Downy Mildew in Roses

Downy mildew (DM) is an oomycetes (water mould) closely related to Pythium and phytophthora and also the moulds that appear in damp showers.

Downy Mildew caused by Peronospora SparsaDowny Mildew caused by Peronospora SparsaEach plant species tends to have its own spp. of DM and rose DM is called sparsa because the fungal spores are relatively sparse.

The spores are airborne and are activated when weather conditions are right i.e. cold nights and high RH.

DM can survive for a long time in rose stems as fungal strands so even healthy looking plants can contain the problem just waiting for the weather to change which explains how good looking crops can turn so quickly.

The first sign of trouble on the leaves and stems is a purplish/brown spot which can be confused with nutrient problems. Also the rose DM sometimes does not produce the mat of spores on the back of the leaves hence the name sparsa. Once DM has taken hold it can’t be stopped at that point and it is advisable to remove the damaged stems after you have applied treatment so that any surviving spores are taken from the house. Good management techniques are needed in this job to avoid spreading the spores.

In wet weather with high humidity DM can explode overnight if the spores have had 6 hours of the right conditions. Fog or mist is perfect for infection which is why the problem is often worse under polythene.

In the morning sporangia have formed on the leaves which are spread to new plants by air movement. The best temperature for the DM is around 14C with a RH of 85%.

Above or below this temp tends to hold DM in a static position but does not kill it.DM can also survive as oospores in dead material on the floor and in the top of the pot so good hygiene is essential as with many other problems.

A good drench with E-CleanTM will help keep the spores down although once the DM has taken hold this will need to done very regularly. The E-CleanTM will have no effect on the DM within the plant.

Some growers see a proliferation of mildews and other problems when there is a background weed problem so it makes sense to keep the area as clean as possible. Also poor light from dirty glass/polythene or old polythene can antagonise the situation so the brighter the light the less of a problem you will have. This can also have an effect on un-protected crops where rubbish or unkept windbreaks cause excessive shade.

There is no doubt that healthy plants get fewer problems but this doesn’t mean we can ignore the possibility of infection as even low nutrient levels that are not apparent can lead to problems. This goes back to the argument for raising the brix of the plants which is difficult at any time but especially so in short light periods.

Control Strategies.

Use a wide range of techniques to prevent DM including environmental, organic and chemical.

less sporeson the back of the leaf, middle rightless sporeson the back of the leaf, middle rightThe starting point is to make sure you have the correct feeding regime so that the plants are in good condition without excessive N in the leaves and the other nutrients in balance. In times of poor weather I find it well worth dropping the Nitrogen levels as against the other nutrients plus cutting out a lot of fertigation in favour of foliar feeding which can be a lot more suitable. Foliar feeding can be many times more effective than fertigation.

At certain times of the year and at certain stages of growth foliar feeding builds the strength of the plant without softening growth, which is more susceptible to attack. On many crops a Urea spray chelated with a little Fulvic acid is a far better option than putting in Nitrates through the roots, an idea I will go into further in an article called foliar feeding.

Over the years I have found that a good background rate of Trichoderma (Elite-HKLTM) and Bacillus subtilus (Elite-GoldTM) can alter the starting point of DM [and] have actually cured the disease with a heavy drench after it has broken out, so it is safe to assume, that if these products are kept in the root zone, where they belong they will be able to either prevent infection or drastically reduce the harmful effects.

In the UK Bacillus subtilus are one of the major products used against DM in many crops.  I tend to keep a colony going by the regular application of a brewed inoculum (E-ProtectTM) which gives a whole range of microbes and fungal spores around the root zone.

It is vital to monitor the weather so that the trigger is spotted, preferably a day or two early so that a prevention policy can be put in place. On our nursery I act when I see the weather change and there is a lot of free water around so at that point I start to crowd the root zone with all the good microbes. Again this doesn’t mean you won’t get a problem but it does mean the recovery will be a lot quicker than I would get if I had relied on chemicals, which can build up a resistance very quickly. When it comes to spraying DM whether biological or chemical it can be advantageous to spray when the crop is slightly wet with dew as the stomata on the leaves are triggered to open to absorb the water.

Remember that the DM needs 6 hours of the right conditions to proliferate so look at house drying methods and irrigation systems to make sure the RH is kept as low as possible. Air movement is very important (as with botrytis) but this will be detrimental if DM is already in the crop (don’t put the fans on in the morning once DM is evident) and also it could make powdery mildew worse. Air flow is particularly important at sunset and sunrise as the sudden change in air temperature can trigger a dew point on the leaves leading to many pathogens being triggered. When conditions favour DM spray often to keep spore levels low with a range of products to increase chance of prevention including products that will work within the plant such as GoldrushTM, E-BoostTM and ProAlexinTM all of which build the phytoalexins within the plant and help suppress pathogen growth.

If chemicals are to be used remember It is now advised that when a systemic chemical/product is used it must be tank mixed with a preventative because systemic products have a much higher risk of resistance. Ensure you take chemicals from different chemical groups, to help stop or slow down the possibility of resistance and add a biological stabiliser such as E-Boost to balance microbial life on the leaf.

