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Cambridge Allotments 2000


Pests and Diseases

By Helen Harwood

A draft for our booklet

Comments please to

I’d like to begin this article on a note of encouragement. Namely, that though all we gardeners suffer from pests and diseases, they’re not really a huge problem. Most years, I grow about 24 sorts of vegetable and 8 sorts of fruit crops. Out of these, I would expect 3 or 4 to have problems. This means that we always get a lot of fruit and vegetables, even if the odd one fails.

So be encouraged! You can’t go that far wrong.


It’s good to examine your expectations when considering pests and diseases. You have to consider what you can control and what you can’t.

For example, in a dry season, many crops will show signs of distress. This is just a fact of gardening life - you can’t control the weather, so there is a limit to what you can do.

How much work are you prepared to put in? Again, in a dry season, you can water plants. But unless you do it reliably and regularly, it’s a waste of time.

Many garden problems can be dealt with if you are prepared to put the time in, but if you are busy, maybe you just have to accept them.

Finally, can you cope with slightly marred produce? If you don’t mind maggots in apples, or whitefly or a few holes in your cabbages, then you can accept these things instead of trying to wipe them out. Essentially, your commitment in terms of time and energy will determine how much you want to do about pests and diseases. Often, the simplest thing is to ignore the things which aren’t serious, and to ruthlessly dig up and burn plants which are seriously affected.

Ground Rules


I would maintain that the right plant in the right place, adequately nourished and watered, will survive. This means that your energy should be focussed on healthy growing, rather than on "pests and diseases".

The mycorrhizal relationship

The early organic movement focussed their attention on the fact that, in a healthy soil, the roots form an association with certain fungi that are able to draw nourishment from humus. That nourishment is then available for the plant to take up. This means that growing is more than just plant roots in soil - it includes fungi too. So when we organic gardeners use compost, manure and other organic materials in our soil, we are nourishing both plant and fungi. This is how our plants are made healthy.

If you want to read more about this, try these books, which you can order from the library: Lady Eve Balfour - "The living soil", Albert Howard - "An agricultural testament"

Soil nourishment and type

You will gather, then, that if you feed your soil with compost, or manure or other organic matter, it will become more and more healthy, and your plants will be healthy.

You also need to be aware of your soil type; acid or alkaline; clay, loam or sandy. There are certain crops which do not thrive in certain soils - these individual preferences can easily be checked in a vegetable growing guide.


Some plots have full sun, some sun part of the day, some are shady. Some sites are windy. These factors will affect your crops. For example, lettuces in full summer sun bolt more quickly. Onions like the sun, while parsnips and turnips will tolerate some shade. Brussels sprouts in a windy place need staking or they will blow about. Brassicas need liming to prosper on an acid soil.

To sum up, examine you plot and its soil, and note where the sun falls, and where it’s shady, and plant accordingly.


Some plants need a lot of moisture, some don’t. Very few will prosper in a real drought. Again, it pays to know your soil - whether it dries out quickly or not. The more you dig a soil, the more likely it is to dry out, so that, where dryness is a problem, it may be best to avoid digging. You can retain moisture by applying a mulch to the soil. This acts like a blanket to prevent evaporation.


Cambridge has a naturally dry climate. We often go for periods of weeks when it doesn’t rain at all. It can therefore pay to exploit the rain when it comes. For example, if you have plants to transplant, do it immediately after a rainstorm. Don’t be tempted to wait. It can also pay to cover new transplants with fleece for a week or so in the summer, to protect from hot sun. This will save them from undue stress and allow them to root.

Above all, know what sort of season it is, and adapt accordingly. If there has been a lot of rain, slugs and snails will prosper. It will pay to go round lifting any cover in the garden (bricks, wood, carpet) and collect and destroy them. In a very damp and humid season you can anticipate potato blight (which also affects tomatoes). Keep an eye out for it, and deal with the first signs. Similarly, in a dry season, onions are more likely to get white rot.

There is very little you can do if the season is too wet or too dry except learn the problems you may encounter and deal with them if you can.


Not all wildlife is good news but a good balance of goodies and baddies can even itself out to some extent. You may get mice, rats, deer, hares, squirrels, wood pigeons (and other things too) which are more of a problem- other gardeners on your allotment site or in nearby gardens will know what the local menace is. For rabbits, hares and deer, you will need to net you crops.

Good Husbandry


This is a fundamental tenet of organic gardening. If you don’t grow the same plant on the same spot every year, you are less likely to get disease. It means a bit of planning before you plant. Eventually, you need to consider the different families of plants you are growing, and group them together. You then divide your plot into 3 or 4, and put one plant family on each section. The next year, you move the plants round, so that each replaces its neighbour, and the same the following year, so that it is at least three years before each crop (potatoes, for example) return to where they were grown first. It is well worth studying the subject of rotation in one of the organic gardening books.


