Indiana Department of Natural Resources sent this bulletin at 04/27/2022 07:40 AM EDT
Weekly Review for April 27, 2022
This informal report by the Division of Entomology & Plant Pathology is a commentary on insects, diseases, and curiosities division staff encounter on a week-to-week basis. Comments and questions about this report are welcome and can be sent to your respective Inspector.
Ren Hall (Nursery Inspector & Compliance Officer) - RHall@dnr.IN.gov
The past few weeks I have been finishing up greenhouse inspections in my territory. This is a follow-up on impatiens necrotic spot virus and thrips infestations in greenhouses, an issue I have reported about for the last few years. I visited two greenhouses at which I have found INSV and thrips (the insect vector for INSV) in previous years, and one greenhouse where I haven’t found this problem before.
At the first greenhouse, I found no evidence of thrips on any plants, but they had several coleus varieties riddled with INSV ringspots. After talking to the grower, they got rid of anything symptomatic last year, emptied most of the greenhouse over winter, and have treated for thrips. However, they propagate coleus by tip cuttings, and this is another way INSV can be spread. If the parent plant is infected, the cuttings will be too.
It’s possible that some infected plants were not showing symptoms at the time of cutting but are now. They had also gotten in (from a different grower) a new coleus variety in recent weeks which was beginning to show symptoms. So, unless it was a coincidence that the shipper also has an INSV infestation, there may yet be thrips hiding in the greenhouse.
Thrips are notoriously hard to get rid of because they can be hiding in soil or flowers at the time of treatment (inadequate coverage), multiple treatments are likely needed, they may be harboring in weeds just outside the greenhouse, they may be harboring in “pet plants” that greenhouses refuse to discard, and insecticide resistance is possible as well.
At the second greenhouse, there continues to be a thrips infestation. I did not find any evidence of INSV on any plants, but maybe there will be signs as the season progresses. There were a few varieties of calibrachoa that were very heavily infested with thrips. I thoroughly checked all the New Guinea impatiens and fancy coleus varieties since every time I have found this virus, these two plants show obvious symptoms, especially characteristic ringspots – these are the “indicator plants” I look for when I suspect INSV.
At a third greenhouse, I found necrotic lesions on New Guinea impatiens that had me taking a much closer look. The lesions are similar to a ringspot caused by INSV, but the shape was slightly different, and I was finding more lesions along the leaf margin, which I hadn’t observed with INSV before. On one small lesion, I did find spores of botrytis, which is a fungal infection common in greenhouses this time of year. I did not find any evidence of thrips and the grower stated they have not had any problems with thrips. To be safe, I did send a sample to PPDL who confirmed the plants had a botrytis infection.
I am including pictures I took of INSV symptoms on impatiens from last year (confirmed by virus testing at the lab) along with the confirmed botrytis on impatiens from this year to show how similar it looks. I think there are subtle differences in the shape and placement of the ringspots on impatiens, and other pieces of evidence should be considered before diagnosing the issue. The biggest thing to me - are thrips present? Other things to consider are how is the sanitation in the greenhouse, checking other varieties of plants for symptoms, talking with the grower (often they are aware of pest and disease issues already), and checking for spores of botrytis. At the third greenhouse (where it was Botrytis, not INSV), everything was very clean and I found very little evidence of any plant disease or pests, so it was hard to rule one way or the other. I always encourage sending a sample to the lab for confirmation. Botrytis is a common issue we see in almost every greenhouse depending on time of year and environmental conditions, whereas INSV can be more serious and much more difficult to control due to its insect vector and wide host range (~600 plant species). As shown above, it can take multiple years to deal with and is very hard to be rid of once it is there.
I have already seen several adult bean leaf beetles that have emerged from overwintering. Seems a little early to me, but maybe that is just my perception of our weather. These guys will feed on soybeans and other legumes for a bit before laying eggs near the roots of a bean plant. They produce two generations per year.
Keep an eye on your calibrachoa. In addition to the thrips Ren noted, there have been some reports of chili pepper mild mottle virus (CPMMoV) coming through some suppliers. This virus is similar to tobacco mosaic virus (TMV) in that it can be transmitted by touch, so be careful handling plant materials and use best management practices to prevent spread. CPMMoV will cross-react with TMV test strips, so testing is possible. Learn more about this virus here.
