Original Down East

In some cases you should consider replacing your stock with new plants after a few years. Strawberries, for example, will lose their vigor after a while. This is often a sign that they have become infected with virus diseases and anything you propagate from them will also be infected. Potatoes too can become infected with virus diseases spread by aphids. The seed potatoes you buy are generally grown in areas where aphid attack is rare, so they are likely to be free from virus. So, unless you can be quite sure that your crop has not been attacked by aphids, it is worthwhile buying in new potato tubers every year.

Different varieties of the same plants may have varying degrees of resistance to pests and disease. Some varieties of potato, for example, are less susceptible to slug damage, while others show resistance to potato eelworm. There is quite a distinct variation in resistance to fungus diseases.

Plant breeders are constantly trying to breed pest- and disease-resistant plants, so it is worthwhile checking the current position on new varieties before buying anything that is notoriously disease-prone. There are, for example, several varieties of snapdragon that have been bred specifically to resist rust fungus, there are eelworm-resistant Phlox and virus-resistant varieties of many plants including potatoes, strawberries and tomatoes.

Where it is difficult to breed-in resistance, it is sometimes possible to have the best of both worlds by grafting the required variety on to a resistant rootstock.

Of course, you should also employ the same safeguards with plants you have raised yourself, though here it's much more difficult to be ruthless about weeding out the weaklings. But bear in mind that a young plant that has been infected with a disease or attacked by a pest is at a disadvantage right from the start. Throw it away to avoid infecting other plants.

Keep your greenhouse scrupulously clean. Use plastic seed trays and pots for raising seeds and pot plants because they are easier to sterilize; wooden seed trays and clay pots are porous so can harbor pests.

Time your spring sowing so that plants do not have to remain in the greenhouse getting leggy and pot-bound because the weather is too cold for them to be planted out. The real secret is to get young plants growing away and then to keep them growing steadily. In some cases, this is all you need to control even the most virulent and damaging of diseases.

One final point to bear in mind when buying or raising plants from seed is that F1 hybrid varieties have much more vigor than those raised from open-pollinated seed. An F1 hybrid is the result of a first-generation cross between two selected parents. These first generations always have much more vigor, which will help to carry them over an early attack.