🦠 When we talk about plant diseases, we refer to living organisms that act as parasites, i.e., they live in or on a plant host and benefit from exploiting it to obtain their nutrients. There are three main categories of diseases, depending on whether the parasite is a fungus, a bacterium or a virus. Here I want to focus on fungal and bacterial diseases.
🔬 Bacterial and fungal parasites can be divided into two classes: facultative and obligate. Obligate parasites need a plant host to survive, whereas facultative ones have the option of eating dead organic matter too.
🍁 Let’s have a look at how fungal diseases attack leaves. The way a fungal parasite gets into a leaf is via the germination of a spore, from which a fungal thread (hypha) emerges and penetrates the leaf tissue, and in particular the space between cells, which contains a solution of nutrients and other compounds (this solution is called the apoplasm, as opposed to the one contained inside cells, called the cytoplasm). The fungal thread then releases substances that dissolve the membranes, kill cells and allow the spread and uptake of nutrient from the plant. Bacteria spread very similarly, but without spores or hyphae, just by swimming into the plant nutrient solution and multiplying.
☘ Three factors are really important in preventing this from happening: the nutritional content of the leaf, the strength of the walls and membranes that protect the outer and inner tissues of the plant, and the ability of the plant to defend itself
Thus, the mineral content of the leaf is really important for all three of these resistance factors, because certain nutrients attract, encourage and support the parasite spread, whereas others have the potential to prevent it and slow it down. A detailed and extensive treatment of the connections between individual minerals and the occurrence of pests and diseases can be found in the books “Mineral nutrition and plant disease” (Datnoff et al, 2017) and “Marschner’s mineral nutrition of plants” (Rengel et al, 2022).
Here I'm not going to touch upon Calcium, Silicon, Boron and Sulphur and Potassium, because I've written about those in previous posts. But I want to spend a few words about Nitrogen.
🌿 Nitrogen is the most controversial mineral when it comes to disease resistance. In fact, high levels of nitrogen increase a plant’s susceptibility to obligate parasites, whereas nitrogen deficiency makes plants vulnerable to facultative ones. Why is it so? It depends on the nutritional requirements of the pathogen. As a rule of thumb, high nitrogen encourages fast and tender growth, which benefits obligates, because these need the metabolites produced by living cells. On the other hand, facultatives prefer senescent material and can revert to eating rotting or dead tissue, so they thrive on old leaves and can also release toxins that accelerate the death of the plant’s cells.
High quantities of amino acids (linked to high nitrogen content) induce the germination of fungal spores on the leaves. Excess nitrogen also slows down the production of some plant secondary metabolites, and in particular phenols, which are the precursors of lignin - the woody material that makes plants more impervious to attacks.
👨🌾 In practice, this means that we have to be aware of the nitrogen supplied to our plants, and we should learn to identify and differentiate between the major facultative and obligate pathogens that can attack plants. Facultatives such as some leaf spots (Alternaria, Cercospora, Xanthomonas, etc.), Rhyzoctonia, Tomato, potato and other blights (Phytophthora, Pythium, etc.), Fusarium wilt, are more likely to be a sign of low nitrogen; whereas obligates such as Powdery mildew (Oidium), Pseudomonas leaf spot, Downy mildew (Peronospora) are often seen in plants with an excess of nitrogen.
🥕 How do we ensure that the key nutrients (Ca, K, Si, S, B, N) are not only available but abundant in our gardens? There are many answers to this question, but here is what we at Living Soil Garden do and recommend.
🍅 A soil test can help us see if there are any underlying deficiencies. As an alternative, close observation of bioindicator weeds and close inspection of disease occurrences can give us lots of clues.
Then, we need to provide a rich, diverse, complex, living source of food for our plants. Plant-based compost and small amounts of worm castings can provide both the nutrients and the microbes needed to unlock them and deliver them to the plant.
🚿 When a deficiency is present and the soil is too unbalanced, dry or slow to give immediate results, we can bypass it by applying a fast-acting foliar feed; in this case, good solutions are sea weed (rich in many minerals and hormones), compost extract (rich in microbes and enzymes), anaerobic horsetail tea (high in silicon) and lactic acid bacteria (which can prevent spore germination by coating the leaves).
As discussed in previous posts, foliar inputs should be limited if not avoided in the long term, as the soil becomes ecologically balanced. Allowing soil microbiology to mediate nutrient uptake is also a great way of avoiding excessive nitrogen (especially in the form of nitrates, which have a huge impact on insects and humans as well as diseases) in the leaves.
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