When Fungus Strikes!
Fungal infections can strike pretty much any plant, and often quite suddenly. This can really take the gardener by surprise, and remedial action is often necessary to prevent the death of the plant and spread to other plants in the area.
Weather conditions and the growing environment can definitely play a role in the development of fungal disease, especially in wet and humid conditions with poor air movement. It is a combination of these conditions which favor fungal growth, and other stresses to the plants that cause an outbreak to occur.
In this article we’ll examine the best chemical options available to the home gardener, list some proven natural and organic methods to tackle fungal diseases, and look at important safety aspects of spraying plants.
Acting preventatively is always better than clearing up an outbreak, and this can consist of basic husbandry, such as ensuring adequate ventilation, humidity levels, carrying-out regular hygeine tasks, and maintaining good watering practices. But these are not always enough. Fungal spores can drift for many miles, and the ambient outdoor humidity and weather conditions often fall outside of the gardener’s control. Sometimes disease-prone plants will need to be treated preventatively when a disease is common in your area, and this can include chemical treatments. Many chemical treatments work best as a preventative measure, as established infections are very hard to control.
In commercial horticulture, there are some strong chemicals which can be used on ornamental plants such as Epoxiconazole, kresoxim-methyl, pyraclostrobin, boscalid, and prochloraz.
But these ‘hard drugs’ of horticulture are not available to the home gardener. Why? Because they are too dangerous if they are not used properly, and often have special storage and disposal requirements.
This does not mean that the chemicals available to regular folk are entirely safe either however, and utmost care should be used with any spray – synthetic or natural – as we’ll see later.
Several off-the-shelf formulations are available from the usual suppliers of garden chemicals, and many of these are very effective, however they need to be applied as directed on the label, and often have the best effect as a preventative measure. Special precautions must be taken with food crops, as retained chemicals in the crop can be damaging to health. Only use a garden chemical on a food crop if it is specifically indicated and approved for that use, and follow the instructions to the letter.
Ready-to-use formulations may contain the following approved fungicides: Triticonazole, Trifloxystrobin (particularly for lawn diseases), Difenoconazole, Tebuconazole and Myclobutanil. These chemicals are reasonably safe when used correctly, however they are still potentially hazardous if they are improperly used, or are allowed to contaminate the wider environment. Aquatic organisms, insects, and birds are often the worst effected so ensure you use them responsibly. Brand names to look out for are (this may vary by region) :
Triticonazole: Fungus Clear Ultra (Scotts), RoseClear Ultra (Scotts, with insecticide),
Trifloxystrobin: Fungus Fighter Plus (Bayer), Lawn Disease Control (Bayer),
Tebuconazole: Fungus Fighter Plus (Bayer)
Myclobutanil: Multirose 2 (Bayer, with cypermethrin insecticide), Systhane Fungus Fighter (Bayer)
Difenoconazole: Plant Rescue Fungus Killer (Westland)
Propiconazole: Less often used in EU, but can be found in US brands.
Organic and Home-made Remedies
There are some chemicals specifically approved for use in organic farming and gardening, however it should be emphasized that these are not safer, only that they are based on simple mineral formulations. These tend to use Copper and Sulfur as active ingredients, which can build up in the environment over time and poison the ground. This is the case in many wine-growing regions where the traditional use of Copper fungicides has caused a damaging build-up in the soil and groundwater.
There are also other options which are natural, and which do not lead to long-lasting environmental damage, but first let’s look at the copper/sulfur solutions.
Copper sulfate – This simple compound of copper is found as a naturally occurring mineral, and is allowed for organic growing applications, usually in combination with lime in the formulation known as ‘Bordeaux Mixture’ or combined with Sodium carbonate, a formulation known as ‘Burgundy Mixture’. Copper sulfate can also be sprayed on its own on dormant plant material at a concentration of 80g/L and on growing plant material at lower concentrations. Copper octanoate is a compound sometimes called copper soap, and can be sprayed at 0.08% on living plant material. Copper ammonium carbonate (Cheshunt compound) can be sprayed at a maximum of 25g/L, but lower concentrations will also be effective and less harmful to growing plant material. The UK Soil association puts a maximum limit of application of Copper at 6kg per hectare per year. Copper compound accumulation are toxic and kills earthworms, beneficial organisms and is toxic to people in higher or chronic exposures. Copper works by preventing the germination of fungal spores, so is useful as a preventative measure only.
