RHODODENDRONS AND POWDERY MILDEW

• Date: 22nd January 2016
• author: Graham Price
• tags: pest & diseases,plant care

I currently have a nasty infection of powdery mildew (PM) on many of my vireyas and decided that something needed to be done. Surfing the Web I was surprised to find there is relatively little information about PM on rhododendrons in Australia and even less for vireyas. Lots about PM on other plants (citrus, grapes, strawberries, roses, cucurbits, etc.) and a little about PM on rhodos in other countries. However, PM is host specific, meaning different fungi infect different plants, so I am not confident advice from other countries or for other plants will be appropriate. Consequently, I assembled a set of info and now think that other rhodoholics might be interested. It is not absolutely comprehensive and it may not all apply to rhodos, but here it is. Please note: I do NOT make any recommendations of what anyone else should do. If you are interested, I suggest you do your own search.

PM is probably the most common and widespread plant disease and one of the oldest known to man -- the ancient Greeks described it. Probably every gardener, unless they live in the Arctic or on a high mountain, has witnessed its symptoms: a delicate white-grey felt that covers leaf surfaces and stems, giving the appearance of a dusting with flour. Photosynthesis is impaired, leaves turn yellow or reddish brown, distort, die and fall off. On rhododendrons PM often attacks new growth as well as more mature leaves and stems. Subsequent growth is dwarfed and distorted, often covered with the mildew. It affects a wide range of plants and is caused by many different species of fungi. The RBG in Sydney says that in rhododendrons it is caused by the fungus Microsphaera penicillata. An important issue is to be able to distinguish between PM and Downy Mildew (DM) which is another, and somewhat similar, fungal infection that can affect rhododendrons. (example page 15)

PM characteristics: Light, powdery growth covering shoots and leaf surfaces. Infects plant tissue by sending hollow tubes from a spore on the leaf surface into the plant to suck out nutrients. It spreads in a radius from the point of infection and, after about four days, its own spores start forming, produced in chains on upright stalks. The spores are what actually give the powdery effect. PM may survive from season to season in infected buds and unlike most fungi, PM spores do not need abundant water for germination. PM infection starts when plants are stressed, such as when grown in crowded or shady locations without sufficient air circulation. Airborne spores can spread very quickly, from a single infected leaf to an epidemic within 3-4 days, with spores produced on the leaf surface (particularly young green tissue) within as little as 60 hours after initial infection. Wet weather and very hot, dry weather do not favour PM.

DM characteristics: DM differs from PM in a number of significant ways. Unlike PM, which appears on both sides of the leaf surface, DM develops primarily on the undersides of leaves. The tops of leaves will have yellow blotches, while the undersides develop a faint frost of grey, white, blue or violet fuzz. Under humid, cool conditions, spores appear copiously on the lower surfaces of leaves, growing in tree-like formation on branched fruiting structures, unlike the PM spores which are produced in chains. In the presence of water from a recent rain or fog, the spores will germinate within 4 hours and sporulation on leaf surfaces may occur in 3 days under ideal conditions of 18oC temperatures. Below 4.5oC the spores won't germinate and they're killed by exposure to 26.5oC temperatures for 24 hours, so dry winds and warm, clear days inhibit spore production. Unlike PM spores, which are spread by the wind, DM spores are spread by splashing water.

Recommended Strategies: There are three broad strategies recommended for PM. (i) Prevention of infection. (ii) Treatment with contact fungicides to prevent infection and kill fungus already present. (iii) Treatment with systemic fungicides to prevent infection and provide a cure if it is already present.

(i) Prevention: The most frequent recommendation for PM is that prevention is better than treatment it after it is established. Important steps are to grow resistant cultivars and avoid conditions of high humidity. For example, avoid planting in shady areas and prune away all vegetation that limits air circulation. Do not over-crowd the plants and do not use overhead irrigation late in the day. All infected plant debris should be removed to reduce the amount of fungus available to infect other plants. Plants with a severe infection should be monitored closely the following spring so that if infections reoccurs, they can be treated early. A possible additional prevention strategy is to increase the plant’s resistance to fungal attack with the following products.

