Crop protection through breeding

Pest and pathogen resistance is vital in the battle for profitable and sustainable brassicas, says Geoff Dixon.

Field of brussel sprouts - photo: HW
Field of brussel sprouts - photo: HW

Brassicas are some of the most amazing plants serving mankind. Collectively, they deliver leaves, flowers and roots for fresh, cooked and processed meals, along with much more: low-fat edible oils, industrial lubricants, lighting oils, animal feed, condiments and flavourings, soil conditioners and ornamentals.

Recent realisation of their benefits for long-term human health raises mankind's debt to brassicas still higher. But they are very vulnerable to damage by pests and diseases.

Why are brassicas so good?

Understanding why brassicas are good for human health helps plant breeders develop pest and pathogen resistance in the crops themselves.

Twenty million years ago the brassicas' ancestors grew where Ethiopia is now. These lands were even then nutrient- and water-poor places. The wild forms of brassica moved slowly northwards towards the fertile crescent of today's Syria, Turkey, Iraq and Afghanistan.

Natural evolution formed two key species about 10 to 15 million years ago: B. oleracea, which eventually became European vegetables, and B. rapa, eventually Asian vegetables. These are still spectacularly flexible and adaptable plants. By natural genetic modification, each produces new species by transferring its entire chromosomes content. Consequently, all the genetic material of B. oleracea and B. rapa combined, producing B. napus (swedes and oil seed rape), 10,000 years ago in northern Europe.

At about the same time, B. rapa took off across Asia to China and ultimately Japan. Then evolution directed by man resulted in the array of Asian Chinese cabbage and its numerous variants.

In Europe, B. oleracea evolved in cultivation to cabbage, Brussels sprout, cauliflower and broccoli. These are the work of growers over the past 2,000 years. The ancestral brassicas evolved chemical armaments of mostly sulphur containing organic substances for protection against pests and pathogens. It is these compounds that are beneficial to human health.

Domestic downside

Domestication resulted in brassicas losing much of their ability to cope with resource-poor soils. Cultivated brassicas rely on huge artificial supplies of fertiliser and irrigation water. The resultant soft, thin foliage offers easy targets for pests and pathogens. And global warming is encouraging the quick spread of some particularly damaging parasites worldwide, such as Plutella xylostella (diamondback moth caterpillars).

Reviving brassicas' frugal use of nutrients and water is one longer-term way of developing more sustainable pest and disease resistance. Breeding for more effective use of resources makes good commercial sense as fertiliser and water costs rocket. It also makes medical sense by enhancing brassicas' benefits to human health.

Currently, brassicas have high nitrogen demand and rapid uptake into their tissues. Roots grow fast and penetrate deeply - taking up far more nitrogen than, for example, carrot or leek crops. Uptake of soil nitrogen continues throughout brassica growth and is reflected in large amounts present in crop residues. Rapid root expansion allows the absorption of lots of soil moisture, hence brassicas have a high demand for irrigation water. Cauliflower roots, for example, penetrate 70cm into the soil, comprehensively filling the profile, and cabbage roots will grow at 1.2mm to 1.5mm per day as compared with 0.2mm for onion and leek.

Rather than providing fertilisers and water ad lib and thereby encouraging susceptibility to pests and pathogens, brassica roots should be encouraged to search. Searching is a primitive characteristic that plant breeders can exploit much more effectively. Recently evolved over-reliance on artificial nutrient and water supplies should be curbed, as suggested by Kristian Thorup-Kristensen from Aarhus University, Denmark, during the recent Brassica 2008 symposium in Norway.

Resistance sources

Crop cultivars possessing pest and disease resistance are the simplest and most environmentally friendly means of crop protection. They place no stress on the environment, operating by simply blocking off the growth and reproduction of pests and pathogens. The current loss of pesticides adds further edge to the need for resistant cultivars.

Early brassica breeders sought resistance from genetic variation within each crop species. Resistance of this type, when available, tends to be short lived. Pests and pathogens evolve strains capable of overcoming it. Resistance is best obtained from wild types allied to domestic brassicas or from hybrids across species. This research is very long-term and demands access to collections of wild brassicas.

