Bacterial Soft Rot of Clivias

Mark Laing, Professor, Plant Pathology, University of Natal, South Africa
22 April 2000


Causal Organism:

A bacterium called Erwinia carotovora pathovar carotovora

Hosts:

The bacterium has many hosts, attacking any fruit or vegetable with a juicy, sugary head or stem. Commonly, this bacterium attacks cabbage, cauliflower, broccoli, lettuce, chicory, Swiss chard, carrots, parsley and celery. Another strain attacks potatoes causing potato blackleg. It also attacks the bulbs of many Amaryllidaceae, especially if another agent such as the Amaryllis worm, has already attacked the bulb and made a wound into which it can enter. This year (2000) was extremely wet in summer, and several of my Clivias which were in a dense growing medium, have succumbed to this bacterium. The symptoms are initially a yellowing of one or two of the bottom leaves. On investigation, one finds a dark, water soaked lesion (wound) at the base of the plant. This may spread right up infected leaves. As the disease progresses, it often completely dissolves the base of the plant and the root crown, so that the plant falls over, leaving the bundle of leaves lying horizontal.

 

It is a very serious pathogen of Arum lilies (Zantedeschia species), being transmitted in bulbs, cuttings and even tissue cultured plantlets. This year I also isolated it from the heads of Agapanthus flowers, in the bud stag, just before and after the bud opened up.

Epidemiology and Disease Cycle

Ideal Weather Conditions: hot :30-400C, and wet : summer rainfall

Carryover: The bacterium lives in the soil or in plant debris in between attacking hosts. It may also be carried in the gut of insect and snail vectors.

Infection process: it usually infects hosts through wounds made by insects, nematodes (eelworms), snails, birds or people. However, under ideal weather conditions, it infects without the need of injuries.

Vectors (agents of movement): It is moved from plant to plant by insects (especially houseflies), snails, birds and people. It is also commonly moved on pruning secateurs or a knife for harvesting vegetables. Houseflies are perhaps the most efficient of its vectors. When this bacterium rots a plant, it does so with external enzymes (extracellular cellulases and pectinases), creating a smelly, soft, watery rot, to which flies are very attracted: a rotting cabbage really stinks. The flies feed on the semi-digested plant, then fly off and settle on a nearby plant. There they clean themselves, which involves a regurgitation process, depositing thousands of bacteria on that plant, and initiating another infection process.

When snails feed on infected plants, they ingest copious quantities of the bacterium, then having moved onto fresh plants, defecate there. Their faeces may contain thousands of bacterial cells, which again initiate another infectious process.

I would guess that the Amaryllis worm can vector this bacterium, carrying the bacterial cells in its gut from the stage of caterpillar to adult moth, to transmission at egg laying. However, it is unlikely that it carries through into eggs.

Raindrops also splash-disperse bacteria between plants. This form of disease spread is usually over a distance of 1-2m, but in a heavy rainstorm, aerosols can form, carrying bacteria up to a 1km.

Control on Clivias

It can be difficult to control, if conditions suit the bacterium, i.e., under hot, wet conditions.

For outdoor plants, use a very porous growing medium so that no water logging ever occurs, even when heavy rains occur repeatedly. Water logging is probably the single biggest cause of this problem, creating a stressed plant, with roots dying from lack of oxygen. For indoor plants, make sure that one does not over water the plants.

Have adequate shade levels, ideally under trees. Shade cloth does provide shade, but increases heating, especially black Shade cloth, and this favours soft rot.

Control Amaryllis worm, with weekly sprays of a pyrethroid insecticide such alphamethrin or cypermethrin (Ripcord is available in South Africa in 100ml quantities for the home gardener). This spray will also control houseflies. Spray diseased plants with a pyrethroid to stop further disease transmission by houseflies. For example, this year I stopped the spread of soft rot of Agapanthus flowers by spraying cypermethrin onto Agapanthus flowers and buds, killing the flies that were moving the bacterium between plants.

Control snails with regular applications of snail bait.

Eliminate other diseased plants, and their debris from the garden (especially vegetables with soft rot). Compost the debris in a deep compost stack which gets hot. This will eliminate the harmful bacteria. Move compost heaps well away from Clivia plants.

This bacterium prefers plants with a high nitrogen fertilization, and high sugar content. Altering the fertilization regime is therefore an important part of controlling it in vegetable crops. The trick is to increase the ratio of calcium, magnesium and potassium to nitrogen being applied. The first three elements make for stronger cell walls, making it harder for the bacterium to break them down. Dolomitic lime is a source of both calcium and magnesium, and potassium can be applied as potassium chloride or potassium nitrate, or by using a 3.1.5 fertilizer (3N : 1P : 5K).

When using knives or secateurs to harvest, prune or cut plants, the cutting implements must be sterilized between plants. Use a 70% alcohol (use methylated spirits diluted 7:3 with water), or Jik (3% sodium hypochlorite) diluted 1:3 with water to sterilize cutting surfaces. This is clumsy, slow and difficult to do in practice. But it is worth it if the bacterium is present.

Where a Clivia plant becomes lightly infected, remove the infected leaf or leaves using a razor blade or scalpel. Sterilize the blade between cuts using one of the above sterilants. Doctor the wound site directly with sulphur dust or copper oxychloride dust.

Where the plant is severely infected and has fallen over, remove it from its pot. Remove any soft or dead tissue from the base of the plant. Dust with sulphur dust or copper oxychloride dust. Plant into sharp silica sand (swimming pool sand). Treat it like a plant cutting, giving it frequent light watering, and keeping it under 60-70% shade. Clivias are remarkably tough, and they recover quite well.