From our perspective deep in the soil, this is a crisis of suffocation and decay. We, the roots of this Grevillea, are designed to breathe and absorb water and nutrients. A pathogen, usually from the Phytophthora or Pythium genera, has invaded our zone. These water molds thrive in saturated, poorly drained soil, attacking our delicate root hairs and cortex first. They physically block our vascular tissues, preventing water from moving up to the stems and leaves. More critically, they release toxins. As we rot and die, we cannot perform our function. The plant above shows distress—wilting, yellowing leaves, leaf drop—not from a lack of water in the pot, but because we, the roots, are drowning and diseased, unable to transport it.
The first and most critical step is to halt the attack's progression. This means you must immediately stop all watering. The saturated environment is what the pathogen desires. You must physically remove the plant from its waterlogged prison, its pot. Gently wash the soil from our root system to assess the damage. Healthy roots on a Grevillea are firm and often have reddish tips. We, the infected roots, will be brown or black, mushy, and will likely slough off, emitting a foul, decaying smell. You must sterilize your cutting tools and ruthlessly prune away all of us who are infected. This amputation is vital to save the healthy parts of the system.
After the diseased roots are removed, we need a completely new environment to have any chance of recovery. You must prune the canopy back significantly. With so much of the root system gone, the remaining healthy roots cannot support the same amount of foliage. This balance is crucial to reduce stress. Do not simply place us back into the old, contaminated soil or pot. The pot must be scrubbed clean with a bleach solution, and you must provide a brand new, extremely well-draining potting mix. For a Grevillea, this is non-negotiable; a native plant mix with added sand, gravel, or perlite is ideal. This new medium will provide the aeration and drainage we desperately need to recover and eventually regrow.
Once repotted, you must resist the urge to overwater. Water us sparingly, only when the top layer of soil feels dry, and never let us sit in a saucer of water. Our recovery depends on oxygen reaching our remaining root tips. While chemical fungicides exist, their effectiveness against these soil-borne diseases is often limited and they can be harsh on a stressed plant. A better long-term strategy is to foster our natural defenses. After we have shown some new growth, you can consider using a gentle, natural antifungal like a diluted potassium bicarbonate drench. More importantly, treatments with phosphite solutions can be highly effective. Phosphite boosts our own natural defense responses, helping the plant to wall off the pathogen and resist future attacks, giving us a fighting chance to thrive once more.
