The most effective way to control the disease is to subject the seeds to a hot water treatment of 51°C for 30 minutes.
Capsicums are also susceptible to pepper mild mottle virus (PMMoV). This also occurs on the seed, and is spread by aphids. If the plants are infected, yet you still wish to keep seed for replanting, the seed must be treated.
Do this by soaking in a 10% solution of trisodium phosphate for two hours, then rinse for 30 minutes in running water. This can be done by placing the seed in a container, then tilting it slightly and using a hosepipe to allow water to slowly circulate and run out.
PMMoV-resistant varieties are also available. Tomato plants suffer from three notable seed-borne diseases: bacterial speck, spot and canker. These are all very difficult to control in the field and it is worth the small effort to treat the seed – simply place them in water heated to 50°C for 25 minutes.
In the same way, cabbage, brocolli and cauliflower seed can be treated for black rot and blackleg. Heat the cabbage at 50°C for 25 minutes, and the broccoli and cauliflower at the same temperature for 20 minutes.
If you can afford it, buy a water bath to treat the seeds. This is a thermostatically controlled unit that keeps the water treatment at exactly the right temperature for as long as you need to.
Alternatively, you can use a pot on your hob, an accurate thermometer (such as a dairy thermometer), a jug of cold water, and a kettle of boiling water. Heat up some water in the pot to the correct temperature, measuring it with the thermometer. Place the seed in the pot.
This will lower the temperature of the water. Rectify this by pouring hot water from the kettle into the pot while stirring. Check the temperature; if necessary, add cold water to bring the temperature down to the correct level. Add hot and cold water as needed to maintain the correct temperature for the required time.
When the time is up, pour the seed and water through a sieve. Then hold the sieve under cold running water to bring the temperature down, as excessive exposure to a high temperature will affect germination. Remove the seed, place on newspaper and dry in the shade.
Ideally, you should dry the seed to the same weight as it was before placing it in the water. However, if the seed has to be planted immediately, dry it enough to handle it. This will greatly speed up germination.
Take care when doing hot water treatment on old seed or seed that has poor vigour. In this case, it is better to do a small batch first to determine whether germination will be reduced.
These days, seed is often coated with a coloured polymer, and various products can be added to this to afford protection against potential diseases. Some growers also prefer coated seeds as they can easily see any gaps where the planter has missed.
Coated seed can be water-treated in exactly the same way as uncoated seed.
Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.