From now until mid-November is when you’ll most likely see plant losses in poinsettia due to severe wilting. These losses can be considerable: anywhere from 2-15% in Niagara operations in past years, with disease pressure seeming especially high this year.
Pythium may be considered the most “likely” culprit in Poinsettia in this area, but this isn’t always the case, and misdiagnosis can mean wasted fungicide applications. Read on for a list of likely suspects and appropriate control measures.
Common Culprits:
The below chart can help serve as a guide to what could be causing your poinsettias to wilt on the bench. This poinsettia diagnostic key from NC State may also help narrow down the issue. Unfortunately, many root rot diseases have extremely similar symptoms, and many are almost impossible to properly differentiate correctly without further testing.
In all cases, once you suspect one of these diseases, it’s best to have the pathogen confirmed at least once by DNA analysis by sending it Lab Services at the University of Guelph or another facility, or by having an OMAFRA agent send in samples for you (a free service if the answer isn’t readily apparent after an on-farm visit).
Timing |
Symptoms* |
Likely Culprit |
Response** |
Can occur at any stage of production but may not show up until November |
The base of infected stems appear soft and wet. Roots are brown and water soaked. Under dry conditions, the internal pith of the lower stem is brown when cut; the stem has a gray canker. |
Phytophthora root/stem rot (Phytophthora nicotianae or drechsleri) |
Rogue infected plants. Clean tools, keep hose ends off the ground. Apply a fungicide to protect plants, e.g. Truban (etridiazole), Torrent (cyazofamid). Control of this pathogen is difficult; exclusion is key; consider treating water source. |
Can occur at any stage of production |
Early in the season, rooted cuttings are stunted, yellow, and wilting. Roots are dark brown and can look “wet”; the outer layers of root tissue strip off leaving a bare strand of inner vascular tissue exposed. Later in the season, lower leaves yellow and drop; plants are stunted. |
Pythium Root Rot (various Pythium species) |
Rogue infected plants. Clean tools, keep hose ends off the ground. Do not over-water or over-fertilize plants. Apply a fungicide to protect plants: e.g. Subdue Maxx (metalaxyl) or Truban (etridiazole) |
Often kills plants immediately after transplant; if disease progresses slowly than may not die until November |
Early in the season, cuttings wilt and yellow. Roots are rotted. Lower stems below ground may have a shredded appearance. Later, lower leaves may yellow and fall off; plants flower prematurely, and die. Sunken dark brown cankers on stem may reach slightly above soil line. Brown lesions visible on roots. |
Rhizoctontia wilt (Rhizoctonia solani) |
Rogue infected plants. Clean tools, keep hose ends off the ground. Do not over-fertilize. Apply a fungicide to protect plants, e.g. Heritage (azoxystrobin). |
September to October, usually |
Plants may be stunted; will wilt suddenly and die; may defoliate. Roots and crowns may develop brown-black discoloration and become soft. Rotted areas may develop cream-orange, wet-looking, spore masses. |
Fusarium wilt (Fusarium oxysporum) |
Rogue infected plants. Clean tools, keep hose ends off the ground. Apply a fungicide to protect plants, e.g. Senator (thiophanate methyl) or Medallion (Fludioxonil). These products need to be applied frequently (e.g every 14-21d) to provide control once an infection occurs. |
Late in Season under cooler weather conditions |
Roots turn black. Plant wilt. Longitudinal splits form at the stem base at and below soil line. Leaves yellow and fall. |
Black Root Rot (Thielaviopsis basicola) |
Rogue infected plants. Clean tools, keep hose ends off the ground. Apply a fungicide to protect plants, e.g. Senator (thiophanate methyl) or Medallion (Fludioxonil). These products need to be applied frequently (e.g. every 14-21d) to provide control once an infection occurs. |
* Symptom list adapted from Penn State, NC State, and Texas A&M Extension** Other fungicides may also be effective; these are only some suggestions. Always check the label before applying. |
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Unfortunately, once symptomatic plants are already present, it may be too late for fungicides to actually cure much. Your best bet is to reapply the chemicals as often as is allowed on the label, and make sure to rotate as many chemistries as possible to avoid resistance.
Other problems not included in the above chart, like leaf yellowing, curling or overall stunting, are likely due to nutrition issues. Check out this previous post on poinsettia nutrition and ask yourself if the problem is widespread (likely nutritional) or with a more patchy distribution (diseases tend to hit a group of plants in the crop).
Planning for Next Year: Prevention
Experiencing significant losses from one of the above culprits this year? Or have a smaller issue year after year that just won’t go away? Then it’s likely time to consider a preventative application on the entire crop next year. Diseases like Pythium and Fusarium are especially hard to control once symptoms actually appear. Getting on top of the problem early is always a better choice, both efficacy-wise and pocket-book wise.
Unfortunately, for most of these diseases, a single fungicide application likely isn’t going to achieve much, so be prepared to make a spray schedule and stick to it. Here’s an example program:
- Intial drench of fungicides while poinsettias are still in the trays. Although some growers are hesitant to apply fungicides to poinsettia at this stage due to fears this may delay rooting (especially when grown in Oasis strips), this appears to be unfounded for most fungicides (see this study). This helps kill any pathogens initially colonizing the roots.
- Drench of Microbial fungicides (like Actinovate, Cease, PreStop, Rootshield, Taegro and Trianum on cuttings) several days after chemical fungicides. The beneficial organisms they contain occupy the space where pathogenic fungi would normally grow, and also actively fight them off. They can also help improve rooting.
- Reapplication of both chemical and microbial fungicides at potting (1 day apart). The active ingredients in many fungicides don’t offer protection for long, so they WILL need to be repeated. Same with microbials: they act entirely preventatively, so once you’ve put the cuttings in a bigger pot, you need to add more.
- Another chemical drench 2 weeks after potting.
Since a spray schedule like this isn’t likely a long-term answer, the next step is to see if you can identify where your pathogen issue is coming from.
Where does your water coming from? Have you had it tested Do you treat? Is your treatment being effective? Could your sanitation be improved? What about fungus gnat control in the greenhouse (notorious for spreading pathogens)?
Stay tuned for upcoming posts on santitation in the greenhouse and common sources of disease.
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