Beating fungal disease

First of the season’s pawpaw (papaya) for breakfast this morning, with Cavendish banana, black passionfruit and homemade yoghurt. The pawpaw is a winter one – still very delicious but not as sweet as the spring ones will be. But it’s my very favourite fruit so I’m a bit biased.

The pawpaw in last year’s winter fruit bowl had more black spot – a fungal disease that they’re susceptible to in cool, wet weather. This winter has been a bit drier but not much. The main difference is that I cut out all the worst affected trees last year, and saved seed from the ones least affected. It is unlike me to take any biomass offsite, but I put all the leaves from the cut out trees in the green bin for council collection. It’s easier for Council to make the large batch hot compost necessary to kill fungal spores. My cool compost, deep litter chook-made system risks allowing spores to survive.

We pruned all the food forest trees, especially the tamarillo nursery trees quite heavily to let more sun in, and more air to circulate. Then I fed the resistant trees well with seaweed brew. All the fruit trees get a few shovelfuls of compost every few months too, and some worm castings every so often, but I think it is the micronutrients in the seaweed brew that have the real effect on disease resistance. Just like humans, plants may grow big on a diet of macronutrients, but they need micronutrients too to build good cell structures and immune systems.

The classic non-organic solution for black spot is Mancozeb fungicide, but Mancozeb is a nasty chemical, good evidence for reproductive and developmental toxicity to humans and DNA damage in rats. As if that isn’t enough, there’s also good evidence that it knocks around the soil microlife I depend on for fertility and the predatory mites I depend on for pest eradication. I don’t want it in my garden.

The classic organic solution is Bordeaux mixture or one of the other copper based fungicides. But copper harms bees and other insects, and builds up in the soil over time to levels that harm plants, microlife, animals. Organic gardeners also try neem oil, wettable sulphur, and baking soda mixed with detergent. But they all have big drawbacks besides the obvious one of being a lot of work. Baking soda is alkali, so over time will change the pH of soil. Sulphur acidifies soil, so does the same in the opposite direction and can lead to leaching of toxic metals. You can save fruit by dunking it in warm water ( 45°C) for 20 minutes or so but that doesn’t help the tree.

Permaculture goes for more holistic, less work solutions to problems like this. And my breakfast shows that it works, for pawpaw black spot but the same strategies work for powdery mildew and most other fungal diseases.

• Choose resistant varieties. Get seeds or seedlings from local or knowledgable sources, and save your own so each generation is more resistant. If you buy plants from Bunnings, they are unlikely to have been bred for local conditions, or for resistance. You could even speculate they’ve been bred to sell Mancozeb.
• Get rid of infection sources. It’s not easy in suburbia. Neighbours have roses that they may or may not spray. But if you have a tree that is badly affected, really, in the long run it’s going to be better to cull it than to treat it. Unless you can include the leaves in a large batch of hot hot compost, you’re better off sending them offsite.
• Fungal diseases like cool, humid conditions. They least like dry leaves. So the more you can water the soil rather than the leaves, build soil water holding capacity so you water less often, let sun dry leaves, and let air circulate, the less fungal diseases will like it. Attention to solar aspect will allow you to cram fruit trees into a food forest, and still get enough sun on leaves.
• Micronutrients allow plants to build strong cell walls that resist fungal infection. You could go to lengths to identify particular micronutrient deficiencies, but my strategy is to use seaweed brew and worm castings and compost made from a nice variety of ingredients including weeds and leaf litter from deep soil mining trees. Then, whatever the particular deficiency, it doesn’t matter. We’re lucky that it is easy to collect seaweed, but you don’t need a lot to make a good seaweed brew. I used to make it a few times a year even when the beach was a camping holiday away. And a little goes a long way.