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Fungus disease is often caused by too much water and not enough air circulation. Another common reason is that foliage remains damp too long. It takes 8 hours of moisture for a fungus to insert it's feeding tube into leaf cells. If the infection is not too advanced, correcting these conditions may halt the spread of the fungus and blighted parts of the plant can be removed.
Fungi need moisture, certain specific temperatures, and a certain pH range to be present before it can propagate. Sodium Bicarbonate prevents fungus from thriving by creating a toxic surface. The fungi remain dormant waiting for suitable conditions.
You can change the plant's location and growing conditions in addition to spraying it with the following preparation.
3 tablespoons baking soda mixed with 3 tablespoons oil (horticultural, vegetable, olive, or corn) mixed with a few drops of dish washing liquid in a gallon of water. Spray the entire plant, repeated applications may be necessary.
This preparation has been shown to prevent blackspot, various grass fungi, and powdery mildew. Some plants are hypersensitive to oil so test the oil on a portion of one leaf and wait 24 hours before you spray the entire plant. It has been said that this solution is not a cure as it may not kill the fungi, but it is a preventative measure that has been shown to work well.