'Disease resistance in fruit and vegetables'; There are a number of lines of defence against pests (that is, those animals that cause damage to the plants we grow) and diseases in the organic garden, principle among these being the practice of good husbandry, creating healthy soil and ensuring high standards of garden hygiene. But no matter how diverse and healthy the garden eco-system may be, there will always be a degree of disease and pest presence. In many ways, some level of pathogen population in the garden can be not only acceptable but desirable as they are indicative of a generally healthful and diverse environment, and add to the overall robustness of the system as an immunity to such detrimental influences will build up, particularly in a balanced polycultural regime. Indeed, most of the plants we grow will tend to be selected because they are trouble free, and those that are more susceptible to attack will have fallen by the wayside over time. However, most farmers find it unacceptable that the food crops they grow are damaged by pests. Generally this has involved finding suitable genetic material amongst existing stocks or in the wild, which is then incorporated into commercial varieties.

In the case of apples, in which research is being carried out in order to develop resistance to diseases such as black spot ('Venturia inaequalis'), powdery mildew ('Podosphaera leucotricha'), orchard fireblight ('Erwinia amylovora'), woolly apple aphid ('Eriosoma lanigerum') and collar rot ('Phytophthora cactorum'), the main sources of resistant material used in breeding programmes such as those being run by East Malling in England or Hortresearch in New Zealand are major gene resistance’s derived from crab-apples. The Vf gene for black spot resistance is derived from the ornamental crab-apple species 'Malus floribunda'. Most black spot resistant cultivars developed around the world carry this gene, but there are some selections that carry the Vr (from M. micromalus) gene. Major gene resistance’s to powdery mildew are derived from M. zumi (Pl2), and the apple cultivar 'Northern Spy' has a long-standing reputation for its major gene resistance to woolly apple aphid. Much later it was shown that the cultivar was also very resistant to collar rot and a useful breeding parent for this resistance.

Some plants can tolerate the presence of large numbers of insects without being severely affected. Other varieties are less attractive to pests, but this can be difficult to sustain or demonstrate. In some cases this can actually make the plants immune to attack, as is the case with the lettuces 'Avoncrisp' and 'Avondefiance' which were bred at the Institute of Horticultural Research, Wellsbourne during the 1960s, which are fully resistant to lettuce root aphid ('Pemphigus bursarius'). For several of the most damaging plant diseases, such as Potato blight ('Phytophthora infestans') and white rot ('Sclerotic cepivorum') of the Allium family, no acceptable resistant cultivars are yet available.

In general it is probably fair to say that resistance will not fully guarantee total crop protection, but choosing resistant varieties should rather be considered as a part of an overall integrated control strategy, especially against virus diseases. In particular they can be especially useful where the threat from specific pests and diseases is high.