Thanks to Dr. Celeste Welty, OSU Department of Entomology, for contributing to this post.
We have seen an unusually high number of tomato and pepper samples with Tomato spotted wilt virus and related Tospovirus diseases this spring and summer. Symptoms on foliage include necrotic and/or chlorotic spots, and necrotic spots or streaks on stems and petioles. Ringspots may be observed on leaves. Plants tend to be stunted and may wilt. Fruits develop a range of symptoms, from chlorotic blotches to ringspots.
TSWV on pepper – notice necrotic spots on leaves.
Closeup of TSWV on pepper leaf – note ringspot symptoms.
Tospoviruses are transmitted from plant to plant by thrips. Thrips larvae feeding on plants acquire the virus and become infective for the lifetime of the insect. The viruses overwinter in infected plant debris and weeds. Plants infected at the seedling stage are likely to have more severe symptoms than plants infected later.
These viruses are managed differently than aphid-transmitted viruses because they have a longer development cycle within the thrips, so there is hope of getting the problem under control if prompt action is taken. If isolated plants show symptoms, these should be rogued and destroyed as soon as possible. Insecticides can be effective if applied as soon as the problem is diagnosed, however, note that thrips are one of the more difficult pests to kill with our current choices of insecticides. We have Radiant (spinetoram), Movento (spirotetramat), and Exirel (cyantraniliprole) that generally do the best; beware these are quite expensive. Lannate (methomyl) can do ok; the pyrethroids (such as Warrior, Mustang, Baythroid, Asana) are generally poor for thrips control. Movento is systemic but needs an adjuvant such as Dyne-Amic or LI-700 to get it into the plant. Note Radiant and Movento are not allowed in greenhouses or high tunnels.
TSWV symptoms on green tomato fruit (cluster type) in greenhouse (photo courtesy of Anna Testen).
Counties in which downy mildew has been reported on cucurbits. Most recent outbreaks are in red. Map from cdmipmpipe.org.
So far downy mildew has been confirmed on cucumbers in Wayne, Huron, Sandusky, Medina, and Fulton counties in Ohio. We also found it on cantaloupe in our sentinel plots in Huron County on July 28. We have not had reports of downy mildew on squash or pumpkins in Ohio to date. With dry or very dry weather in much of Ohio (1/3 of Ohio is currently under moderate drought conditions), we expect that downy mildew will continue to be less frequent and less severe than in previous years. However, in gardens and fields where it has occurred and fungicides have not been applied, downy mildew has been very damaging. We are recommending Orondis Opti alternated with Ranman, Gavel, Zing! or Zampro (tank mixed with a protectant fungicide like chlorothalanil or mancozeb if not included in the product). Presidio has been an effective fungicide in the past, but several studies have shown that it has lost efficacy in the last few years. Growers should be aware, especially with pumpkins, of other diseases that may be confused with downy mildew. Bacterial spot of pumpkin can cause leaf spots that converge and kill the leaves; however, fungicides are ineffective against bacterial spot. Despite the dry weather, we have seen bacterial spot of pumpkin and other bacterial diseases of vegetable crops this year in Ohio. Bacterial diseases are best managed in vegetables by using clean seed and reducing bacterial populations during transplant production by limiting moisture and applying bactericides.
Bacterial spot symptoms on pumpkin leaf.
Downy mildew symptoms on pumpkin leaf.