Grevillea species, while generally robust, can be susceptible to several pathogens and environmental stress-related diseases. From our perspective as plants, these ailments disrupt our vascular systems, damage our photosynthetic tissues, and impede our ability to absorb nutrients and water, threatening our health and beauty.
This is a devastating soil-borne water mould, not a true fungus, that attacks our root systems. From our roots' point of view, it feels like a relentless suffocation. The pathogen destroys the fine feeder roots responsible for water uptake, leading to a systemic drought within us despite moist soil. You will see our leaves wilt, yellow, and die back from the base upwards. The most telling sign is a dark brown or black lesion girdling the stem at the soil line. Treatment is difficult once advanced. The primary action is to improve soil drainage drastically. Fungicides containing phosphite can be applied as a foliar spray or soil drench; we can absorb this and translocate it to our roots, boosting our own defence mechanisms to suppress the pathogen.
These pathogens, such as *Cercospora* or *Alternaria* species, directly target our foliage. From the leaf's perspective, it begins as a small, localised invasion—a tiny spot where the pathogen has breached the surface. These spots expand, often with dark margins and a lighter centre, disrupting chlorophyll production and effectively creating dead zones on our solar panels. For treatment, we recommend removing and destroying severely infected leaves to reduce spore spread. Ensure good air circulation around our canopy by pruning neighbouring vegetation. As a protective measure, apply copper-based fungicides or bactericides early in the season, especially during wet, humid weather, to create a protective barrier on our leaf surfaces.
Sooty mould itself is not a direct pathogen but a clear indicator of another problem: a sap-sucking insect infestation. Scale, aphids, and mites feed on our sugary sap and excrete a sticky waste product called honeydew. This substance coats our leaves, and the sooty mould fungi grow on it. From our perspective, it is like a blackout curtain being drawn over us; the mould physically blocks sunlight, severely reducing our capacity for photosynthesis. The treatment focuses on the root cause. Control the sap-sucking insects with horticultural oils or insecticidal soaps. Once the honeydew production ceases, the sooty mould will weather away or can be washed off with a strong jet of water.
While not a infectious disease, chlorosis is a common ailment we suffer from. It manifests as a yellowing of our leaves while the veins remain green. From our cellular view, this is a cry for help—a critical shortage of chlorophyll due to an inability to uptake essential nutrients, primarily iron and manganese. This is almost always caused by soil that is too alkaline (high pH) for our preferences, as we thrive in slightly acidic conditions. In high pH soil, these micronutrients become chemically locked up and unavailable to our roots. The treatment is to correct the soil pH by applying soil sulphur or using a fertiliser formulated for acid-loving plants. Chelated iron supplements can provide a short-term corrective measure by supplying iron in a form we can absorb more easily.