Silicon: Strengthens plant tissue so it can better resist PM infection. Can be used as either a foliar spray and/or by application to soils. Silicon is considered to provide protection against fungal infection by depositing silicon in epidermal cell walls and stimulating an indirect host defence response. Trials have shown that silicon can delay onset of infection by 1-2 days and reduce the severity of infestation by around 50%. Soluble silicon is available from hydroponic outlets as potassium metasilicate (K2SiO3) and is used at a concentration of 100ppm Si.

SeaWeed Extract: PM is common where plants are deficient in potassium and some trace elements, as when the plants have exhausted their supply of fertiliser or when soil is too dry. Seaweed extract is rich in both potassium and a range of trace elements (including sulphur) and spraying foliage with seaweed tea can be effective against PM, not because it kills the fungus, but because it quickly provides the nutrients plants require to resist the fungus.

Although prevention is good advice it is not particularly relevant when one is building a collection of rare plants or where they cannot be easily relocated to better situations. In these circumstances, the only option is for treatment with a fungicide – but which one? Lots of research has gone into the control of PM in agriculture using different fungicides and there are many new commercial ones available, all with difficult-to-pronounce names. However, very little has been targeted at the backyard gardener for small-scale application and none are specifically targeted at rhododendrons.

Treatments: There is a bewildering array of fungicides available but most are intended for agricultural crops. I report here mostly on the backyard treatments and the few commercial fungicides that are likely to be available through local gardening shops.

(ii) Contact Fungicides: These fungicides are intended to kill the fungus on contact.

Powdered Sulfur (sulphur): Gardening Australia presenter Jerry Coleby-Williams recommended the use of powdered sulfur for PM: Simply apply over the leaves when they are dry. Reapply after heavy rain. Powdered sulfur is perfectly safe to use and has a long-lasting effect. Don't use it on really hot days as it can burn the leaves.

Addition information is provided for it’s use on grape vines (which may/may not apply to rhodos). Sulfur is man’s oldest fungicide and to date no fungicide resistance has developed despite centuries of use! Sulfur is cheaper than most other fungicides and works well if sprayed well - best used at the rate 600 g/100L. Because PM grows on the surface of plant tissue, it is killed readily by any contact fungicide active against the pathogen. As a result, sulfur has post- and pre-infection activity; that is, it both kills the pathogen and protects new foliage. Sulfur has effective volatile (fumigant) activity at temperatures >20ºC but in cooler temperatures, <15ºC, this activity is limited. The contact activity occurs at any temperature but for this, good spray coverage of upper and lower surfaces is needed. Sulfur may cause phytotoxicity at high temperatures if high humidity - sulfur in solution is weakly acidic and burns the foliage if, over a long period, the spray solution does not dry. The best time to apply sulfur is on a calm evening after a warm day, allowing maximum volatile action of the fungicide as it permeates the sprayed canopy. A single application with good spray coverage can restrict PM for 40 days. Lime Sulfur: This is a variant on powdered sulfur and it controls a range of fungal diseases and pests on crops and ornamentals. There is no withholding period. It is considered less toxic, yet provides effective control of PM and other fungal diseases. Best to begin applications early in the season, since sulfur is more effective as a preventative. In subtropical areas it should be used before the high humidity season. It is ideal as a winter clean up spray.

Baking Soda: Concoctions containing Baking Soda (sodium bicarbonate or Baking Powder) are claimed to be effective on a wide variety of diseases and especially PM. It disrupts fungal spores that land on the leaf surface, making them unable to infect the plant. In 1992, a study tested baking soda alone and in combination with oil - baking soda alone was ineffective but when oil was added the mixture became very effective. Besides needing oil, baking soda also needs to be applied at an early stage of infection – or even before you see it – to get the best results. Formulation: add 1 tablespoon of baking soda, 1 or 2 teaspoons of dish soap, and 1 or 2 teaspoons of vegetable oil to 4 litres of water. Baking soda has potential to burn plants, so be sure to test it on a small portion of a plant to see if it is safe. The RBG in Sydney found this to be effective on PM, rust and black spot on roses. So, oil, detergent and bi-carb is all you need for a great preventative fungicide.

