Impatiens and Downy Mildew


Experienced nationwide by many gardeners in 2011 but far fewer for 2012, concerns have been raised about planting in 2013.  The HDC (Horticultural Development Council) conference on the matter, while not giving any clear solution to the problem does give some guidelines for producers in 2012/3.  Seed raised production is possible as there is no evidence of the disease being seed born.  Only Impatiens walleriana hybrids are affected, the disease has not been seen to affect New Guinea hybrids or SunPatiens.


  Ensure good hygiene, disinfect well before start of production.

  Avoid vegetatively produced Impatiens walleriana.

  Grow under optimum environmental and nutritional conditions.

  Grow with good air circulation and avoid plant foliage remaining wet for long periods.

  Check production regularly for signs of the disease.

  While the disease does show resistance to fungicides it is still important to carry out a recommended preventative spray program.

Additional notes for Landscapers and Gardeners

    Due to the possibility of resting spores being present in areas planted with Impatiens walleriana in 2011 avoid planting Impatiens in the same beds or compost.

  Ensure all plant material from 2011 is removed and destroyed.

    Sterilise containers or baskets to be reused.  Use fresh compost.

We have slightly increased our production of impatiens from 2012 for 2013. Reports from several of our customers who planted impatiens during 2012 conclude that those who followed the guidelines above experienced very few problems.

It is still wise to not replant the same beds 2 seasons running but to alternate with another plant.

If you are still concerned about planting in 2013 then there are plenty of well proven bedding plants to consider, including Begonia, Fuchsia, Geranium, New Guinea Impatiens, SunPatiens, Petunia, Verbena, Zinnia etc...

The above notes are for guidance only and there still remains a risk that the disease will reoccur during the 2013 season - hygene is the key !

For more information, please read information from HDC below:

Fungicide resistance changes growers' approaches to impatiens mildew control

Protected ornamentals growers are being advised to avoid producing impatiens from imported cuttings, following the confirmation during HDC- funded research of a strain of impatiens downy mildew (Plasmopara abducens) resistant to metalaxyl-M, the most effective fungicide used
against the disease.

Researchers Martin McPherson of Stockbridge Technology Centre and Philip Jennings of Fera say imported cuttings are the source of the new strain in the UK.

A 'straw poll' taken at the HDC/BPOA/HTA impatiens downy mildew seminar in October suggested that growers were 2:1 in favour of continuing with impatiens but that no-one intended using
cuttings-raised material for the 2012 crop. Some propagators have said they will not be
delivering cuttings-raised plants into the UK market, and will keep cuttings- and seed-raised
plants separate in their supply networks.

McPherson: maintain fungicide protection for seed-raised crops "Our research in PC 230a
identified several alternative non-metalaxyl fungicides that can provide moderate to good activity against plasmopara in impatiens when applied as protectant treatments in advance of downy mildew infection occurring," McPherson told HOC News.

"Until further information is available, growers must maintain fungicide protection on seed-raised impatiens crops from sowing onwards."

Based on the project results, McPherson suggests an early drench or compost incorporation treatment using Fenomenal (fenamidone + fosetyl-aluminium), fosetyl- aluminium alone (in products such as Aliette or Plant Trust), or propamocarb HCI (in, for example, Filex or Proplant or in Previcur Energy which also contains fosetyl- aluminium).

This early treatment should then be followed by protectant sprays using a combination of different
products in an alternating programme to minimise the risk of further resistance problems developing.

Of the alternative products tested so far mancozeb (products such as Dithane or Karamate),
Paraat (dimethomorph), Amistar (azoxystrobin) and / or Signum (boscalid + pyraclostrobin) appear to offer the best protection, suggests McPherson.

It is important to note that none of the fungicides tested were effective once infection was established in the crop so early treatment is essential.

Fenomenal was recently approved for use on protected ornamentals and McPherson points out that data on crop safety in bedding plants is very limited. It was included as an experimental product in HDC's trials and was effective and crop safe but this was on larger impatiens plants so
growers may want to trial it on a small scale for crop safety.

"It could be a key product to maintain effective control," said McPherson.

