A Quick Review of Poinsettia Downy Mildew

Nora Catlin

After a long while of not seeing powdery mildew on poinsettias, several cases were seen last season. Perhaps that was a fluke and we won’t see it again—let’s hope that’s the case. But let’s use the mildew seen last year as a reminder and take the opportunity to review this disease.

Typically, you see the white dusty or powdery fungal growth on the surfaces of leaves; however, the early symptoms on poinsettia are pale and subtle yellow or chlorotic spots on the leaf surface with white/light gray fungal growth on the leaf undersides. As the disease progresses, you can see the fungal growth develop on the leaf surfaces and even the bracts.

When to look for symptoms and signs

You should be scouting for symptoms and signs of the disease throughout the whole crop cycle; however, be particularly watchful early in the season. Finding and identifying them earlier in the crop means easier and effective management. Waiting for the epidemic to build means it will be much more challenging to get the disease under control, plus your fungicide options are more limited later in the season. Realize that some labels warn against applying to bracts, and residue close to sale is another consideration.

Epidemics can very quickly develop in greenhouses once conditions are favorable. The disease can be slowed or halted with high temperatures. Temperatures under 86F (30C) and relative humidity of 85% or higher will favor the disease and will cause rapid development.

■ Scout regularly and carefully, starting early and continuing through the season, paying especially close attention in the fall when temperatures become cooler (68 to 86F/20 to 30C) and ideal for disease development.

■ Seek the assistance of a diagnostic lab or your local extension specialist when needed.

■ Keep humidity under 85%.

■ Timely applications of effective fungicides is key to management. Don’t wait for this disease to explode before trying to get it under control; you’ll want to ensure good management prior to bracts forming. Repeat applications at regular intervals as per label instructions and be sure to rotate between products with different modes of action.

Pay attention to notes on plant safety. If using a product new to you for poinsettias, test on a small number of plants prior to treating the entire crop; realize that some products may not cause injury on leaves, but may injure bracts. Active ingredients that have shown good efficacy in published trials with poinsettia powdery mildew include strobilurins (trifloxystrobin, azoxystrobin, pyraclostrobin), triflumizole, myclobutanil, triadimefon and piperalin. Many other products are labeled and can be useful in your fungicide rotation programs. GT

Nora Catlin is a Floriculture Specialist at the Cornell Cooperative Extension in Suffolk County.