Pest Watch: Know your enemy... Strawberry red core

Strawberry red core is a long lived fungal-like disease which attacks the roots of strawberry plants, causing them to rot. Its stealthy mode of infection and survival mechanism is second to none so it can be a tough customer. Luckily knowledge of its methods of invasion and favourite habitat means gardeners can keep it at bay and all is not lost if you find it has infiltrated your plot.

Causes and symptoms

Strawberry red core is often first noticed in spring with the appearance of stunted plants with reddish-brown, greenish-blue or purple tinged leaves. A reddish band can appear around the leaf edges and this may grow to cover the whole of the central leaf area. Growth can be patchy with individual plants becoming weak and stunted and eventually dying off as if affected by severe drought. On inspection roots are stunted, you will find the white feeding tips have rotted away and the remaining roots may be dark brown or black with outer layers that easily peel off. When the root is split open with a knife, the classic symptom is a reddish-brown core running down the centre.

The red core is most noticeable in spring and autumn but not as defined during the summer. If in doubt, always lift a plant and try this test on the roots from early spring to summer. Affecting anywhere in the UK, this destructive pathogen, Phytophthora fragariae var. fragariae, is actually a water mould or oomycete rather than a true fungi. Loving cool, wet conditions, it will strike more often in wetter areas or those with heavy soil.

Ecology and lifecycle

As with all water moulds, life begins as a spore released into the soil from the decaying roots of an infected plant. Spores can be introduced on plants, soil and tools or boots. These spores are incredibly tough and can lie dormant for at least 12 years and have been reported to re-infect up to 17 years later, even if no strawberries are grown in the intervening years. Infection of healthy roots occurs in wet conditions and the pathogen can travel in movements of soil water, often travelling from higher areas to lower ones as water drains through the soil.

The ideal conditions for the spores to develop and thrive are in poorly drained, cool, wet soils and often a period of heavy rainfall can trigger activation. The developing water moulds are attracted to the chemical exudates produced by the growing strawberry rootlets and grow towards their tips where they start to digest the root cells. Following infection, rot begins to spread from the tips up towards the crown of the plant causing the characteristic reddening of the inside section or ‘stele’ of the root. Infections spread most rapidly during early spring and late autumn when conditions are most favourable. The damage caused to the root prevents proper mineral and water uptake causing the leaf symptoms which are normally detected first.

Prevention and Control

There is no cure once plants are infected so exclusion and prevention are essential. The number one consideration is that the pathogen thrives in damp conditions, so site your strawberry bed in well-drained soil or in a raised bed. This could mean adding compost, well rotted manure, leafmould and grit or sand to heavy soils, growing on heaped up ridges or using standard raised beds or containers. A height of at least 25cm (10in) is best for raised beds to ensure good drainage, especially if the soil below is compacted or heavy clay.

Digging over the ground to relieve compaction is also advisable. Next, always buy the healthiest, certified plant stock you can to avoid bringing red core in to your garden. Strawberries are good fun to propagate but always be slightly suspicious of plants donated by friends just in case. You might want to check the roots in early spring before accepting them if they have had any issues the season before.

There are a number of cultivars that show resistance to red core so selecting ‘Symphony’, ‘Rhapsody’ or ‘Pegasus’ would be good while some are less resistant so avoid ‘Christine’, ‘Royal Sovereign’ and ‘Honeoye’ and check with your grower which varieties they would recommend. If you do find that red core has somehow made its way in or you have it in an existing strawberry patch, destroy all infected plants by burning or binning. You will need to grow new plants in a fresh strawberry patch prepared as above, ideally as distant and uphill from the original as possible. Do not grow strawberries on a patch that has been infected as the longevity of the spores ensures reinfection. Get rid of any plants propagated from infected mother plants too.

Finally, it is a chore but always clean equipment well after use especially if you take or lend your tools to other gardeners, say on a neighbouring plot, as you wouldn’t want to bring anything nasty back by mistake.