The approval for Plant Trust is also new this year and again information about crop safety in
bedding plant species is limited so growers should undertake their own small-scale trial for crop safety on their own range of varieties.

"As this product potentially provides longer term protection due to its slow-release nature it rnay be best used during six-pack production where it may provide some continued protection post-planting outdoors," said McPherson.

Growers should note that current approvals are due to expire in October 2012 for Aliette and in March 2013 for Filex while Previcur Energy has a 21-day harvest interval.

Downy mildew symptoms on impatiens.
Several non-metalaxyl fungicides can provide protection.


Downy mildew - where next?

Philip Jennings and Martin McPherson look at the implications  for growers of the arrival of impatiens downy mildew strains resistant to what had been the most effective fungicide.
Infections associated with imported cuttings this year spread quickly onto seed-raised crops.

This time last year results from the ongoing HDC project on impatiens downy mildew had enabled us to identify a number of fungicide spray programmes able to control the disease (HDC News November 2010). The best were those in which the systemic fungicide metalaxyl-M (in Subdue on its own or with mancozeb in Fubol Gold) was included as one of the early sprays or drenches. All performed best when applied as a protectant programme initiated before the pathogen arrived - none performed well as curative treatments when the sprays began after the disease had already become established.

But then, this spring, we began getting reports of impatiens downy mildew (Plasma para abducens) infections associated with imported cuttings, with the infection quickly spreading from these to seed-raised crops. What was particularly worrying was that these outbreaks were not responding to spray programmes based on metalaxyl-M. In the light of experience with lettuce downy mildew, we felt the most likely reason was that the pathogen had developed some resistance to the fungicide. 

Experiments to help us track any changes in sensitivity to metalaxyl-M had been included in our research from the start because we knew this type of pathogen is prone to developing fungicide resistance. Before this spring we had one strain of P abducens, collected in 2009, which we were
using to test fungicide performance. Tests showed that it was highly sensitive to fungicides containing metalaxyl-M and we are also fairly confident, based largely on the effectiveness of recommended spray programmes before this year, that all strains originating in the UK have remained metalaxyl sensitive.

This year we've undertaken more comprehensive resistance tests using 15 samples collected from 11 different nurseries - and the results confirm that a metalaxyl-M
resistant strain of P abducens has developed, introduced into the UK on cuttings.

Glasshouse trials carried out on all 15 samples showed that no disease control was achieved following a Subdue soil drench treatment applied at the manufacturer's full recommended rate, even when the plants were inoculated and treated on the same day. Some reduction in disease levels was seen following the application of full-rate Fubol Gold WG, but control levels were less than 50% and probably a result of the mancozeb component. 

Often fungicide resistance in a particular pathogen comes at a cost in terms of its ability to infect or to survive adverse conditions. But this does not seem to have been the case for the new strain of P abducens; if anything, it seems more aggressive than the metalaxyl-M sensitive strain. Whether it will pay a penalty in terms of the ability of its oospores to survive the winter remains to be seen.

The appearance of this metalaxyl-M resistance will undoubtedly cause problems for impatiens downy mildew control, as what had been the most effective systemic component of control programmes is now effectively rendered redundant.

The effectiveness of the other alternative fungicides against the metalaxyl-M resistant strains has still to be established, though the chances of finding an immediate replacement with such a high level of systemic activity is doubtful. Unfortunately though, the true extent of the problem will only be fully revealed over the coming years as we gain a clearer understanding of the potential of the resistant strains to overwinter in the UK.

With good hygiene (including removal of all infected plant debris followed by disinfection) it should be possible to minimise the risk of a metalaxyl-M resistant strain persisting on propagation and production nurseries where there have been problems this year - as long as the pathogen is not reintroduced on a fresh batch of cuttings. Given the fact that imported cuttings account for just 3 to 4% of total impatiens production, the risk they pose to seed-raised crops is far too high and they should be avoided.

However, later in the year when crops are planted out, they will potentially be re-exposed to both metalaxyl-sensitive and resistant strains harboured in the soil over winter. Depending on the infection potential from such soil-borne oospores to semi-mature six-pack plants we may see a more regular and significant problem arising in parks and gardens in the future. Here, the
pathogen is likely to develop unhindered as fungicides are not used after planting.

