Pests and Diseases Workshop

The February workshop, presented by Teresa and Andrew Widd, focused on garden pests and diseases, with the emphasis on prevention. As Andrew pointed out, prevention is better than cure – and cheaper! However, prevention doesn’t always work so the latter half of the talk dealt with possible cures.

The workshop kicked off with the usual quiz when Teresa showed us a series of pictures and asked us if we could identify the various pests and diseases displayed. Some were easy (the grey squirrel), some were debatable (the blackbird who’s a welcome visitor – until he starts digging up your seedlings!) and some were difficult (Antirrhinum rust and the keeled slug). A few were impossible such the Rosemary beetle, which is a recent introduction to the UK and is now spreading North. If you fancy practicing your pest identification skills you may want to help out with the RHS survey looking into the beetle’s spread across the UK. Changing climate means that we can expect quite a few of such new nuisance arrivals in our gardens – RHS also has information on those potential new pests and diseases.

Once we’d guessed or bluffed our way through the quiz, Andrew took over and covered pests and diseases in more detail. We were delighted to learn about the many ways in which problems can be avoided before they begin. These include simple things such as good hygiene, e.g. cleaning your tools and pots, particularly if you work in more than one garden, keeping weeds down in the garden and, if you have a greenhouse, cleaning it out annually. Andrew also stressed the importance of correct composting, ie not composting diseased plants or peelings from potatoes with scab or parsnip with canker (crimes of which we’re probably all guilty).

Crop rotation is also a good way to avoid diseases. By not planting brassicas, for example, where you’ve grown them recently, you can avoid the build-up of brassica pests and diseases in the soil. Crop rotation was covered more fully in an earlier workshop, so check this out when you plan your 2016 garden. It may save you a crop!

Buying clean stock is another obvious way to avoid introducing pests and diseases into your garden so carefully check any plants before you buy and avoid any that show signs of infestation or infection.

Companion planting – this is a big topic, one that might usefully be covered by an entire workshop, but Andrew went over some of the better known plant combinations. In some cases the mechanism by which one plant can inhibit pests and diseases in another is understood. For example, some plants emit volatile chemicals which can inhibit fungal diseases. In other cases, a companion plant may attract pests away from a food crop. For example, Nasturtiums attract cabbage white butterflies and divert them from your cabbages etc (although this can have a down-side in that you may end up increasing the pest population for next year.) In many cases, however, the mechanism is unknown, but empirically companion planting seems to work, and here are a few of the better known combinations:

  • Onions and carrots; onions deter carrot root fly while carrots deter onion root fly – a marriage made in heaven!
  • Marigolds inhibit whitefly in tomatoes, so a good thing to grow in the greenhouse.
  • Garlic is known to inhibit a number of diseases, such as black spot and leaf curl.
  • Chives inhibit apple scab.
  • Brassicas grow well next to celery.
  • Fennel, however, shouldn’t be grown near dwarf beans, kohl rabi, caraway or tomatoes.
  • Peas and onions shouldn’t be grown together.
  • Nasturtiums and Hyssop attract cabbage whites (but see above).<
  • Foxgloves stimulate the growth of other plants.

This just gives a taste of the possibilities of companion planting. For more information, check out companion planting guides by Thompson&Morgan  or Gardener’s Calendar.

Grow healthy plants

It’s not sufficient to buy healthy plants; you have to keep up the good work by keeping them that way since a healthy plant is better able to withstand pests and diseases. So grow your plants in the right conditions, e.g. plant brassicas in firm soil, legumes in organic-rich soil. Protect them from wind but, on the other hand, make sure they’re well ventilated by growing at the right spacing, and pruning to encourage air-flow. Improve drainage if necessary and water correctly, neither too little nor too much.

Erect physical barriers

This technique is designed to combat pests rather than diseases, and ranges from wire mesh (to deter deer and rabbits), to fly paper which you can use to ‘catch’ flea beetles. One of the key factors of this approach is to understand your enemy. If you know something about the biology and life-cycle of whatever is threatening your precious crops you’re half-way to dealing with it. Some pests are active only at certain times so avoiding having a vulnerable crop at that time will go some way to preventing the problem. Other pests, particularly those of apples, crawl up the trunk so you can stop them with a grease or glue band. Yet others, such as the carrot root fly, don’t fly much over two feet above the ground so can theoretically be stopped by a fine mesh barrier more than this height. You can, of course, cover your plants completely and there are lots of materials on the market designed for this purpose, ranging from netting to fleece which, with the right mesh size, will keep out anything from birds, through butterflies and down to the tiny carrot root fly. Even snails and slugs can be deterred by physical barriers. Coarse grit may slow them down but they really don’t like copper so putting a copper strip around a favourite pot can protect a precious Hosta. There are also discs impregnated with copper that you can put around individual plants.


