In organic growing we don’t use chemicals that damage the produce, people or the environment, but that doesn’t mean we don’t spray!

Pink Lady apples with Black spot
Pink Lady apples with Black spot

All deciduous fruit trees are prone to fungal diseases, some more than others, and if they’re not properly prevented or controlled in spring, they can be devastating.

We don’t have room here to go into a lot of detail about individual diseases, so we’ll focus instead on the basic principles of fungal disease prevention.

Each fungal disease has trigger points that stimulate an outbreak of disease – a unique combination of temperature, the number of hours the tree has been wet, and the amount of spores present, amongst other things (we go into more detail about individual diseases in the Keep Your Fruit Trees Free from Disease short course).

Regardless of the disease we’re trying to prevent, the aim is to keep your trees protected at all times while they’re flowering with what we call a “cover spray”

This means we’re aiming to have the trees covered with something that will prevent the fungal spores from causing a disease outbreak.

The regime differs a bit depending on what disease is posing a risk, but the same principles apply. Put an organic fungicide spray on, and then if you’ve had a lot of rain (more than about 25mm, or consistent rain for days on end) assume it’s been washed off and replace it.

Mixing Bordeaux spray
Mixing Bordeaux spray

An effective spray available to organic growers is copper, normally mixed with builders lime (when it’s called a ‘Bordeaux’ mix).

However as always, it’s important to consider the pros and cons. Copper is such an effective fungicide that it can have quite a detrimental effect on the soil because it kills the soil fungi, which are vital to the health of the soil and your trees. So, our compromise is that we only ever use copper once or twice a year.

Peach buds covered with copper spray
Peach buds covered with copper spray

In an ideal world, the tree’s natural defense system (including naturally occurring microbes on the leaves) would prevent a disease outbreak, but for many growers, their trees still need some protection.

Another spray we can use in organic orchards and gardens is sulphur, which is much ‘softer’ and less damaging to the soil, but also less effective at preventing outbreaks of disease. It’s likely to get washed off by rain more easily though, so it may need replacing more often.

Whey is another spray that is often recommended for organic fruit trees, but we’ve not used it ourselves and it’s hard to get any good data about its effectiveness. If you plan to use it please do some ‘citizen science’ and keep good records (including photos) of what you use, when you use it, and your results – and then send them in to us, we’d love to see them and share any new-found knowledge.

On a side note, one of the orchards in the Australian Network of Organic Orchards reported back on a trial he did with potassium silicate and potassium bicarbonate to prevent Black spot fungal disease in apples – and it was almost a total failure! So be warned, if you’re trialing an unproven product set up a small controlled experiment rather than risking your whole crop.