Reducing Infection risk

Measures to reduce the risk of infection and spread of downy mildew are outlined in the HDC factsheet Il109 lmpatiens downy mildew and in the HDC leaflet Good horticultural practice but of course this will need interpreting in the light of metalaxyl-M resistance. The key measures include:

    Start with disease-free cuttings or seedlings - the pathogen may be present in a latent (symptomless) form. Results from PC 230b indicate that the pathogen is not seed-borne.

    Avoid importing cuttings.

    If cuttings are used, ensure adequate quarantine from seed-raised crops, including separate labelling so you can trace supply. At least 14 days quarantine will be required, the period from infection to sporulation. Once sporulation occurs, air-borne spread is likely.

    If cuttings are imported, check immediately on receipt, before transfer to quarantine. If there are signs of downy mildew infection, reject the batch immediately and seek confirmatory diagnosis.

    For seed crops, ensure adequate air circulation around plants by spacing and ventilation to prevent prolonged periods of leaf wetness. Avoid overhead watering, especially late in the day, but if it's necessary, do it early or on days when solar radiation levels will dry the leaves quickly.

    Check susceptible crops regularly; send suspect plants for diagnosis. Where infected plants are found, remove them immediately by carefully placing in a plastic bag in situ to avoid dispersing spores. Destroy infected plants by sending to landfill or incineration.

    Maintain an effective protectant fungicide programme using a range of products with different modes of action to minimise the risk of resistance. Details of alternative (non-metalaxyl) fungicides can be found in the HDC factsheet. For detailed advice on specific spray programmes, consult a BASIS qualified consultant.

    Practice good nursery hygiene, clean up crop debris between crops and maintain effective weed control (including 'volunteer' impatiens plants) in and around the growing areas. Use appropriate disinfectants to help minimise carry-over.

    If wild impatiens grow nearby, monitor them carefully and send suspect samples to a diagnostic laboratory.

    Finally, please inform the PC 230 project team of any findings of infected material and if possible send us a sample - it will help us ensure the project is up-to-date with how the disease is developing in the trade.

Although these measures should help reduce this risk of infection during propagation and production, infection can still occur after the plants leave the nursery and are planted out, especially if used in areas where infected impatiens have been grown before. Results from experiments in PC 230b suggest that oospores remained viable even after some of the extreme
conditions last winter.

Future work
Testing spray programmes based on non metalaxyl-M products is clearly a priority. At the same time, we need to look at novel chemistry to try to identify alternative contact and systemic fungicides to replace metalaxyl-M, on the assumption that the resistant strain is sufficiently adapted to survive over winter and to reinfect.

Although there were widespread reports of impatiens planted outdoors this year that were 'not looking too good', probably as a result of infection by downy mildew, it is not clear whether these were the result of an infection caused by the metalaxyl-M resistant strain or the older sensitive one.

Testing the sensitivity of strains isolated from any future outbreaks (from both nurseries and landscape plantings) would help determine the prevalence, persistence and geographical distribution of the aggressive metalaxyl-M resistant isolate in the wider environment.

While we know P abducens oospores survived last winter's harsh conditions, their potential to infect plants is not yet known. As a result, work is still needed to determine the susceptibility of plants from six-packs or similar, transplanted into civic display beds, parks and gardens, to infection by overwintered oospores.

Finally, while the pathogen does not have a natural 'green bridge' to carry-over on plants in the UK, there is the possibility that indigenous or introduced 'wild' impatiens may harbour the disease. There is an urgent need to understand this risk as it could be critical in terms of future decision-
making for effective disease control.

PC 230a Control of downy mildew (Plasmopara obducens) an economically important foliar disease on impatiens.
Term: May 2010 to April 2011 PC 230b Source of downy mildew (Plasmopara obducens) infections on impatiens
Term: August 2010 to July 2011 Project leader: Philip Jennings Industry representatives: Mike Smith and Chris Need (Fera, Stockbridge Technology Centre and
commercial nurseries)

Discolouration and leaf curl associated with early stages of downy mildew infection (an infected plant left, a healthy plant right)