We, the Grevillea family, are a diverse and resilient group of Australian natives, prized in American gardens for our vibrant, spider-like flowers and fine, often silvery, foliage. While we are generally tough and adapted to well-drained soils and sunny climates, our transplantation to the USA exposes us to new environmental stresses and pathogens that our evolutionary history did not prepare us for. Our distress signals manifest as discolored leaves, stunted growth, and wilting, often misinterpreted by our caretakers.
Perhaps our greatest threat is the water mold Phytophthora cinnamomi. This pathogen lurks in poorly drained, heavy, or overwatered soils, attacking our root systems. From our perspective, it is a silent suffocator. It clogs the vascular tissues we use to transport water and nutrients, leading to a rapid and devastating decline. Our leaves wilt, turn pale green or yellow, and our branches die back. The most tragic part is that this often occurs when our caretakers, meaning well, provide too much hydration, creating the anaerobic conditions this mold thrives in.
On our softer new growth and beneath our leaves, tiny invaders like aphids and scale insects pose a significant nuisance. These pests pierce our epidermal layers with their needle-like mouthparts to suck out our vital sap. This direct loss of nutrients weakens us, causing our leaves to curl, yellow, and become stunted. Furthermore, they excrete a sticky, sugary substance called honeydew, which coats our surfaces. This honeydew attracts sooty mold fungi, which, while not directly parasitic, blackens our leaves and further impedes our ability to photosynthesize effectively.
Our foliage is our pride, but it is vulnerable to fungal attacks, especially in regions with high humidity or overhead watering. Fungi such as Cercospora and Alternaria species cause unsightly leaf spots. These appear as brown or black circular lesions, often with a yellow halo. From our point of view, each spot is a dead zone, a area where photosynthesis has ceased. Similarly, powdery mildew coats our leaves in a white, powdery film, directly blocking sunlight and stressing our systems. These infections are not just cosmetic; they drain our energy and compromise our overall health.
It is crucial to understand that many of our ailments are secondary, stemming from cultural missteps. We are profoundly sensitive to phosphorus; high levels found in many general-purpose fertilizers are toxic to us, causing leaf scorch and locking up other essential nutrients. Planting us in clay soils or locations with poor drainage is an invitation for root rot. Furthermore, while we are sun-worshippers, extreme heat combined with inadequate water can cause severe wilting and sunscorch on our leaves, making us more susceptible to the pests and diseases mentioned above.
