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Diseases & Pests

 

We’ll concentrate on the diseases first, not least because they’re actually harder to diagnose than the pests which ravage your plants and are easily visible rampaging around. No, plant diseases are much more subtle and their stealth-like approach means it can often be too late, or at best difficult, to repel them once they take hold.

But it’s not all bad news, you’ll be pleased to hear! Although the garden disease line-up sounds like a rogue’s gallery – step forward bacteria, viruses and fungi – with a bit of preventative care and general good husbandry, keeping on top of common diseases is relatively simple. Strong, healthy plants are far less likely to succumb than weak, over-fed and sappy specimens; practicing crop rotation will help prevent soil-borne problems from taking a foothold and there are many disease resistant varieties of vegetables on the market.            

So who exactly is the vegetable gardener up against, disease-wise? Here are some examples of the most common foes you’ll meet, but don’t go thinking this is a definitive list – far from it! But for now, let’s get to know the enemy...

Downy Mildew - Fungus

Symptoms & how to recognise
Develops in damp conditions and thrives on young plants, causing leaves to die. Initial symptoms similar to powdery mildew, but this is more prevalent in dry conditions – downy likes the wet. Look out for furry grey growth which appears on the upper surfaces of leaves. 

Treatment
Remove and destroy infected leaves. If growing under glass, make sure the greenhouse is well ventilated and don’t water from above. Outside, keep the air circulating between plants with generous spacing and weed control. Get infected leaves off quickly and burn them and water the soil, not the plants.

Any other info...
Certain strains affect specific plants (lettuce, onion, spinach and sprouts) and the disease will also target ornamentals and other vegetables. There are plenty of resistant vegetable cultivars around so go for these if possible.

Powdery Mildew - Fungus

Symptoms & how to recognise
Powdery version also affects leaves but will also affect fruits which split and crack. Plants suffering from the disease will show a powdery white fungal growth on the upper leaf surfaces.

Treatment
The fungal spores enjoy humid and stagnant air, so try and avoid these conditions by watering the soil and not the leaves. If any leaves begin to show symptoms of the disease, get rid of them straightaway.

Any other info...
It affects many garden plants – both vegetable and ornamental - with brussels sprouts, cucumbers, courgettes, strawberries and other soft fruits particularly at risk. Make sure you don’t compost diseased plant material – burn it!

Clubroot - Bacteria

Symptoms & how to recognise
Not something you want to find in your garden – ever. Soil-borne Clubroot affects root crops along with brassicas and the roots of infected plants are grossly swollen and distorted. Up top, foliage is pale and washed out and plants readily wilt – they may even die.

Treatment
Unfortunately, there’s not much you can do. The bacteria can survive in the soil for up to 20 years and can be spread on tools, boots, compost – anything which comes into contact with the soil. The key is prevention – practice crop rotation, keep weeds down (some are a host for the disease) and be careful where you get your plants from.

Any other info...
Careful planning and gardening should keep this disease away.  You can still grow but you'll need raised beds.  There are some resistant varieties available. Acid and waterlogged soils are more likely to play host to this soil-borne nasty and try not to spread it by cleaning boots and equipment.   

Blight - Fungus

Symptoms & how to recognise
You’ve probably already heard of blight – it’s the disease that caused the Irish potato famine, but what you might not know is that it affects tomatoes (grown outdoors) as well. Both crops show brown lesions on leaf tips and fruits, accompanied by white, fluffy fungus. Potatoes suffer more than tomatoes, the entire top of the plant can collapse. Finding a blight infected potato tuber isn’t much fun either – it’s a slimy wet mush!  

Treatment
You’re really in the hands of mother nature on this one. Blight spores need very specific weather conditions to germinate; suffice to say humid, damp and warm weather over two consecutive 24-hour periods is most likely to trigger off the disease. Prepare for every eventuality by planting potato tubers deeply and earthing up; if the worst comes to the worst, cut down infected growth (or haulms) and burn them.

Any other info...
There are blight checkers available, which will tell you if your plants are in the early stages of the disease. Local radio stations may warn of ‘Beaumont’ periods, when weather conditions are spot-on for outbreaks, and some blight resistant cultivars are also available. Watch out for similar symptoms on outdoor tomatoes – they belong to the same family as potatoes so are equally at risk.  

