There are some interesting poinsettia symptoms showing up lately. Here’s why they required a closer look from out pathologists to avoid misdiagnosis of the causal pathogen.
PROBLEM: Over the last couple of weeks, Ball Technical Services team member Nick Flax has received an increase in reports of symptoms consistent with Pythium in poinsettias ... at least at first glance. Chlorotic, curling, wilting leaves and bracts, discolored roots and eventual collapse of symptomatic plants ... sounds like the Pythium we all know and love, right? Interestingly, not so, in quite a few cases!
NICK’S TIP: After our plant pathology team took a closer look, they found a less-common pathogen for this time of year: Rhizoctonia solani. While root and crown rot caused by Rhizoctends to be more prevalent during propagation and earlier in the season, monitor your crops closely for symptoms and get out in front of this issue now, rather than fight an uphill battle right before plants ship or move to retail.
What To Look For
As a soilborne pathogen, Rhizoctonia infects most often near the soil line.
• Dark-colored, sunken cankers develop on stems first.
• As cankers develop, they can become tannish, while borders typically remain darker colored.
• Disease progression typically starts in the lower part of the canopy and causes branches and foliage to wilt and die.
• When lesions girdle the stem, this completely cuts off the flow of water and nutrients to shoots above, and total plant collapse will occur.
This pathogen prefers warm conditions (68–90F [20–32C]) and moderate to high substrate moisture and high relative humidity in the lower part of the canopy.
Maturing canopies, inappropriate crop spacing, wet foliage late in the day and poor air flow through the crop can all contribute to high humidity in the lower canopy.
Though many growers are starting to drop their greenhouse temperatures to tone crops, quite a few regions in North America have experienced unseasonally warm temperatures these past few weeks. This may explain the sudden, out-of-season uptick in disease pressure.
Management
This pathogen must be managed preventatively. Focus on exclusion and crop cultural best practices from the beginning of the crop cycle, such as:
Start with clean inputs. Source cuttings and young plants from trustworthy sources. If disease occurs early in propagation or the finished crop cycle, reach out to your supplier for support.
Only use fresh growing media. Do not recycle potting mix from previously unsold crops. Similarly, plant into new containers or thoroughly clean and sanitize reused pots before planting into them.
Maintain strict footwear sanitation. Have employees clean off their shoes before entering the greenhouse, provide “greenhouse only” footwear for employees, and/or place sanitizing footbaths in high-traffic areas or near the entrances of each greenhouse.
Water early in the day. Set a goal to finish watering by lunchtime and stick to this as much as possible. Put plants on drip irrigation as early as possible to reduce canopy wetness. Or use subirrigation, if you have that type of infrastructure in place.
Minimize crop movement. Rhizoctonia needs an entry point to infect. Excessive handling or movement of plants can lead to small cuts or breaks in the stem and open the door for infection.
Provide preventative protection. Be sure to include an appropriate fungicide in your rotation that provides protection against Rhizoctonia. See fungicide recommendations below.
Outbreak Management
If you confirm the presence of Rhizoctonia in your poinsettia crop, start by roguing out symptomatic plants ASAP. Do not compost infected plants or save growing media and container—throw them away! Next, treat the crop with an appropriate fungicide, including (but not limited to):
• Fludioxonil (ex. Medallion)
• Iprodione (ex. Chipco 26019)
• A strobilurin (FRAC group 11 fungicides; ex. Heritage, Pageant, Mural, Broadform)
• Triflumizole (ex. Terraguard)
Monitor the crop for new symptoms and continue to rogue out affected plants. Also, remember that one pathogen can open the door for secondary infection. Oftentimes, Pythium can also join the party, so it would be best to treat for both pathogens together with a tank-mix fungicide drench or in close succession as separate drenches. Fungicides with active ingredients such as cyazofamid (ex. Segway), etridiazole (ex. Banrot, Terrazole, Truban), or mefenoxam (ex. Subdue MAXX) are my go-tos for Pythium prevention.