For those who do not want to make their own, there are commercial fungicides based on potassium bicarbonate. Eco-fungicide is a proven organic product for treatment against PM, black spot and most mildews in vegetables and ornamentals. It controls disease without damaging the biological balance of your garden (safe for good bugs and bees) with no residual effect. When applied to plants, it alters the pH and the osmotic balance of the leaf surface. This inhibits fungal spores from germinating and growing. It also damages the cells of fungal spores, resulting in dehydration and death of fungi, providing effective disease control. Use 3-4g (approximately 1 teaspoon) per litre of water mixed with 2ml of emulsified vegetable oil.

Milk: In another Gardening Australia webpage Jerry Coleby-Williams writes as follows. Using chemicals against a disease in the garden, such as a fungicide to get rid of PM, often cause other problems. They can affect beneficial micro-organisms in the soil and can kill pollinators like bees. I like to use milk as a fungicide - organic milk because it contains all the antibiotic qualities necessary to make it work. The mix is 1 part organic milk to 10 parts water. Give it a good stir and it's ready for use. The objective of spraying is to cover every part of the plant, both sides of the leaves and coat the stems. It only works as a preventative, not as a cure. Milk has been proven to work on plants within the cucumber family, it’s also good on begonias and effective in controlling mildew on grapevines. Too much milk in the solution will encourage sooty mould, so stick to the recipe; one part milk, 10 parts water. (GP – Jerry seems to think that milk is not a chemical – if it’s not a chemical then what is it?).

Chamomile Tea: It is claimed that PM can be treated with chamomile tea. For each 500 ml of spray required, steep one teabag in a cup of boiling water for 15 minutes, then dilute to 500 ml with cold water. Remove and destroy severely affected leaves, then spray the both sides of the leaves and stems early in day so that leaves have time to dry before nightfall.

Chlorothalonil: This is a polychlorinated aromatic mainly used as a broad spectrum, non-systemic fungicide, with other uses as a wood protectant, pesticide, acaricide and to control mold, mildew, bacteria and algae. Chlorothalonil changes fungal intracellular glutathione molecules to alternate forms which cannot participate in essential enzymatic reactions, ultimately leading to cell death, similar to the mechanism of trichloromethyl sulfenyl. Sold in Australia with the trade names (among others) of Bravo, Barrack, Unite and Dacogreen.

Copper Fungicides: When formulations of copper are dissolved in water, copper ions are released into solution. Copper ions are toxic to fungi and bacteria because they destroy proteins in fungal tissues. However, because copper can also kill plant tissue, there is a risk of injuring foliage. Factors promoting this injury include: 1) the amount of actual copper applied, and 2) cold, wet weather (slow drying conditions) that apparently increases the availability of copper ions and, thus, increases the risk of plant injury. Copper fungicides have largely been replaced with conventional fungicides that are generally safer to plant tissues and often more effective.

Several terms are used when discussing copper as a fungicide. The original material used was copper sulfate. When combined with lime in French vineyards, the combination became known as Bordeaux Mixture.

Bordeaux Mixture: It has a long residual action and has been used for years to control many diseases, including DM and PM. It can be made (mixed) on site by combining copper sulfate with hydrated lime. It is also commercially available as a dry wettable powder.

Fixed Copper Fungicides: Following the discovery of Bordeaux Mixture, several ‘fixed copper’ fungicides were developed, which release less copper ions and are generally less injurious to plant tissues than Bordeaux Mixture.

Oils: To eradicate mild to moderate PM infections, use a horticultural oil or one of the plant-derived oils such as Neem or Jojoba Oil. Be careful not to apply an oil spray within 2 weeks of a sulfur spray or plants may be injured and not when temperatures are above 32°C or to drought-stressed plants. Some plants may be more sensitive than others so the interval between sulfur and oil sprays should be longer.

Biological Fungicides: Biological fungicides (such as Serenade) are commercially available beneficial microorganisms formulated into a product that, when sprayed on a plant, destroys fungal pathogens. The active ingredient is a bacterium, Bacillus subtilis, that helps prevent PM from infecting the plant. While this product kills the PM organism and is nontoxic to people, pets and beneficial insects, it has not proven as effective as the oils or sulfur in controlling

PM. Mancozeb Plus: Mancozeb Plus Garden Fungicide & Miticide is a multi-purpose, dual-action formulation combining the benefits of mancozeb and wettable sulfur. Both ingredients have a contact fungicidal action, helping to control and prevent the entry and spread of a range of common fungal diseases. Wettable sulfur has the added benefit of providing control of a range of mite species. This formulation is suitable for use on a range of vegetables, fruit and ornamentals. It controls petal blight, PM, rust, dollar spot in lawns and others.