It is always advisable to add a sticker to fungicides as they fall off the leaves very easily so MeltdownTM and E-CleanTMcan be added provided you have tested the crop for phytotoxicity beforehand. Also most fungicides work much better at a low pH of around 4.5 so the addition of a little phosphoric acid to the spray water can work wonders as well. It has been shown that if many fungicides are applied at pH 7 or above the half life of the product can be measured in minutes. As there is an internal aspect to DM it is important to drench the plants to attack the hyphae within the stems although the use of biological products will probably give the better results leaving the chemicals for emergencies where they are most likely to work if resistance is not be allowed to form.

 

Hygiene.

Constant spraying with E-CleanTM will also help reduce all harmful pathogen spores and two spotted mite eggs.

It is important in none biological programmes not to use too much E-CleanTM on the foliage, as it will deter the good pathogens as well as the bad.

As far as the ground is concerned it is important to use E-CleanTM lightly so that it does not build up in the soil where it will again be detrimental to natural flora and fauna and to help with this I drench with E-ProtectTM to keep the numbers of microbes well up. Remember that after an attack every effected stem that is left on holds the possibility of re-infection the next time conditions are right.

Also remember that secateurs used for cutting will transmit disease if they are not dipped very regularly (at least after each row and after each plant where a problem exists or if cuttings are being taken); E-CleanTM is ideal for this.

Keep the nutrition of the plants up to the maximum as an actively growing plant will automatically fight off pest and disease on its own, provided the right balance is maintained.

In the UK and Europe Bacillus subtilis has long been used on many crops as part of the DM control programme. Indeed on seedlings it became almost the sole product used as very little chemical is now used on seedlings at all. With this in mind it may be worth using Elite-GoldTM mixed with Kelp 372TM and Fulvic acid (Carrier1400TM) plus a source of Nitrogen – preferably urea or biological as in (Aero-LiberateTM) as a simple type of compost tea to build the Bacillus numbers in the root zone. If this is tried it must be pointed out that it would have to be applied regularly (at least every 10-14 days) and over a long period of time to build up positive effects.

 
For those still relying on, or those using in an emergency I have listed below all the chemicals that can have an effect of DM. There may be others that can also help but this list helps to make sure you are getting the right mix of chemical groups. Please note that you must use chemicals in accordance of manufacturers’ instructions and rates and test for safety on your particular crop.  
 

PRODUCT ACTIVE GROUP ACTION

ALIETTE SUPER Fosetyl aluminium
Thiram
Thiabendazole Alkyl phosphonate
Disulphide
Benzimidazole Systemic/curative
ACROBAT Dimethomorph
Mancozeb Morpholoine
Dithiocarbamate Systemic/protectant
AMISTAR WG Azoxystrobin Strobilurin Systemic/protectant
ANTRACOL Propineb Dithiocarbamate Organic protectant
CAPTAN Captan Cyclicimide Protectant
CHLOROTHALONIL Chlorothalonil Chloro-nitrile Protectant
COPPER HYDROXIDE Copper hydroxide Inorganic copper Protectant
COPER OXYCHLORIDE Copper oxychloride Inorganic copper Protectant
CUPROFIX DISPERSS Copper sulphate and Lime Protectant
DELAN WG Dithianon Quinone Protectant
EUPAREN MULTI Tolyfluanid Sulphamide Protectant
FOLPAN Folpet Cyclicimide Protectant
FOL-R-FOS /PHYTOS K Phosphorous acid Inorganic Systemic/curative
GEMFluazinam Pyridinamine Protectant
MANCOZEB Mancozeb Dithiocarbamate Protectant
MANKOCIDE Mancozeb
Copper hydroxide Dithiocarbamate Protectant
MAX MZ Mancozeb
Metalaxyl Dithiocarbamate
Phenylamide Protectant
Metalaxyl Metalaxyl Phenylamide Systemic/curative
NORDOX 75WG OLEO 40 Cuprous oxide Inorganic copper Protectant
POLYRAM DF Metiram Dithicarbomate Protectant
RIDOMIL GOLD MZ WG Metalaxyl
Mancozeb Phenylamide
Dithiocarbamate Systemic/curative
REASON Fenamidone Imidazolinone Protectant
SERENO Fenamidone
Mancozeb Imidazolinone
Dithiocarbamate Protectant
SHIRLAN Fluazinam Pyridinamine Protectant
SOVRIN FLO Dimethomorph Morpholine Protectant
 
 
Nick Woodyatt
Research & Development Manager
Franko Solutions

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Disclaimer

Any information and advice provided is given in good faith and is considered complete and accurate at the time of publication. All growing methods, products and chemicals should be tested on a small area before wide-spread adoption. All chemicals and products should be used in accordance with the manufacturer’s instructions and all health and safety protocols must be followed. Franko Solutions as far as possible do not accept liability for any error(s) or omission(s) in the content, nor damages, accident or other loss(es) arising from the use of product(s) or growing method(s) listed herein. Information and/or guidelines published on this website should not be acted upon until the product(s) have been purchased and the manufacturer’s instructions followed. Please note that as this website is internationally available, it is your responsibility to ensure that each product is registered for your crop for the country in which you intend to use the product. Franko Solutions cannot take any responsibility for product registration requirements being met.

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