Root rot (Phytophthora root rot)

Symptoms

The disease develops in overly wet, poorly aerated substrate and attacks the roots and the base of the stem. The roots turn brown, rot and lose their bark, which slips off easily, and a dark, watery rot appears at the stem base. The above-ground part wilts, yellows and dies back despite moist soil – a characteristic symptom, because the damaged roots no longer take up water. Especially at risk are houseplants, succulents and cacti (from overwatering), and in the ground conifers, rhododendrons, ericaceous plants and strawberries on waterlogged sites. The process begins inconspicuously at the root hairs and root tips, which darken and disappear, after which the rot advances towards the framework roots and the root collar. Healthy, pale, active roots waste away — what remains is a dark, rusty-brown or blackish, watery mass, and the outer layer of the root bark slips off like a glove, exposing the discoloured central cylinder. In trees and shrubs, infection of the collar and the base of the trunk (stem-base rot) shows as a dark, often streaking patch, sometimes with oozing, and the bark there is moist and sunken; in conifers and rhododendrons, browning and dieback are seen from the bottom or from the centre of the crown while the soil is still moist. Characteristic is the nested (patchy) occurrence of symptoms in the bed — where water and the pathogen accumulate locally. It is worth distinguishing the two causal oomycete genera: Pythium mainly attacks seedlings and fine roots, causing damping-off and root rot in cool, wet substrate, whereas Phytophthora also attacks older, woody plants, framework roots and the collar, more strongly in warmer, water-saturated soil. Key for diagnosis is the contradiction of the symptoms: the plant wilts and looks 'dry', although the substrate is wet — this results from the destruction of the roots, not from a shortage of water.

Treatment

Hard to cure once it appears – the key step is to stop watering immediately and let the root ball dry out. Potted plants are best removed from the pot, their rotten roots cut away, and repotted into fresh, free-draining substrate and a clean container; heavily infected specimens are discarded together with their soil. Fungicides containing fosetyl-aluminium or metalaxyl slow the oomycetes, but they do not replace improving the water and air conditions. Management starts with isolating the diseased plant and cutting off the source of excess water: in the ground this means exposing and drying out the root collar, improving drainage, and, when growing in beds, forming raised beds or mounds so that water does not linger around the base. When repotting potted plants, the entire brown, disintegrating root system is removed down to healthy, pale tissue, the roots are rinsed free of the infected soil and set in a scalded container in fresh, loose substrate; the old soil and infected debris are not composted but removed from cultivation. An important practical note: oomycetes differ biologically from true fungi, which is why typical foliar fungicides against fungi (e.g. from the triazole group) are ineffective against them — only products targeted at oomycetes help (phosphonates, e.g. fosetyl-aluminium, and phenylamides, e.g. metalaxyl/mefenoxam), applied to the soil or as a drench and treated as support, not as a substitute for drying out the substrate. These products limit active infection but do not disinfect the soil of resting spores. After the intervention is finished, tools and containers are disinfected, because the resting spores (oospores) remain viable in the soil and plant debris for many years.

Prevention

The basis is free-draining substrate and drainage – plants are set without standing water, overwatering is avoided and pots are not left sitting in water in saucers. Use clean soil, disinfected tools and healthy nursery stock, and plant sensitive species (rhododendrons, conifers) on sites that do not flood. Moderate, needs-based watering is more effective than any treatment. Prevention rests on the principle of 'the right plant in the right place': sensitive species are not planted in places with a high water table or on compacted, impermeable soils, and on waterlogged ground raised beds, planting mounds and loosening material (grit, coarse sand, bark) are used. Susceptible species should not be replanted in substrate where dieback from root rot has already occurred, because the pathogen's oospores survive in it for many seasons. Watering is done in the morning with water from a clean source — water from a closed circuit or from containers collecting run-off can carry motile zoospores between plants. It is worth limiting excessive nitrogen fertilisation (soft, over-lush roots are more susceptible), choosing healthy nursery stock and, where possible, tolerant cultivars or rootstocks (e.g. in growing strawberries and fruit trees), and putting new acquisitions through a short quarantine before they join the plants in the collection. Biological products with antagonistic microorganisms are introduced preventively, counting on support, not on a guarantee of protection.

How to tell it apart from look-alikes

Control — organic and chemical methods

We give active-ingredient groups and modes of action, not brand names — specific products and their approval vary by country and are sometimes withdrawn. Always check and follow the current label.