Sulfur/Sulphur – This common element is essential to all life, but in excess it is toxic. Applied at 0.4% it is a reasonably effective fungicide, however it is best used as a preventative measure, and has limited effect on active disease. Sulfur can cause breathing problems for anyone, and especially in those who have asthma, so avoid breathing dust or fumes. Sulfur applied to the ground will lower the pH, making ti more acidic. Sulfur (dioxide) fumes are used to fumigate against mites and other greenhouse crops, however it will also damage the plants, possibly killing them. Sulfur has a strong smell and taste, so avoid using on food crops.
Potassium Bicarbonate – Not to be confused with the similar Potassium carbonate, this ‘Bicarb’ is an effective treatment against Powdery Mildew, however the treatment needs to be repeated regularly to prevent new infections. It is sprayed as a solution on the affected foliage, where it kills active fungal growth and prevents spores from germinating for a period of time. A 5g/L solution is the best start, although some crops can tolerate up to 20g/L.
There are a handful of recently-developed ‘biofungicides’, microorganisms which naturally prevent fungal attack and can help plant recovery. Such organisms are naturally present in the environment, especially in the rootzone, but have been selected and bred especially for spray-on and watering-in formulations. These include strains of Streptomyces griseoviridis (eg. Mycostop) and Bacillus subtilis as well as Bacillus amyloliquefaciens strains. These have a promising future for home and professional horticulture, being harmless to the wider environment, but very effective.
Milk- Spraying plants with a milk solution has been proven to be reasonably effective at preventing fungal attack and even treating infected plants. The mechanism of action is uncertain, but it may be due to natural antimicrobial factors in the milk, or because of the production of lactic acid by bacterial fermentation.
Water- Spraying with water has also been demonstrated as reasonably effective at preventing fungal infections by simply rinsing off spores before they can penetrate the plant.
Neem Oil- Spraying with neem oil is known to kill bugs, but a sprayed solution/emulsion may also help treat fungal infections as the oil contains antifungal agents. Various neem products are available on the market in most of the world (but not in the UK) for the purpose of killing insect pests and fungal infections. There is nothing stopping you from making your own mix if required, just don’t use it on consumeable crops and don’t sell it.
Sesame Oil- Various oils can help prevent and treat fungal infections, and some formulations include Sesame oil as their main ingredient. Sprayed at a 0.1% concentration, Sesame oil can be effective at preventing and treating fungal problems.
Horticultural Oils- The term ‘Horticultural Oil’ refers to a wide range of plant and mineral oils used to treat plants. They are sprayed in a solution to treat plants that are infested with pests or infected with fungal disease. Special grades of mineral oil are often used, with care taken not to spray in extreme heat. Typical concentrations of mineral oil are 1% or below, with an emulsifying agent or surfactant employed to keep the oil in solution (really in ‘suspension’) with the water. Various plant oils are also used, including Rapeseed (Canola) and Cottonseed oil. It isn’t certain exactly how these work against pathogenic fungi, but Horticultural oils are pretty effective, especially as a preventative measure.
Essential Oils- Many plants naturally produce antifungal chemicals, and these can be employed by the gardener by using the essential oils of those plants. The most effective of these are Garlic oil, Palmarosa Oil, and Thyme oil.
- Garlic – This pungent oil also dissuades insects from attacking the plant. Garlic contains various antifungal and antibacterial compounds.
- Palmarosa – The oil of the Palmarosa grass (Cymbopogon martinii) is rich in geraniol, a potent chemical which is both repellent to many insect pests and an antifungal against some pathogenic fungi.
