right)
Read more here.