The Q&A Archives: Petunia Flowers Rotting

Question: We were told that we had botrytis blight on our petunias, after noticing spotting and wilting on our greenhouse petunias. The areas grew and within days wilted and were sticky slimy. We treated our 100 x20 greenhouse with 3 cans of exotherm that was suggested by our greenhouse supplier. We plucked hundreds of flowers off and they'd look better until the new flowers opened and then they do the same thing. We've treated them twice in the past week but it dosn't seem to be doing anything. The foliage on the blue petunias seem to be nice and green though a little spindly. The lilac and whites have yellowing on the tips. Most of the flats seem to be o.k. so far but I'm sure it will go to them soon. The baskets were very full and beautiful before this started but not anymore. I'm thinking of just throwing out all the baskets and hopefully can save the rest. The lobelia, nemophilia, ageratum, geraniums seem to be o.k. Please, any advice would be appreciated. This is our first year with the greenhouse.

Answer: That's a large greenhouse! Any time you have a large number of plants--especially the same type of plants--growing in one place, you're bound to have some pest problems.

If it is indeed botrytis blight--and this is a common disease, also known as gray mold--then you have provided the disease with perfect conditions; the fungus likes relatively low light levels and high humidity. Botrytis spores enter plant tissue through wounds--or even slight bruises--and aging leaves and petals.

There are a few things you can do to limit its spread. First of all, remove all affected plants. Try not to water from overhead, to keep leaves dry. Space plants as widely as possible, and prune as necessary so leaves don't touch neighboring plants. Increase air circulation by opening vents, using a fan, etc. Pick off aging flowers immediately. Unfortunately, few if any plants are immune to the disease, so your whole crop may be at risk.

I suggest you get a good reference book, such as "Greenhouse Gardener's Companion", by Shane Smith (Fulcrum, 1993). You will certainly encounter lots of challenges in your new venture, and this book should help you form a plan of action. If you are trying to start a commercial venture, I suggest you contact some of your colleagues, if possible, to inquire about pest management strategies. Your Cooperative Extension Service may also be able to help.

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