Gardening in a changing climate
Diseases
The changing climate will affect plant diseases already present, as well as those yet to arrive. Drier summers will mean a welcome decrease in downy mildews, leaf blights, fruit scabs and other diseases that require water to disperse their spores; tomato and potato blight and rose black spot, for example, all need water to complete their life cycles. Apple and pear scab also thrive in wetter weather. An exception is powdery mildew, which is able to infect even in dry conditions and can be severe on drought-stressed plants.
Phytophthora is giving horticulturists - and gardeners - cause for concern, due to predictions of wetter, warmer winters. While the best-known Phytophthora is potato leaf blight, it is unusual in being airborne - other species of this important pathogen group are soilborne and attack roots. They need water to infect and, like all pathogens, are limited by low temperatures. If winters become warmer, Phytophthora will have longer periods to infect. Work by scientist Clive Brasier and colleagues at Forest Research (the research agency of the Forestry Commission) gives unsettling predictions for the spread and severity of P. cinnamomi, an important species thought to originate in Asia.
Future disease problems
Phytophthora (pictured above)
Warmer, wetter winters may prove a boon to Phytophthora root diseases. One of the most worrying is P. cinnamomi, originally from Southeast Asia, and so named because one of its first recorded hosts was cinnamon. In the past 200 or so years it has inadvertently been spread around the globe by mankind and, despite its tropical origins, P. cinnamomi can make its home in northern Europe. Its host range is immense and one of its most popular UK victims is common yew. Predicted climate changes for northern Europe would enable P. cinnamomi to spread northwards and deeper into the colder areas of central Europe, and also to become more severe in areas such as southern England where it already exists.
Pear rust (Gymnosporangium sabinae)
The laboratory at RHS Garden Wisley has noticed an upsurge in the frequency of pear rust. This striking disease was once a rarity in the UK, with none recorded in some years, but now the laboratory can sometimes see more than 30 cases a year. The orange spots on the upper leaf surface and surprising brown growths below are dramatic and unmistakeable, though not yet sufficiently damaging to warrant control. Could this upsurge be due to climate change? As yet this is unknown, but the two seem to be associated.
Athelia rolfsii
Among diseases that currently cannot quite establish, perhaps because the climate is too cool, is Athelia (formerly Corticium) rolfsii. This pathogen has a wide host range and is damaging in warmer climates such as the southern USA. Plants yellow and wilt, and develop rotting brown lesions at soil level. Its biology is similar to our indigenous Sclerotinia sclerotiorum, which does turn up in gardens and is an important pathogen of commercial crops such as lettuce, sunflower and oil seed rape. It spends the dormant season as sclerotia, small seed-like fungal structures that germinate in spring to reinfect and can easily be spread accidentally on bulbs or in soil. This disease already occurs in continental Europe and there have been outbreaks in the Channel Islands. It is close to making landfall in mainland Britain. There is one report that it did in fact cause serious damage to an experimental crop of imported bulbs in the UK, but the infection died out and has not recurred.
Camellia petal blight
This disease originates in Japan and is present in New Zealand, USA and much of mainland Europe. It was first detected in Cornwall in the 1990s. The fungus infects flowers and causes them to go brown and sometimes fall prematurely. It forms a resting structure in the fallen flower, which remains dormant until next flowering season when it germinates to release airborne spores. In 2002 it was reported more widely through southern England, and RHS pathologists detected it for the first time at Wisley, though it must have been present undetected for at least a year. It is a major problem in the USA and threatens to become a problem in Britain. Little can be done to prevent its spread. As the northern limit for frost-sensitive species extends with a warming climate, the disease is likely to follow.
Canna viruses
Cannas are increasingly popular in the UK and a longer, warmer growing season will mean that they can be grown more widely. However, with new plants come new problems. The 2002 RHS trial of cannas (for AGM award plants) was heavily infected with virus. Bean yellow mosaic virus was confirmed and a second one, canna yellow mottle virus, was suspected. Some severely affected plants were rogued before planting out, but even so, at least 70% of the trial was affected. Because cannas are vegetatively propagated these viruses are very easily spread in infected planting material. It will be important for gardeners and the retail trade to source healthy plants.
Found a new disease?
If you think you have found a new-to-Britain disease and are an RHS member, you can send or bring samples to the Members’ Advisory Service at RHS Garden Wisley, Woking, Surrey GU23 6QB, or samples can be sent to the Plant Health and Seeds Inspectorate, a division of Defra that is responsible for dealing with new plant pests and diseases. Write to: PHSI, Central Science Laboratory, Sand Hutton, York YO4 1LZ.
Samples of the disease should be sent with the host plant in a sealed polythene bag in a stout container. The covering letter needs to give details of where the plant is being grown, from where it was obtained and when the problem was first noticed. A contact telephone number should be provided.