(iii) Systemic Fungicides: These fungicides are intended to penetrate into the plant tissue and move throughout the plant, killing any fungus that is present and preventing new infections. There can be a major problem with this type of fungicide in that the fungus can develop a resistance and the fungicide can become ineffective. There are about 15 different active ingredients and ~65 products registered in Australia for systemic control of PM.

DMI’s: Demethylation Inhibitors were developed in the late 1980’s and include Bayfidan, Mycloss and Topas. They have translaminar activity, ie. they move across and/or within the leaf and are partly systemic. However, they do not move in the plant’s sap in sufficient concentration to be effective beyond the sprayed zone. Topas has a limited volatile action so is the most effective fumigant of the DMI’s. The DMI’s are single-site fungicides, acting at only one point of the PM biology. As a result, there is a risk of the fungus developing resistance. The DMI’s are excellent pre- and post-infection fungicides for PM. They protect the plant and kill the fungus and have potential for use in early season when young shoots are most at risk. DMI’s are also valuable tools where spray coverage is limited, but they are more expensive than sulphur. Resistance management strategies recommend that the DMI’s not be used more than three times in a season.

Triforine (also called Triforine Rose Fungicide). I pick out Triforine because it is readily available from the large hardware chain (you know the one) and it is targeted at the home gardener. Triforine is a systemic fungicide that is both preventative and curative. Triforine will also treat rust and black spot on roses. It is applied as a foliar spray every 7-10 days while the conditions for PM are favourable. It is absorbed by the sap stream and transported to all tissues throughout the plant where it destroys diseases already present and prevents new diseases from entering. Triforine can be toxic (Poison schedule S5-caution) so care and appropriate equipment should be used, even though commercial promotions claim it is environmentally friendly and safe for both people and the environment - a low hazard to beneficial insects and animals. It has a half-life in soils of approximately 3 weeks. Some garden experts recommend adding a wetting agent in with the Triforine to reduce run off and help the Triforine stick to the plant. A cheap way to get a similar (but not as good) effect is to add 2 ml of dishwashing liquid per litre, but a proper wetting agent like the Kenwet 1000 or Agral is by far the best way to stop runoff. They recommend the Kendon brand of Triforine, which was tested by Choice magazine and found to be the best.

Strobilurins: (I am not sure if these are available in small packs for the home gardener in Australia). A relatively new group of fungicides (developed in the 1990’s) called strobilurins are used for preventative treatment of fungal diseases, including PM. They have been applied to agricultural crops such as grapes, but may also be useful with rhododendrons. A strobilurin shown to be particularly effective against PM is trifloxystrobin, which in Australia is sold under the trade name Flint. Other trade names for strobilurins are Amistar and Cabio.

Strobilurins were extracted from the fungus Strobilurus tenacellus and have a suppressive effect on several different plant pathogenic fungi, reducing competition for nutrients and they inhibit electron transfer in mitochondria, disrupting metabolism and preventing fungi growth. Like BMI’s, they have translaminar activity, which means they move across and/or within leaves thereby providing control on both leaf surfaces, but they need to be applied with good coverage and dose to compensate for the dilution of active ingredient as it moves within the leaf. Strobilurins are purely for prevention of infestation and have almost no curative benefits, but, as an added advantage, strobilurins also control DM.

A degree of resistance to strobilurins has developed in fungi that affect grapes and cucurbits. Management of resistance is by limiting their use and by using them as a component of an integrated program with other fungicides. Strobilurins should be applied in alternation with other systemic fungicides that have a different mode of action including multi-site contact fungicides that have a low risk of resistance, such as chlorothalonil and copper hydroxide.

That’s all folks. I intend using Triforine on my vireyas (which I bought at that large hardware chain) so I hope one day I will be able to report on successful treatment of Powdery Mildew.