- Thyme (Thymus vulgaris) – This outstanding herb produces an oil which kills various species of mites and is a potent antiseptic and antifungal.
Potassium compounds- The potassium ion has an interesting antifungal effect and seems to suppress powdery mildew infection quite effectively. Examples of this include:
Potassium bicarbonate– Both Sodium bicarbonate and Potassium bicarbonate are effective at preventing fungal infections, particularly against powdery mildew. Potassium bicarbonate is probably more effective however against Powdery Mildew, whereas Sodium bicarbonate has been shown as more effective against some other fungi. Potassium bicarbobate can be sprayed at 3 to 5g/L, or at higher concentrations on more tolerant plants.
Potassium silicate– The addition of this compound to sprays and irrigation has the effect of increasing the amount of silicon available to the plant – spraying is most effective but both methods are beneficial. Silicon is not an essential plant nutrient, but plants with access to soluble silicon are stronger, as the silicon is incorporated into the plant cell walls, allowing the stems to become thicker, stronger, and the leaf cuticles harder to penetrate. This gives protection against pathogenic fungi and juice-sucking pests.
Horticultural Soap- This liquid soap is based on Potassium salts of fatty acids, mostly Potassium laurate and Potassium palmitate. The dual effect of the Potassium ions and the binding effect of the saponified fatty acids prevents and possibly helps treat fungal infections.
Reservoirs of Disease
Powdery mildew is a good example of a ‘typical’ plant pathogen, and consists of a wide range of related ‘species’ which are, in reality, probably better described as a diverse and related group of tribes which are possibly capable of breeding with one another, but are generally specialised to particular hosts.
Despite being able to infect and show symptoms on a range of plants, many species of powdery mildew require an ultimate ‘ideal’ host plant in order for them to complete their lifecycle and actually produce spores – for example Berberis.
Berberis and also some other plants can act as a reservoir and breeding ground for spore-producing powdery mildew which will then go on to infect plants from spores over a several mile radius year after year. Sometimes the host plant will show little if any signs of disease, but it might be worth checking which plants might be harbouring the fungus in your area!
Similarly, Mint and Groundsel plants can sometimes act as a reservoir for rust fungus (Puccinia), which can infect other plant species, often affecting them worse than the original hosts. Keep an eye out for host plants that might be causing repeat infections, and consider treating them as well, or removing them altogether in severe situations.
Finally, some important Safety Points
- Use treatments only when needed, but preventatively is better.
- Use the right chemical for the job – do your research and check that the treatment will work on your target fungus.
- Read the label and follow instructions to the letter.
- Don’t exceed application rate, and don’t water down chemicals unless advised to do so.
- Spray at the right time and intervals for the best control.
- Don’t mix proprietary chemicals yourself – only use mixtures that are supplied by the manufacturer. Certain combinations may counteract each other, or cause damage to the plants.
- Avoid contact with skin, eyes and don’t breathe the spray. Don’t eat drink or smoke when spraying, and wear appropriate protective clothing.
- Use the spray efficiently to get total coverage on all surfaces of the plant without excessive runoff.
- Spray in the evening or early morning on a day that is not windy, rainy or too hot and dry.
- Be careful to avoid non-target or sensitive species, and keep away from aquatic organisms. Avoid situations where runoff might enter ponds or waterways.
- Don’t use the same apparatus used for herbicides or other chemicals. Clean your equipment and wash up properly afterwards. Do not pour left-over chemicals down the drain.
- Store chemicals in their original packaging. Keep in a cool, dark , frost free place and away from where they might be accessed by pets or children.
Some useful links:
A study comparing Sodium and Potassium carbonate and bicarbonate solutions against citrus green mold.
Information from the American Phytopathological Society about fungicides.
Information Sheet (PDF) from the RHS regarding Fungicides for Home Gardeners.
Plus information on withdrawn chemicals and the Safe Use of Garden Chemicals (also from the Royal Horticultural Society).