There's a growing interest in eating food that doesn't have spray residues and is nutri' />

Is there such thing as a fruit tree that doesn't need spraying?
There's a growing interest in eating food that doesn't have spray residues and is nutrient-dense. However, we tend to avoid fruit with any sort of blemish that may result after disease or insect attack. Can we have it both ways - good looking, healthy fruit and no sprays?
Plant and Food scientist Dr Vincent Bus leads a team at Plant and Food Research (PFR) looking at developing resistances to pests and diseases in pipfruit.
On one level explaining their work is easy - they expose cultivars to common diseases, the seedlings that either don't get sick or recover well are crossed with others.
In theory, trees could be developed with multiple resistances and growers need never spray again. Hoorah!
If only it were that easy.
Vincent said the crabapple was a common source of resistance genes; however the fruit wasn't great, so was crossed with better-tasting cultivars and the offspring tested for resistance and fruit quality.
"It takes four to five generations to cross the resistance back into eating apples - to increase size and quality - and that's the slow bit of resistance breeding," said Vincent.
Sometimes old cultivars with resistance are available. For example, Northern Spy apples have good resistance to woolly aphid; but most people now expect fruit of the quality of, say, Jazz or Pacific Rose.
The plant and food researchers cross Northern Spy with one of these new varieties hoping to achieve tasty fruit on a tree immune to this pest.
"We look for texture like Jazz and anything good is selected. For every 10,000 seedlings, we may get one cultivar."
Another trait they are breeding for is a red-fleshed apple.
The seedlings are inoculated with woolly aphid or scab. Survivors are grown out to see if the apples have good enough quality. It takes on average five years to get fruit from a tree, so it's not a job for the impatient.
The crosses come from the age-old practice of pollinating one flower with another, though today a technology called cisgenesis is available to transfer genes directly from one apple to another. Both methods are time-consuming, so there is no quick fix.
Defining resistance is also complicated. "We want durable resistance and only one gene for resistance in an apple isn't enough; the pathogen has a nasty habit of mutating and trying to overcome the resistance; and it happens quite readily.
"So we make resistance durable by putting several, say three, resistance genes for each pest or disease in the cultivar. For scab, we aim to have a pool of 10 genes at least, to have the variation to combine these genes and achieve durable resistance. You have to do it for scab, you have to do it for mildew and fire blight - although perhaps not necessarily 10 genes for the latter two diseases."
It's called pyramiding resistance genes, and you'll be getting the idea now of why it's such a slow process.
PFR works on the major worldwide apple diseases - including scab, mildew, fire blight and European canker. "Resistance breeding is regarded as the major alternative to chemical control, which currently is the predominant means to combat these diseases.
"We want our cultivars to be successful all over the world; so we focus on the major problems first and we are making progress."
Vincent says there is no useful resistance for brown rot, and stonefruit are highly susceptible to leaf curl.
There may come a day when PFR develops a great tasting apple on a tree resistant to all the major diseases and growers need never spray again. But in the meantime for people who weren't going to spray, Vincent said some old as well as very new resistant cultivars were available. "A major effort has gone into resistance breeding over the years," he said.