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Disease resistance in fruit and vegetables

Disease resistance in fruit and vegetables; There are a number of lines of defence against pestss (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

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