Mosaic Virus

Symptoms & how to recognise
Although often referred to as cucumber mosaic virus, this nasty disease will attack courgettes, aubergines and a list of ornamental plants as long as your arm! The distinctive yellow mosaic pattern appears on the leaves of infected plants and flowering practically stops – not what you need when you’re trying to grow edible veg. In fact, any fruits which are produced will be small, dark, hard and inedible – nice!

Treatment
There’s not a lot you can do once that mosaic appears. Again, it’s all down to prevention and viruses are transmitted by aphids – a lot more of which later – so keep these pests down to minimum, especially in the greenhouse. Keep a close watch on weeds and don’t grow susceptible veg plants near any flowering ornamentals to be safe. Burn infected plants when doing so, and don’t handle healthy specimens without washing hands or gloves first.    

Any other info...
As with most vegetables, resistant varieties of cucumber, marrow, squash and aubergines are readily available. Dahlias, delphiniums and primulas are often hosts of the virus and although aphid control is more difficult to enforce outdoors, these critters really are the ones to watch. As viruses are transmitted in sap, be aware of using the same knife or secateurs when cutting or removing diseased material.  

Blossom End Rot - Deficiency

Symptoms & how to recognise
Not quite a disease, more a disorder but something you’ll almost inevitably come across if you grow tomatoes. Blossom end rot is the unsightly brown patch which develops at the base of the fruit as it ripens. Rather confusingly, it can develop on individual fruits on a truss, leaving the others perfectly healthy!  

Treatment
It’s a lack of calcium which leads to blossom end rot in tomatoes, and the main reason for this is a lack of water around the plants’ root zone. Acidic soils exacerbate the condition but removing the affected fruits and improving irrigation is often enough to combat the problem.

Any other info... 

As we said earlier, this is not an exhaustive guide and there are many other weird and not-so-wonderful ailments which can afflict your plants. On the whole though, vegetables are a hardy lot and prevention is certainly the best cure when it comes to diseases. Here’s a quick recap of the first steps to creating a disease-free garden;

  • Crop Rotation - growing the same crops, in the same place, year after year is a real recipe for disaster (find out more on Crop Rotation)
  • Healthy Plants; grow your plants ‘hard’, not soft and lush. This gives them a greater natural resistance to disease and will help fend off pests, which, as we’ve seen above, are virus carriers
  • Healthy Soil; standing in waterlogged or parched soil puts plants under stress and weakens them, so caring for your soil is essential. Water the plants when they need it and dig in plenty of organic matter to help drainage
  • Feed Properly; feed your plants as directed. Giving them unnecessary, and unbalanced, nutrients – especially nitrogen – will only lead to the soft growth pests just love sinking their mouthparts into
  • Sow Seed; sowing your own gives you the peace of mind that you‘re not bringing infected plants into the garden. If you do buy young vegetable plants, make sure they’re from a reputable source
  • Weed Regularly; weeds are unsightly, they compete for water and nutrients and they harbour pests and diseases. They’re fit only for the compost heap (before they set seed) so get rid of them
  • Clear Up; old plant material and organic debris lying around the garden sees pests and diseases rubbing their hands in glee. This ‘garden litter’ is the perfect host for diseases which can easily spread to plants, so clear up after you!
  • Mix and Match; planting your vegetables amongst ornamentals (obviously try to avoid the situations listed in the table above) to help slow the spread of diseases. Make sure you know what you’re doing before you try this though!
  • Clean your Tools; certain diseases can survive in soil on tools, boots and secateurs blades so clean these regularly, especially if you think clubroot may be a problem

And if you follow that little lot, you shouldn’t have too many problems! Although good husbandry is important, don’t get too hung up on the risk of disease; as long as plants are strong, healthy and well-cared for, they are likely to withstand most outbreaks – and there’s always resistant varieties to fall back on. Pests are another matter...

Common Garden Pests  – A Quick Guide

There’s no point lying about it – the vegetables you’re attempting to grow for your own consumption are also much coveted by the millions of pests which exist in your garden. From the smallest whitefly and aphids right up to pigeons, rabbit and even deer – all are out for a piece of the action and if you don’t put the brakes on them, you won’t have a crop to harvest! If you can see the bigger picture, you can’t help but be fascinated by the way the different ecosystems and food chains interact; but for now it’s the smaller image of your garden we’re concentrating on, and what – and who – is hungrily licking their lips.

Let’s meet some of your new neighbours...

