Disease resistance in fruit and vegetables

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.

For these crops there has been considerable research and selective breeding carried out in order to find cultivars that are resistant or immune to pest and disease damage. Generally this has involved finding suitable genetic material amongst existing stocks or in the wild, which is then incorporated into commercial varieties.

Example : the apple
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 fire blight (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. pumila) or Vm (from M. micromalus) gene. Major gene resistance’s to powdery mildew are derived from M. robusta (Pl1) and M. zumi (Pl2), and the apple cultivar Northern Spy has a long-standing reputation for its major gene resistance to woolly apple aphid. Since early this century this resistance has been used to develop woolly aphid resistant rootstocks such as MM.106[?] and M.793. Much later it was shown that the cultivar was also very resistant to collar rot and a useful breeding parent for this resistance.

Resistance and immunity
Some plants can tolerate the presence of large numbers of insects without being severely affected. This is not very satisfactory however as insects will still cause damage, and in fact further breeding and population expansion of the pest species is supported. Other varieties are less attractive to pests, but this can be difficult to sustain or demonstrate.
The most valuable form of resistance is where the pest cannot survive as well on one variety as on another. 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).

Trade-off of breeding for resistance
Sometimes however there can be a trade-off, for those varieties which have increased immunity or resistance may be lacking in other qualities such as flavour, yield or quality. Celery resistant to the Fusarium[?] fungus (Fusarium oxysporum spp.) may not succumb to this disease, but may also be unacceptably short, ribby and low yielding.
Further, a cultivar that is resistant to one disease may be more susceptible to another that is equally important. A lettuce cultivar that is resistant to mosaic virus[?] may be sensitive to corky root disease[?] (caused by Rhizobius suberfaciens), whilst another that resists corky root may be vulnerable to downy mildew[?] (Brim lactic).
Another drawback to resistance is that depending on the host pathogen system, resistance is sometimes not long lasting as new pathogen strains quickly develop, and further research and breeding is constantly needed.

Availability of resistant varieties
Resistant varieties are not available for all crops. For several of the most damaging plant diseases, such as tomato blight[?] (Phytophthora infestans) and white rot[?] (Sclerotic cepivorum) of the Allium[?] family, no acceptable resistant cultivars are yet available. In addition, commercial seed companies and plant breeders rarely invest resources into developing resistant cultivars for more minor or speciality crops, which often tend to be those of greater interest to the organic grower.

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.

The table below illustrates a number of pest and disease resistant vegetable crop varieties, although should not be considered comprehensive.

Crop	Variety		Comments
French Bean	Aigullion		Resistant to bean mosaic virus and anthracnose
	Hildora		Tolerant of bean mosaic virus
	Maxi		Tolerant of bean mosaic virus
	Nektar Queen		Resistant to bean mosaic virus
	Farba		Resistant to bean mosaic virus
	Hilda		Resistant to bean mosaic virus
Broad Bean	Futura		Tolerant of chocolate spot
Brussels sprouts	Cavalier		Resistant to light leaf spot
	Braveheart F1		Resistant to powdery mildew and light leaf spot
Cabbage	Stonehead F1		Resistant to mildew
Calabrese	Emperor F1		Highly tolerant of black rot and downy mildew
	Sampson F1		Highly resistant to black rot and downy mildew
Carrot	Fly Away F1		Bred for resistance to carrot fly
	Newmarket		Good resistance to splitting
Cucumber	Bush Champion F1		Resistant to cucumber mosaic virus
	Marketmore		Resistant to cucumber mosaic virus
	Burpless Tasty Green F1		Resistant to mildew and tolerant of heat
	Slice King F1		Resistant to downy and powdery mildew, gummosis and angular leaf spot
	Cumlaude F1		Tolerant of powdery mildew
	Media F1		Tolerant of powdery mildew
Leek	Swiss Giant, Zermatt		Moderate rust resistance
	Alvitta		Rust tolerant
	Conora		Good resistance to rust
Lettuce	Alexandria		Highly resistant to downy mildew and bolting
	Lisbusa		Resistant to downy mildew, tolerant of mosaic virus, root aphid and tipburn
	Sylvestra		Resistant to downy mildew and aphids
	Little Gem		Resistant to root aphid
	Barcelona		Resistant to aphids and mildew
	Iglo		Resistant to downey mildew
	Revolution		Resistant to mildew and bolting
	Smile		Resistant to aphids, mildew, tipburn and bolting
Parsnip	Avonresister		Very good rsistance to canker
	Gladiator F1		High resistance to canker
	Tender and True		Good canker resistance
Pea	Ambassador		Resistant to powdery mildew, entation virus and fusarium wilt
	Cavalier		Good resistance to powdery mildew
	Greenshaft		Resistant to downy mildew and fusarium wilt
	Rondo		Resistant to fusarium wilt
Potato (first early)	Colleen		Good resistance to blight and scab
	Premiere		Resistant to blight and spraing
	Pentland Javelin		Good all round disease resistance, including golden eelworm and scab
	Swift		Resistant to golden eelworm and tolerant to blackleg
Potato (second early)	Cosmos		Resistant to blight and common scab
	Osprey		Very resistant to scab and eelworm
Potato (maincrop)	Harmony		Very resistant to scab and partially resistant to white and golden eelworm
	Milva		Blight resistant
	Remarka		Good all round disease resistance
	Admiral		Good resistance to blight and scab
	Cara		Blight resistant
	Valor		Good overall disease resistance combined with high eelworm and blight resistance
Pepper	Bell Boy F1		Mosaic virus resistant
	Mandy F1		Highly resistant to mosaic virus
Sweetcorm	Golden sweet F1		Highly resistant to smut
Tomato	Alicante		Good resistance to greenback and mildew
	Libra F1		Resistant to fusarium crown and root rot
	Shirley F1		Resistant to fusarium, cladosporium and TMV