But supposing all these preventative measures don’t work and the pesky pests and diseases do get into your garden? Is there anything you can do that doesn’t involve spraying with nasty chemicals?

Yes, was the answer. There are indeed a number of alternative solutions, biological controls being an increasingly favoured method. Some biological controls occur naturally, such as the larvae and adults of the lacewing, the ladybird and the hoverfly, all of which gobble up aphids. Larger ‘biological controls’ include the thrush, toad and hedgehog, all of which just love to eat slugs and snails and, rather more controversially, the domestic cat who can keep your mouse population down and make serious inroads into the blackbird and pigeon population. So making your garden friendly towards these ‘good guys’ can help you keep pests and diseases at bay. But there are also commercial solutions to pest control that don’t involve chemicals. Nowadays you can buy a wide range of nematodes which parasitise some of the worst garden pests, including slugs, leatherjackets and the dreaded vine-weevil. The nematodes arrive in a powder form and are watered onto the soil where they multiply and get to work. You can even buy a mixture which claims to deal with a wide range of pests. However, it’s worth researching the life-cycle of your chosen target before applying the nematodes to check you’re applying them at the right time since they aren’t cheap and, once purchased, don’t keep for that long.

Physical solutions can also be used to deal with pests that get into your crops as long as the infestation is light. Butterfly eggs can be located on brassica leaves (look underneath) and squashed. If you miss some and they end up as caterpillars, these can be picked off, as can gooseberry saw-fly caterpillars, which tend to attack the lower leaves of gooseberries and jostaberries and are easy to see along the edges of the leaves. Traps can be made to capture slugs and snails (see this website on how to make your own), although commercial traps are available. This is a good way to use all that stale beer! Another physical trap which caused some controversy at the workshop is the syrup trap for wasps since for most the year the increasingly rare wasp is a real help in the garden, particularly as a predator of cabbage white caterpillars, but in autumn they can cause some damage to fruit. Earwigs aren’t much of a problem but if you’re growing prize dahlias it might be worth making an earwig trap out of an upturned pot, stuffed with straw which the earwigs crawl into and can be shaken out of later. Fly paper, mentioned above for flea-beetle, is another physical trap but may also catch beneficial insects.

However sometimes nothing works and you may have to resort to chemicals.  Andrew had some useful advice which should be followed if the worst comes to the worst, the most important of which is to read the label. So, use at the right time on the right crop and in the right concentration. Check your chosen chemical doesn’t kill beneficial insects. Alternate between chemicals. Consider using an ‘organic’ chemical.

Andrew finished up his talk by showing us some pictures of some of the major ‘nasties’, both pests and diseases. ‘Crawly’ pests included chafer grubs and leatherjackets, both of which attack grass roots, but can be dealt with by using nematodes. Vine weevil grubs, which attack many plants, can also be treated with nematodes. The leaf-cutting bee damages rose leaves but isn’t a serious pest. Aphids cause a number of diseases, some more serious than others. The currant blister aphid causes leaf damage in currants (hence the name) but this isn’t too disastrous for the plant. The predations of slugs and snails are well-known but there are a number of possible solutions such as nematodes, traps or organic slug pellets. Carrot root fly can be deterred by a physical barrier. The pea moth damages the peas inside pods. Cuckoo spit caused by the frog hopper is unsightly but although the insect sucks sap it isn’t considered a major pest. The mealy bug and red spider mites, however, can be serious pests of indoor and greenhouse plants. Fungal and bacterial diseases can be equally disastrous and we all recognised some of the common ones, such as scab on potato and canker on parsnips. Strawberry grey mould and gooseberry mildew were also familiar but can be prevented by ensuring good air circulation around the plants. Erratic watering or rainfall can give rise to a number of conditions which pests and diseases can take advantage of. For example fruit split in apples or tomatoes can lead on to mould. A similar physiological disorder, blossom end rot in tomatoes, is caused by water-logged roots.

We left the workshop having learned not only how to recognise a number of pests and diseases but how to deal with them, either before they arrive by prevention, or after they’ve turned up by cure. This is clearly a huge topic which can’t be covered in two hours, but a number of books on the subject are available and Andrew and Theresa recommended the RHS book on Pests and Diseases. RHS website also has a good collection of pest and diease fact sheets and Garden Organic provides useful advice on chemical-free controls and prevention.

And thank you to our baking volunteers for supplying the home-made goodies based on local ingredients – this time Janice delighted us with gluten-free goosberry meringue pies (goosberries from our very own Fruit Tree Walk!) and Kaska made some dairy-free Polish gingerbread cookies (with a secret ingredient of her bag-grown potatoes for extra fluffiness).

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