GARDENING / Unhealthy attitudes: Some gardeners spend all their time worrying about diseases. Anna Pavord explains why this is madness

Hungry Joe in Joseph Heller's Catch-22 collected lists of fatal diseases and arranged them in alphabetical order so he could quickly put his finger on the one he most wanted to worry about. Some gardeners are the same, not about themselves but about their plants. They see their roses only in terms of the presence or absence of black spot, their Michaelmas daisies only in relation to mildew.

At any hour, day or night, they can reel off a list, memorable only for its ghastliness, of all the diseases rampant in their garden, and an even longer list of the toxic substances they are deploying in pursuit of their ultimate goal: a garden in which no leaf has a blemish and no fruit a spot of mould.

This is madness. The world is an imperfect place, and the only sane way forward in the garden (as with anything else you love) is to accept its occasional imperfections. By and large, with plants as well as people, contentment and health go hand in hand. Plants will be less likely to cough and spit if they are growing in the right kind of soil, with just enough sunshine and shade, and the right amount of food and drink permanently at hand.

This requires forethought. It means some swotting up before you plant. Matching plants to positions is the key to good gardening, but the plants also need a say in the business.

The second thing a gardener can do is choose plants with known resistance to disease. This is becoming easier. In the past, breeders, especially of roses, concentrated on flower size and colour to the detriment of the plant's overall health and vigour. Now, at last, the balance is being redressed. The difficulty sometimes lies in knowing which varieties are suspect and which are healthy.

A list of plants that have been judged worthy of an Award of Garden Merit (AGM) has recently been published by the Royal Horticultural Society and goes at least some way towards sorting the sheep from the goats. The criteria by which AGM plants are judged are wide. Disease-resistance is only one part of a package that includes general appearance, weather-resistance, flower colour and ease of cultivation. But it is unlikely that any plant on the AGM list is going to be a complete duffer in terms of health. It is at least a starting point.

Roses seem to cause the most sleepless nights among gardeners for whom black spot is a moral slur. Peter Beales' book Roses (Harvill, pounds 35 h/back, pounds 17.99 p/back) is honest about vices as well as virtues. In choosing a floribunda, for instance, you would need to have a very good reason - masochism, perhaps - to go for the soft yellow 'Charleston', which is known to be especially prone to the disease. The rich salmon-pink 'Elizabeth of Glamis' is similarly afflicted. So is 'Gold Bunny' and the velvety red 'Lilli Marlene'. One of the ironies of our concern with the environment is that the huge increase of black spot on roses and the consequent huge increase in the amount of fungicide being chucked about stem from the various Clean Air Acts of the Fifties. There was a lot of sulphur in the air then, especially in industrial areas, and it acted as a powerful deterrent to black spot.

Black spot is caused by a fungus called Diplocarpon rosae, which stretches in radiating strands under the cuticle of the leaf. As the disease progresses, the spots join up until quite large areas of the leaf are affected. Then the rest of the leaf turns yellow and it drops to the ground. The fungus does best in warm, moist summers and it spreads when rain splashes on to the foliage of roses, dispersing the spores on to fresh pastures.

The fungus winters on the stems of roses. Reproduction takes place by means of a special cell called the ascus. 'The ascospore- producing stage develops on fallen leaves in the US, but does not usually occur in the UK,' says the authoritative New RHS Dictionary of Gardening (Macmillan, pounds 550, four volumes).

There was much sucking of teeth at the Royal National Rose Society (RNRS) over that sentence, the point being that if the spores do not develop on fallen leaves, we are all wasting our time carefully picking them up and burning them, which is the usual advice on black spot. The RNRS believes fervently that the leaves do matter.

Roses of all kinds, not just floribundas, vary greatly in their resistance to black spot, but the fungus is good at adapting to different conditions and different kinds of rose. Only the oriental R. bracteata, 'The Macartney Rose', introduced from China in 1793, seems to have anything like immunity to the disease. But it (and its child, 'Mermaid') is slightly tender and gets cut down in cold winters. It also has vicious thorns but is an elegant rose: single pure- white flowers, each with a pronounced boss of golden stamens, very like 'Mermaid' in all except colour and vigour. It reaches only 8ft, and is happy in shade on a north wall.

The villain of the piece is Rosa foetida, a bright-yellow species native to the Middle East. Along with its colour, which has been an important element in rose breeding, it brought less welcome baggage to the party, a fatal susceptibility to black spot.

The flowers are large, single, a rich golden yellow, and appear in early June. Their genes survive in many modern hybrid tea roses, for the first-ever yellow HT rose, 'Soleil d'Or', raised in 1900, came about as the result of a cross between R. foetida and a red hybrid perpetual called 'Antoine Duchet'. This helps to explain why no mention of black spot appears in any of the late-Victorian gardening books that I have. It had not yet been introduced. And air was at its foulest then, packed with enough noxious substances to fell even the bravest Diplocarpon rosae.

To beat it you need a sprayer welded on where your right arm usually is. Spray heavily with Nimrod-T (ICI) after pruning, and continue at fortnightly intervals through the season. Drench through the winter with dilute Jeyes fluid (techically illegal, but widely used) or Armillatox.

If you want to take the other route and plant roses with good resistance to black spot (and mildew and rust, the rose's other great enemies), choose floribunda varieties such as 'Amber Queen', 'Arthur Bell', 'Mountbatten', rich apricot 'Southampton', or orange- red 'Trumpeter'.

Among climbers, the blood-red 'Dublin Bay' can usually see off most fungi, as can 'Golden Showers', which blooms continuously from June to October, the strong- growing 'Maigold', which has semi-double flowers, 'New Dawn' with pale flowers beautifully set off against glossy foliage, or 'Seagull' - the kind of vigorous cluster-flowered rose that needs a large-ish tree to get its teeth into.

But remember that resistance means resistance, not victory. Even the mightiest sometimes sneeze.