If you’re aiming to grow your fruit organically, you won’t want to spray your fruit trees with any chemicals that could damage your fruit, people, or the environment.
But that doesn’t mean you don’t spray at all.
All deciduous fruit trees are prone to fungal diseases, some more than others. If the diseases are not properly prevented or controlled in spring, they can be devastating.
Instead, we’ll focus on the basic principles of fungal disease prevention. The same principles apply to pretty much any fungal disease your fruit tree is likely to get.
What causes fungal disease in your fruit trees?
Each fungal pathogen has trigger points that stimulate an outbreak of disease. They are a unique combination of:
- the number of hours the tree has been wet, and
- the amount of spores present.
We list these variables (and others) for individual diseases in the Keep Your Fruit Trees Free from Disease short course.
Regardless of the disease that you’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 you’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.
What should you spray your trees with?
In an ideal world, the tree’s natural defense system (including naturally occurring microbes on the leaves) would prevent a disease outbreak. Unfortunately, for most of us who live in the real world, our trees still need some protection.
An effective spray available to organic growers is copper. This is often used mixed with builders lime to create ‘Bordeaux’ spray.
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 by damaging soil fungi. Healthy soil fungi are actually vital to the health of the soil and your trees.
So, the compromise we recommend is only ever to use copper once or twice a year.
Another spray you can use in organic orchards and gardens is wettable sulphur. This 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.
Trialling new organic fungicide sprays
Whey is another spray that is often recommended for organic fruit trees. We’ve not used it ourselves and it’s hard to get any good data about its effectiveness.
If you plan to use whey please do some ‘citizen science’ and keep good records. Take photos, and make notes of what you use, when you use it, and your results. Please send them to us – we’d love to see them and share any new-found knowledge.
On a side note, a member of the Australian Network of Organic Orchardists (ANOO) reported on a trial in his orchard. He sprayed with potassium silicate and potassium bicarbonate to prevent Black spot fungal disease in apples. The results? Almost total failure!
So be warned, if you’re trialing an unproven product, set up a small controlled experiment rather than risking your whole crop.