Sulfur, organic fungus control, but use with caution

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Mined from volcanic rock and sold either as a dust-fine powder or as thicker wettable micro-granules, sulfur turns to gas on contact with plants, its vapour killing the germinative tube of fungi. Permitted in organic farming, it tackles powdery mildew, scab and mites, but only when sprayed on a bright day between 23 and 25 °C.

What exactly is sulfur?

Sulfur is a naturally occurring mineral that forms around volcanoes during periods of inactivity. It’s a simple substance denoted by the letter S in the periodic chemical table. We extract it from volcanic rocks, distill it, and refine it. This process removes any earth and impurities mixed with it. We thus get sulfur in various forms.

Sulfur acts by sublimation. That means, it turns into gas on contact with plants. It emits a sulfuric vapor that penetrates inside pathogenic fungi and disrupts their development. Fungi growth is hindered, and even stopped. In fact, sulfur kills the germinative tube of fungi without penetrating plant tissues.

Even though organic farming permits sulfur, you should use it sparingly. Despite its natural nature, sulfur remains a treatment and can impact the environment. It carries a degree of ecotoxicity. Among other things, it is toxic to Anthocorids, a family of predator insects which includes bugs whose larvae feed on aphids and mites.

Different forms of sulfur

In garden centers, sulfur comes in two forms:

• A very fine sulfur powder that you can spray as it is with a powder spreader (or a simple kitchen sieve). Since this dust-like sulfur is very volatile and can be swept away with wind, it’s more suitable for treating greenhouse plants.
• Wettable sulfur comes in the form of micro-granule powder that’s much thicker. You dilute this sulfur in water for spraying

• Check the sulfur concentration and formulation type on the label before measuring, since dusts are generally applied dry while wettable powders must be mixed with water at the rate the label specifies.
• Store both forms in a closed container in a cool, dry place away from any flame or spark, since sulfur powder is combustible and cakes badly once damp.

Favorable conditions for using sulfur

Sulfur sublimation requires specific weather conditions to be fully effective: it is not very effective at a temperature below 50 °F (10 °C) and, beyond 80 °F (27 °C), it can burn the foliage. Ideally, spray sulfur on a bright, sunny day (not too cloudy) with a temperature between 73 and 77 °F (23 to 25 °C).

The effectiveness of sulfur lasts for a week. So, it’s essential to repeat the treatment, but without overdosing it because sulfur has an acidifying effect on soils.

• Rain washes sulfur off the foliage, so a heavy downpour means spraying again.
• Never apply sulfur close to an oil-based spray such as white oil or dormant oil, and leave a few weeks between the two, because the combination scorches leaves.
• Some plants are sulfur-shy, notably gooseberries, blackcurrants and cucurbits, so treat a few leaves first and check them a couple of days later before doing the whole plant.

Using sulfur as a fungicide

The main use of sulfur is as a fungicide, meaning that it fights preventively (and also curatively) against the growth of fungi, and therefore key common fungal diseases such as:

• powdery mildew on tomato, squash, rose, grapevine, fruit trees…
• scab on apple and pear trees
• purple strawberry spot
• erinose on linden, lychee and maple
• all diseases affecting grapevine (powdery mildew, excoriosis, grape erineum mite…)

It is essential to strictly follow the right dosage as written on the packaging. You can also mix sulfur with Bordeaux mixture (for example to treat potato blight or black spots on rose bushes). In specialized shops, you can find products that combine 20 or 30% sulfur with helpful powdered plants like nettle, horsetail, allium, and algae, and/or rock powders.

Sulfur against pest insects

Sulfur can also have a miticidal action against red spiders (which are indeed mites!) or the pearleaf blister mite and other blister mites on hazel and tomato. It also works against certain pest insects such as stink bugs and ants.

• Elemental sulfur acts mainly on mobile mites and is generally weak against eggs, so repeat sprays are usually needed, though lime sulfur is more ovicidal and can kill overwintering eggs.
• Wet the undersides of the leaves thoroughly, since that is where red spider mites feed, spin and lay.
• Avoid sulfur in a greenhouse stocked with predatory mites such as Phytoseiulus, which it kills along with the red spiders.
• Some plants scorch readily after a sulfur spray, among them cucumbers, melons, gooseberries and apricot trees, so treat a few leaves first and wait before doing the whole plant.

Sulfur for Fertilizing and Amending

Sulfur also serves as a fertilizer and mineral amendment. It is used as a fertilizer for vegetable plants and fruit trees that lack it. This lack is characterized by the yellowing of leaves.

As an amendment, it helps acidify soil that has too much lime.

• Sulfur deficiency yellows the youngest leaves first, whereas nitrogen shortage starts on the oldest ones, so check which end of the shoot is pale before feeding.
• Brassicas and alliums such as onions, garlic and leeks have the highest sulfur requirements of the common vegetables, so they are among the crops most likely to reveal a soil sulfur deficiency.
• Elemental sulfur only lowers pH once soil bacteria convert it to sulfate, which needs warm, moist soil and takes several months.
• When using elemental sulfur to acidify soil, rake it into the top few inches and water it in; sulfur applied as a fungicide, by contrast, must stay on plant surfaces to work.

Precautions During Application

• Wearing gloves, glasses and a mask is highly recommended during spraying
• You should stand with your back to the wind to do the spraying
• Sulfur should not be applied near a water point
• Applications should stop at least a week before harvesting fruits and vegetables, preferably with a bout of rain in-between to rinse the product off.

→ To go further:

• Diseases and pests, all the tips to treat well
• Organic gardening: learn to garden with nature
• The sprayer, an essential accessory in the garden

• Sulfur powder is combustible, so store it dry and well away from heat or naked flames.
• Rinse the sprayer and its nozzle thoroughly with clean water straight after use, as wettable sulfur settles and blocks the filter.