Q What is soft rot?
A This is a very common rotting problem affecting a wide range of plants. Most plant diseases are fungal infections, but this one is caused by bacteria. The main bacteria involved is called Erwinia carotovora, which lives in the soil. It occurs in strains, each of which only affect a limited host range. Other bacteria are also sometimes present. These may be other kinds of erwinia or a bacteria called pseudomonas.
Caption: Check stored dahlia tubers regularly to avoid soft rot spreading
Q How do I recognise soft rot?
A The stench is unmistakable – it is a far more pungent and unpleasant smell than that given o by fungal rots. The source of the odour is the disintegrating tissues. These become soft and slimy, turning into a watery mass. One early warning sign is sunken, wet spots on affected plants. But these will very quickly spread, so you could easily miss them.
Q Could I mistake it for anything else?
A There are many different kinds of rots, usually caused by fungi. Look out for rots caused by blue moulds, which mainly affect fruits. These have a mould much like that seen on old bread. Other rots are caused by botrytis; these are characterised by a grey, fluffy mould. Sclerotinia rots are recognised by the firm, white mould, with black resting bodies in the mould and affected plant material. Parsnips often rot as a result of the blackened crowns that come from parsnip canker.
Q Can you tell me more about soft rot?
A As diseases go, it’s not very invasive and needs help to get into plants. Once inside, it lives at first between the cells of the plants. The enzymes it produces there break down tissues, disrupt cells and dissolve cell contents.
Typically, wounds left by harvesting, frost, disease damage and insect or slug damage open the way for soft rot. Young, vigorous plants don’t suffer much but older plants, especially those weakened by age or prolonged storage, are highly vulnerable. In fact, when plants are beginning to die naturally or through stress, soft-rot bacteria can move in without the help of a wound left by another organism.
Q Which plants are likely victims?
A Plants with fleshy storage parts are most at risk. Bulbs (eg hyacinths), cyclamen corms and rhizomes, bearded irises and arum lilies, for example, suffer severely. Vegetables, especially potatoes, tomatoes and cucumbers, are also at risk. Soft, fleshy heads and leaves can be attacked. Cabbages, celery and lettuce are common soft-rot victims. Bacterial rots can also affect the soft growth of ornamental plants. Primula crowns, for example, can be damaged, leading to the wilting and collapsing of the plant. The stems of houseplants, such as dieffenbachia, can also rot. Tomato stems can be attacked – there are many tomato-stem problems, but this one results in more slimy stems than most.
Q Are stored plants attacked?
A Yes, they are. In fact, the most annoying soft-rot damage is in stored vegetables. Carrots, swedes, squashes and cabbages are commonly attacked. Be careful not to keep damaged produce – it is likely to rot quickly and spoil the rest. Check over your stored vegetables and bulbs every month or so and dispose of any affected ones. If it looks as though the plants are beginning to suffer badly, they are probably approaching the end of their storage life and should be used up as fast as possible.
Q Where does it come from?
A These bacteria are present everywhere, and can live in the soil during winter in infected plants, on contaminated equipment and even in insects.
Q How does soft rot spread?
A Sometimes planting material, seed potatoes or iris divisions carry the bacteria to new plants at planting time. The bacteria can also be transported inside insects from plant to plant. Contaminated soil or compost is another way it can spread. It is not, however, found in manure.
Q What sort of conditions encourage soft rot?
A Moisture is needed for it to spread. In greenhouses and vegetable stores, keep the plants and harvested produce dry. Water indoor plants from below and ventilate freely to avoid humid, close conditions. Warmth also speeds up the disease. Gather vegetables in the cool morning, store in the fridge and ventilate storage sheds and greenhouses to let in cool air.
Outdoors, badly drained plots or compacted soil are soft-rot havens. Aim to improve drainage or even build raised beds. Increase the spacing between plants to improve airflow. Don’t water from above and water early in the day so that any wetting dries quickly.
Excess nitrogen or insufficient potash in the soil can also make the problem worse. If soft rot affects your plants, go easy on nitrogen fertiliser and add 35g a square metre of sulphate of potash.
Q What can I do about soft rot?
A Disposing of contaminated plants to avoid a build-up in the soil, cleaning or removing contaminated materials and equipment, careful storage, avoidance of injuries and pest damage and crop rotation are the basis of controlling this disease.
Q Can I avoid soft rot?
A Controlling pests (such as carrot fly and slugs) will help limit losses. Also, try to avoid adding too much fertiliser or manure which causes excessively soft growth - soft growth seems especially vulnerable to invasion.
Q What do I do with rotting plants?
A Do not compost them. The bacteria may survive composting, and end up being spread around the garden. Burn all rotting plants or consign to the dustbin. If you are feeling energetic, you can bury rotting plants on a spare plot. To be safe, you need to bury them at least 50cm deep.
Q Are there any sprays?
A No chemical sprays are recommended for controlling soft rot.
Q Will crop rotations help?
A Yes. Try not to grow susceptible plants on the same plot year after year. This is very difficult to arrange in small gardens. Where a serious outbreak has occurred, consider planting sweetcorn, as this is fairly resistant to soft rot.
Q Are there any resistant plants?
A No, there are no completely resistant varieties.