Aphids

What they do and how to recognise them
Sap-sucking mainstays of the garden pest world and a.k.a greenfly and blackfly. With hundreds of species of aphids, there are very few garden plants or vegetables which escape their unwanted attention. As they suck sap from plants, viruses are transmitted and moulds develop on the honeydew they excrete. Aphids can be green, black, red, pink and various other colours and are roughly 2mm long. An idea what you’re up against – a single aphid could in theory produce ten million tonnes of offspring in 100 days of summer breeding!

Treatment
Luckily, aphids have many natural predators. The humble ladybird is a much loved companion of the gardener, due to its fondness for aphids. Both adult ladybirds (adalia bipunctata) and larvae love nothing more than dining out on this prevalent pest, and are capable of consuming large numbers each day. Lacewings and hoverfly larvae also dine out at the aphid cafe along with various beetles. Attracting pest predators such as these could turn out to be the best thing you do in the garden – your own little army fighting back!

Any other info...
It’s fair to say aphids are unpopular, and that’s the reason for the wide range of sprays on the market. Pyrethrum, fatty acid and plant/fish oil based organic sprays cover the aphids, block their breathing holes and they suffocate – other control methods include companion planting (to encourage predators), keeping plants healthy and not too soft and green and cultural tricks like pinching out broad bean tops before the aphids arrive. There are also many non-organic sprays available.

Whitefly

What they do and how to recognise them
The scourge of greenhouse gardeners, the presence of whitefly is indicated by white scales on the underside of the leaves and quite often, a white cloud of flies will take to the air if an infested plant is touched. Affected plants can develop viruses and sooty mould. Adult whitefly are 2mm long small insects with white wings, and they lay eggs on the underside of leaves.

Treatment
Whitefly will recoil in horror at the mention of a tiny parasitic wasp named encarsia formosa. The wasp will lay its eggs directly into the whitefly scale; the eggs then hatch and feed on the pupating whitefly inside, turning the scale black before the adult encarsia emerges. Lacewings, ladybird and even a few small birds are all partial to the scales so help is at hand!

Any other info...
Yes there is! Hanging sticky traps in the greenhouse will snare adults on the wing; regularly applying fatty acid or plant oil sprays doesn’t do them any favours and again, many chemical controls are on the market. If you are using predators, don’t spray anything else or put up traps – you’ll kill them too!

Slugs & Snails

What they do and how to recognise them
Yes there is! Hanging sticky traps in the greenhouse will snare adults on the wing; regularly applying fatty acid or plant oil sprays doesn’t do them any favours and again, many chemical controls are on the market. If you are using predators, don’t spray anything else or put up traps – you’ll kill them too!

Treatment
The nematode phasmarhabditis hermaphrodita likes slugs as much as gardeners hate them! The nematodes will actively seek out the slugs below ground, enter their body and spread bacteria, which causes the slug to stop feeding and perish. The good news doesn’t end there – frogs, toads, hedgehogs and birds are all partial to slugs and snails and will happily devour them all night long – and there are plenty of traps about to catch these A-list pests!

Any other info...
You’ll never rid the garden of slugs completely but you can reduce the population significantly with the various traps, nematodes and natural predators around. Supplement their efforts with organic slug pellets, resistant potato cultivars and you own moonlit sorties to remove them by hand. Chemical slug pellets should be a last resort; they are harmful to pets and children and remain in the food chain, affecting all manner of creatures

Caterpillars

What they do and how to recognise them
The larvae of various moths and butterflies, caterpillars cause extensive damage to both vegetables and tree fruit. The cabbage white larvae in particular is infamous for decimating brassica plants, whilst the codling moth caterpillar has no qualms about ruining entire apple crops on single trees or throughout orchards.

Treatment
Midnight forays with your torch are a good way to keep down caterpillar numbers – squash and offenders you find and destroy egg clusters as well. Along with the nematodes, you can apply various organic contact sprays which see off these pests, and there are some stronger chemical alternatives available as well.

Any other info...
Midnight forays with your torch are a good way to keep down caterpillar numbers – squash and offenders you find and destroy egg clusters as well. Along with the nematodes, you can apply various organic contact sprays which see off these pests, and there are some stronger chemical alternatives available as well. 

Unsure of a particular garden pest and need some mroe help please email our pest control expert Julian Ives.

Meet the Author: Jo Blackwell
Jo  Blackwell

Jo Blackwell is new on the Harrod Horticultural block and has recently taken over her post as Horticultural Advisor and Kitchen Gardener in Stephanie's Kitchen Garden. She caught the gardening bug when she bought her first home 18 years ago.  Her first greenhouse soon followed and she later gained an allotment, where she grows her own organic fruit and vegetables.

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