Phytophthora root rot

What you see

The plant looks drought-stressed no matter how much you water it - sparse, small, yellowing leaves, dieback from the top, and a general failure to thrive. Wash the roots and the fine feeder roots are gone; what remains is dark and brittle, and the tissue under the bark at the collar is red-brown rather than white.

Conditions

Waterlogged or compacted soil. The swimming spores need standing water to move, so it is a drainage disease first and a pathogen second. Rain gardens, low spots and pots sitting in saucers are prime sites.

Treatment

Improve drainage before anything else. Remove dying plants with their root balls. Do not simply water more - watering a Phytophthora plant hastens its death.

Prevention

Plant on mounds or in raised beds. Never let pots stand in water. Use fresh, clean compost and mains water for propagation. Avoid replanting the same genus in the same spot.

Months: October, November, December, January, February, March

Plants

Lavender, Rosemary, Orange, Lemon, Tangerine, Rhododendron, Strawberry, Avocado, Olive, Box

Remedies

Mulch

The thing people get wrong

The cruel trick of this disease is that it makes a plant look thirsty. Wilting, small pale leaves and dieback all read as drought, so the gardener waters more, which is precisely what the pathogen needs to move. If a well-watered plant looks parched, stop watering and dig up a root before doing anything else.