Greenhouse Scout School 2026

It may depend on what the disease is. I'm hoping Margery might jump in here.

Tiffany, I was thinking that you'd for sure have root rot by bringing plants in in field soil to overwinter. A more prudent practice, when you have more time than you did this winter, would be to take the healthiest looking cuttings you can find in early fall and then root them and pot them up into medium-sized pots for overwintering. That would give you less of a chance of getting Pythium or Rhizoctonia or Fusarium along with the roots and soil of a dug-up plant. But perhaps you are being asked to overwinter gigantic geraniums and you need to maintain the big framework? If so, it sounds like you are already following the essential practice of NOT overwatering or overfertilizing, or you wouldn't be seeing white roots by now! Sounds like you are watering them skillfully. I need to understand what you mean by stem rot, because if you meant death of the main stem as something grew up from the roots, you'd be looking at dead plants. Would a better description of what you are seeing be "branch dieback"? Are they sort of withering back from the tip? Like you, my instinct would be to prune that out, but be careful to cut well below any discoloration, or you'll be moving a fungus (I'm thinking Botrytis) from place to place. The open wounds do favor new infections, you are right. You would want to get cut well below anything that you would describe as brown and mushy, and if you find that you have cut into a mushy soft area, dip your knife into 70% alcohol or 10% bleach (1:9 dilution with water) or some other greenhouse disinfectant to sanitize before rinsing and making another cut on that or another plant. Ideally use a group of tools in rotation so that they can all get a 10 minute soak in the disinfestant. Would you be able to take some photos of representative geraniums to show us the dieback? I'm sure they are stressed by winter, so that is a lot of what's behind what you are seeing; it's probably not pure contagious disease. You could delay the pruning operation til late March so that there is some sunlight and better growing conditions - then the plant can "heal its wounds" much better than when it is in low light with low water and low nutrients and winter daylengths. Commercial greenhouse growers of big geranium crops, by the way, usually don't try to take them thru winter: they buy in rooted or unrooted cuttings in the spring so they can start fresh with vigorous young vegetative plants from a cutting specialist. If you still have some plants showing this stem rotting behavior I'd love to see a few pictures. It sounds like stress and Botrytis without seeing exactly what they look like.

Hi Amy, that would be something the scout and the greenhouse owner would have to discuss as far as what you are both comfortable with (and what type of liability insurance you have). At some of my places I have gone up on very high rolling ladders, but at most greenhouses if the plants are out of reach I just look for signs of cast skins and honeydew on the plants below. If I suspect a plant up above is infested I ask a greenhouse worker to get it down for me to inspect. Sometimes I’ve written in the notes “I couldn’t find the source of the winged aphids on the cards - I suspect they are in the baskets overhead” - then it’s the grower’s responsibility to find them.