Cause Plasmopara obducens is a host-specific, fungus-like organism known as an oomycete, or water mold. There have been sporadic reports in greenhouses since 2004, but it was not until summer 2011 that regional outbreaks of this disease were seen for the first time in landscape beds and container plantings in North America. The disease was found in landscapes in Oregon and Washington since 2012. Impatiens business dropped 50% to 60% due to this disease alone.
Oospores survive in plant debris and are released into the soil or media as infected stems decay. These resting spores can survive in the soil for years. Oospores produce sporangia, which are then splashed by rain or carried by wind to new plants. These airborne spores may find their way into greenhouses as well as landscapes. Young plants and immature plant tissues are especially susceptible to infection. Once on the plant, sporangia may germinate directly or release swimming zoospores. Zoospores can swim in moisture on the leaf surface, encyst, then germinate and penetrate the leaf tissue through the stomata by means of a germ tube. The time from infection to the appearance of new sporangia varies from 5 to 14 days depending on the age of plant tissue, temperature, and humidity. In response to changes in light, temperature, and/or humidity, the new sporangia are dislodged and can easily become airborne or splashed by rain or water to other leaves and distant plants. Complete plant loss can occur in as little as 4 to 6 weeks. There is no evidence of seedborne transmission.
All cultivars and intraspecific hybrids of I. walleriana are susceptible to impatiens downy mildew. A few species of wild impatiens are also hosts of this disease, but there are no other known hosts. New Guinea impatiens (Impatiens hawkerii) are highly tolerant and have been successfully grown in greenhouses and landscapes even under high disease pressure.
Symptoms A light-green yellowing or stippling of infected leaves develops first. Very subtle gray lines or markings are sometimes observed on the top of the leaf. Infected leaves may curl downward at the edges, but generally this is a more advanced symptom of infection. Leaf symptoms are often first observed on the younger leaves or terminal growth. Seedling cotyledons are also highly susceptible. Plants infected at an early stage of development may be stunted in both height and leaf size, yet may show no visible signs of sporulation.
A white, downy-like growth (sporangia) may be visible on the underside of infected leaves during cool and moist or humid conditions. Eventually leaves and flowers will drop, resulting in bare stems with only a few tiny, yellow leaves remaining. This symptom is more likely to be observed in a landscape setting and may be blamed on deer or rabbit damage. The remaining green stems then collapse.
Scouting Frequent scouting of the greenhouse crop, looking for early leaf symptoms (subtle stippling or yellowing), is critical for early detection. Turn leaves over to observe the undersides for the white, downy growth.
Cultural control Aggressive scouting, environmental controls, and good sanitation will help manage this disease in the greenhouse.
- Avoid the introduction of diseased plant material. Inspect liners and plugs on delivery. Segregate any newly arriving material to observe for symptoms.
- Grow vegetative cuttings and seed-grown plants in separate greenhouses to limit cross-contamination.
- Reduce greenhouse humidity and limit leaf moisture, especially at night, to limit secondary spread.
- Provide good air movement through plant spacing and horizontal airflow.
- Irrigate when leaves will dry most quickly to help minimize leaf wetness and humidity.
- Promptly bag and seal symptomatic plants and any fallen leaves or petals before carrying them out of the greenhouse. Before planting in the landscape, and at the end of every season, remove as much plant debris as possible including leaves, stems, and roots.
- Rotate landscape planting beds each year so that impatiens are not grown in the same place for 3 or 4 years.
- Consider New Guinea impatiens, begonias, coleus, or other alternative plants where impatiens were grown the previous year.
Chemical control Must use fungicides in conjunction with cultural controls and before the disease is found. Apply fungicides preventively on a 7- to 14-day interval (depending on material) for maximum efficacy. Scouting first then spraying is an ineffective tactic. Alternate or tank-mix products from different groups that have different modes of action.
- Adorn at 1 to 4 fl oz/100 gal water plus another fungicide. Impatiens is NOT specifically mentioned on the label but can legally be used. Has some eradicant activity. Saturate the root zone thoroughly when drenching. Group 43 fungicide. 12-hr reentry.
- Agri-Fos at 1.25 to 2.5 quarts/100 gal water. Saturate the root zone thoroughly when drenching. Group 33 fungicide. 4-hr reentry.
- Aliette WDG at 1.25 to 4 lb/100 gal water. Has not been effective. Group 33 fungicide. 12-hr reentry.
- Alude at 1.25 to 2.5 quarts/100 gal water applied as a foliar spray at 14- to 21-day intervals. Saturate the root zone thoroughly when drenching. Group 33 fungicide. 4-hr reentry.
- Disarm O at 1 to 4 fl oz/100 gal water. Group 11 fungicide. 12-hr reentry.
- Fenstop at 7 to 14 fl oz/100 gal water. For greenhouse use only. Impatiens is NOT specifically mentioned on the label but can legally be used. Has some eradicant activity. Group 11 fungicide. 12-hr reentry.
- Heritage at 1 to 2 oz/100 gal water plus a non-silicone-based wetter sticker. Poor to some control. Group 11 fungicide. 4-hr reentry.
- Mancozeb-based products can be used as mixing partners. Limit total applications as these products can build up on plants causing stunting and yellowing. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Pentathlon DF at 1 to 2 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Mefenoxam 2 AQ at 0.49 to 0.98 fl oz/100 gal water plus another fungicide with a different mode of action. Group 4 fungicide. 48-hr reentry.
- Micora at 4 to 8 fl oz/100 gal water. Group 40 fungicide. 4-hr reentry.
- Orvego at 11 to 14 fl oz/100 gal water. Group 40 + 45 fungicide. 12-hr reentry.
- OxiPhos at 1.3 to 4 quarts/100 gal water as a foliar spray. Group 33 fungicide. 4-hr reentry.
- Pageant at 12 to 18 oz/100 gal water. Do not use more than two (2) consecutive applications before switching to a different fungicide group. May cause discoloration when applied to blooms. Group 7 + 11 fungicide. 12-hr reentry.
- Segway at 2.1 to 3.5 fl oz/100 gal water. Drench rate is based on pot diameter. Impatiens is NOT specifically mentioned on the label but can legally be used. Group 21 fungicide. 12-hr reentry.
- Stature SC at 6.4 to 12.8 fl oz/100 gal water. Do not make more than two (2) sequential applications. Has some eradicant activity. Group 40 fungicide. 12-hr reentry.
- Subdue MAXX at 0.13 to 0.25 fl oz/100 gal water as a seeding drench or at 0.5 to 1 fl oz/100 gal water plus another fungicide as a foliar spray. Group 4 fungicide. 48-hr reentry.
- Vital at 4 pt/100 gal water. Impatiens is NOT specifically mentioned on the label but can legally be used. Saturate the root zone thoroughly when drenching. Group 33 fungicide. 4-hr reentry.
- Zonix at 45 to 76 fl oz/100 gal water. Can be sprayed on crop or injected into irrigation system. Short residual. 4-hr reentry.
Reference Warfield, C.Y. 2012. Downy mildew of impatiens. Grower talks, 2012.