The worldwide network of gene banks holds seed of wild relatives. Brassica relatives are kept at the Warwick HRI gene bank. Largely thanks to the dedication of former curator Dr David Astley and his staff, this bank holds 24,000 accessions, as described by Professor David Pink at the Norwegian meeting.

Pests and diseases that matter

Resistance to cabbage root fly (Delia spp) would be an enormous boon for British growers. Providing woven crop covers that exclude adult flies is an enormous and increasing expense.

Low to moderate resistance is present in cabbage cultivars but it is too weak for commercial use. Some wild Mediterranean brassica relatives contain strong root fly resistance. Botanically further afield, strong resistance is present in white mustard (Sinapis alba). This species, along with field pennycress (Thlaspi arvense) and honesty (Lunaria annua) also offer sources of flea beetle resistance because they manufacture chemicals unacceptable to this insect.

Thrips are causing increasing major losses in Europe, particularly to autumn and winter storage cabbage because of reduced head quality. Dr Roland Vorrips of Wageningen University, The Netherlands, has demonstrated that sugar content and tightness of the cabbage head are correlated with susceptibility by offering ample food and protection to the insects.

Diamondback moth is now a major world pest as increasingly warm winters permit its survival. Outside Europe, where GM crops are used it is controlled by introducing genes from the bacterium Bacillus thuringiensis (Bt genes), resulting in the formation of toxins that poison the insect. This resistance mechanism greatly reduces the worldwide use of environmentally damaging pesticides. In China, there are three million hectares of Chinese cabbage sprayed at regular intervals with insecticides controlling this pest. Using resistant cultivars carrying the Bt genes removes this hazard.

The production of large areas of oil seed rape has increased the risk of fungal diseases causing havoc in brassica vegetable crops. Resistance is known to exist in some wild species and ancient crops such as Portuguese kales for downy and powdery mildews, blackleg and ringspot. Breeding work is progressing steadily but a release of commercially acceptable resistant cultivars is still some way off.

Robust, well-managed resistance could survive for a considerable time. Resistance to Fusarium yellows, a soil borne disease, was developed in the US in the 1930s and is still effective.

Most recently, Syngenta has released cultivars of cauliflower and cabbage resistant to the soil-borne organism Plasmodiophora brassicae, which causes clubroot disease. Resistance is taken from B. rapa using clubroot-resistant Dutch stubble turnips bred in the 1950s. This exemplifies the time periods required for breeding disease-resistant vegetable cultivars.

Minimum periods of 15 to 20 years of research and development are needed for finding a resistance source, transferring this into commercial lines and then ensuring that the product is commercially attractive. Many good resistant cultivars have failed because of quality defects and hence lack of market acceptance. As an example of the time needed, Professor Graham King of Rothamsted Research described current basic studies of extra-nuclear gene regulation that might in 20 years control cauliflower disorders like bracting, riceness, apical blindness and axillary branching.

Managing resistance

Avoidance of "resistance breakdown" is vital and demands integrated management. Syngenta's new clubroot-resistant cultivars require this approach. A pyramid of husbandry and nutritional techniques reduces opportunities for the development of pest or pathogen races tolerant to the resistance.

Conserving clubroot resistance requires the regular use of lime maintaining pH 7.2 or greater, and fertilisers like calcium cyanamide and calcium nitrate. These promote soil conditions favouring the host plant and disadvantage the pathogen. Boron and anionic wetters added to starter fertilisers reduce the movement of Plasmodiophora brassicae to the root hairs. Alternatively, boronated fertilisers should be used.

Resistant cultivars should not be grown continually on one piece of land. Neither should they be used repeatedly on land containing high pathogen concentrations. Both practices encourage the development of physiological races capable of eroding resistance genes. Where insect resistance is available it is wise to grow reservoir strips of non-resistant cultivars. This encourages the multiplication of natural predators of the pest itself.

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