Don’t compost diseased plants - burn them if you can. This breaks the chain of infection. Inspect your plants frequently. If you notice a diseased plant, take it out and destroy it. This may stop the pest or disease spreading. Plants under stress are the most likely to suffer from pests and diseases - you will often see only one broad bean plant in a row has blackfly, the runt of the row - so get rid of it. Try not to let your plants suffer, for example by transplanting when it is very hot. They are more likely to get attacked.

Tour of inspection

I always think this is a good idea. You can dawdle round your plot, admiring your crops and congratulating yourself. If you see a problem early, you are more likely to get on top of it. Learn the predictable things that happen at various times of year: slugs attack seedlings in March and April - look for them and remove them, cabbage white butterflies lay eggs from July to September-pick off the caterpillars as they grow.

Too much and too little

Not everything likes a lot of nitrogen (manure is rich in nitrogen). Too much makes onions store badly. Too much manure on winter brassicas makes them sappy and vulnerable to frost. Too much on carrots and parsnips makes the root fork. Learn which crops like lots of manure, which like compost, which like lime, which hate it.

Some "diseases" are actually problems of deficiency or overdose, such as chlorosis. This makes the leaves pale in colour and means that the plant needs nitrogen (or has a mineral deficiency).

Resistant Varieties

Some plant diseases are so predictable that vegetable varieties have been bred for resistance. For example, the potato variety Pentland Crown has good resistance to blight, whereas Pink Fir Apple variety does not. Parsnip Tender and True has good resistance to canker, White King less so. It is worth picking varieties with resistance when choosing seeds from the catalogue.

You’ve got a problem

Where should you start?

Have a really good look at the plant. Turn leaves over to look for insects on the bottom side of leaves. Look for insects deep in the heart of cabbages or cauliflowers. Gain as much evidence as you can.


You need a good book. I find Hessayon’s "vegetable expert" and "Fruit expert" very good and cheap. They have pictures for identification, though they are not organic in their remedies. Also good is the RHS "encyclopaedia of gardening" or their pests and diseases volume. Or look in the organic gardening section in the bookshops. Ask your neighbours - they might have the same problem.


I find it useful to consider why I might have the problem. For example, is it a dry year? If so, they plant is of course under stress. Did I overmanure that plot? That means the onions are very green and sappy. Because it’s been raining a lot, there are lots of fungi about – that’s why the onions have downy mildew. I’ll be more careful next year.

General solutions

I’m a great believer in ‘encouragement’, so I often spray a sick plant with a feed of liquid seaweed. No-one seems quite sure what seaweed feed does, but it seems to be a sort of tonic, and it makes plants grow better. Sometimes it is enough to strengthen a plant against disease. Always try it, I’m surprised how often it works.

My other tonic is dried blood. It is a nitrogen fertiliser, and will give a lift to plants that have been suffering from slug or flea beetle damage. Try to remove diseased material. See if you can limit its spread. This too works surprisingly often.

After this you have 2 choices - give up, or adopt a remedy from one of the organic gardening books

My list of the most usual problems

These are the most common problems I’ve encountered.

Potatoes Potato blight (wet years), frost
Onions White rot (dry years), Downy mildew (wet years)
Peas Pea moth (caterpillars in pea at certain times of summer), mice, rabbits
Brassicas Whitefly, caterpillars, any year, usually second half of summer
Strawberries Botrytis (grey furry mould, wet years)
Raspberries Raspberry beetle caterpillars in fruit- later in summer, any year
Apples and plums Brown rot (wet years)
Parsnips Canker
Leeks/chives Rust (dry years)
Asparagus Asparagus beetle
Artichokes (globe) Blackfly
Broad beans Blackfly, chocolate spot (wet years)
Carrot Carrot fly (every year)
Courgettes Cucumber Mosaic Virus
Seedlings slugs and mice
Roses Blackspot (endemic on old-fashioned varieties)
Seedling in pots Sciarid fly (fungus gnats), damping off (fungus)
Tomatoes/peppers Tobacco mosaic virus stunted plants (In the greenhouse, if you smoke)

The incurables

There are three really bad problems:

white rot of onions
clubroot of brassicas
eelworm of potatoes

All of these are considered major problems, but even these can be managed. HDRA, in particular, can give advice on what to do. So never give up!!


The important thing about pests and diseases is that, the more you garden, the more you will become your own expert. There are lots of ways of avoiding trouble, and of dealing with it when it occurs. You will learn more and more as you go on. But I will repeat once again my firm belief that no matter how inexperienced you are, you will always get some food from your plot, so concentrate on your successes and don’t let the pests grind you down!

Helen Harwood Jan 2000

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Latest update 03/03/00