Beat Pests and Diseases With Good Garden Hygiene

, written by Jeremy Dore gb flag

Washing a trowel

I recently returned home after camping with my family to find four of my tomato plants had been attacked by blight while I was away.  All the symptoms were there – blotches on the leaves and stems, fruit turning brown and sections of the plant dying.  My first concern was how to prevent it spreading to my many other tomatoes and to keep it from affecting next year’s crop.  Understanding how plant diseases and pests spread is central to this as may plant diseases and pests can survive over winter in a garden, ready to attack young seedlings the following year.

The Usual Suspects

The following pests and diseases all commonly survive in gardens and need dealing with efficiently to prevent their spread:

  • Blight: One of the worst diseases of potatoes and tomatoes, blight is spread by spores blown from one area to another.  Infected plants left on the soil from a previous year or the tubers of infected potatoes in the soil can harbour the disease which then spreads rapidly during warm, moist conditions.  So, disposing of all blight affected plants is essential.  In the case of my blight-attacked tomatoes I seem to have been able to remove the problem before it transferred to other healthy plants in the garden and greenhouse but it often spreads before the problem is spotted.
  • Downy Mildew: Most mildews (which generally affect squash and vine plants) survive in plant debris through winter.  Clearing away all old plants is particularly important, including any diseased leaves on the soil.
  • Viruses: Diseased plants and some weeds can retain virus infections such as cucumber mosaic virus (which is not limited to cucumbers but also affects other squash plants, lettuce and spinach) and infected plants must be destroyed.  Failure to do this makes it likely that the virus will then be spread by aphids when they suck the sap from plant stems.
  • Cabbage root fly and carrot root fly: Common winged insect pests like root fly survive as larvae in the soil around the plant.  Burning infected plants and moving the crops to new areas for at least 3 years is advised combined with the usual protection of these plants using root collars for brassicas and fine mesh for carrots.
  • Aphids: Usually aphids overwinter as eggs on a perennial plant, typically trees.  However, brassicas such as cabbages and broccoli are often grown through the winter and aphids can lay eggs in stalks left in the ground.  It is important to harvest these plants or pull them up before the aphids produce their next generation in late spring.
  • Root aphids: Other aphids attack the roots of plants such as lettuce and these have been found to overwinter in soil so if this has been a problem it is wise to avoid growing the same plant in that space next year.
  • Slugs and snails: These love all ‘nooks and crannies’ and in particular stones, wood and under sheet mulch.  Removing or checking these sanctuaries will reduce the problem of controlling slugs the following year.
Aphid on lettuce

Eliminate Everything?

From this list it sounds as if the best idea would be to completely level the entire vegetable garden over winter, removing everything and leaving just bare soil.  I know gardeners who do this but there are problems with this approach:

  • Beneficial insects need homes over winter too.  I once started cutting back a low hedge in my vegetable garden during winter, only to find that I was destroying the habitat for a number of ladybirds which were sheltering there.  I now make sure that I leave some good places to encourage insects like them to stay.  After all, they are going to be the best defence I have against aphids the next year.
  • Bare soil gets eroded over winter if exposed to harsh rain, snow and wind.  So this is a good opportunity to sow green manures or add a layer of mulch.  Some gardeners cover all dormant areas with black plastic which really helps eliminate weeds but can also harbour slugs and produce a lot of water run-off if it’s not permeable.
Jeremy in his greenhouse

The Best Advice

Garden hygiene isn’t the only method used to control pests. Equally important are crop rotation, encouraging beneficial insects and feeding the soil well.  However, the following principles are a good checklist to follow wherever possible:

  • Remove all diseased plant material as soon as it is spotted.  This should only be composted if you are using very hot composting techniques, otherwise it should be disposed of in a bin or by deep burying.
  • Clean all tools and hands immediately after handling infected plant material to ensure that you are not spreading disease around the garden yourself.
  • Be wary of spreading soil from areas that have suffered from problems such as clubroot, root aphids etc.  Cleaning footwear may be necessary.
  • Keep plants cut back and well spaced so that air can circulate well.  Damp areas with little ventilation help fungal diseases, moulds and many others to survive.  Our Garden Planning Tool can help get the spacing right to prevent overcrowding.
  • Clean greenhouses, cold frames and cloches (row covers) well each year.

Cleaning up at the end of the season is certainly not my favourite activity and it can easily seem like just another chore to add to the list of jobs.  Instead I try to see it as part of the very early preparation for next year: ensuring that the garden is in perfect condition for starting plants in spring and trusting that the rewards next year will be well worth it. 

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Comments

 
"Hello, Jeremy. Thank you for the informative post, as always. I had been wondering whether I should be composting leaves and stems of plants that were spent or completely discarding them. Would tomato blight manifest itself as browning of the leaves toward the bottoms of the plants? So far, the fruit does not show any ill effects. Could something else cause this? Only three of my plants are affected. Also, for the first time this year I used straw as a mulch, which has worked well, I think. Should I lightly till this into the soil in the fall or leave it on the surface until spring? I am in northern Illinois in the US. Thanks again for your terrific site."
Geri on Friday 4 September 2009
"Jeremy - Thanks again for a great post on an important sibject. I was glad to see we are in agreemenet about composting sick or diseased plants. At-home composting sytstems are just too risky so why take the chance! This is one place where better safe than sorry is important. I also just wrote a post on composting diseased plants. Here in AZ we are getting ready for another full gardening season! We seem to never stop gardening so good hygeine is really important as we don't get the cold frosts to kill disease or pests. I appreciate the sound tips and advice! The Garden Goddess at Down 2 Earth Gardens"
Doreen Pollack on Friday 4 September 2009
"@Geri, The way I tell whether it is blight or not is if the stem of the plant is also showing brown/dark patches. Other diseases don't seem to do this, so it is a good indicator. The bottom leaves browning can just be a lack of the right nutrients or the plant starting to reach the end of it's life this season, so it may well not be blight. However, I would advise checking out some pictures of blight-attacked plants to be sure such as these: http://www.hort.cornell.edu/department/Facilities/lihrec/vegpath/photos/lateblight_tomato.htm. To answer your second question, I would wait until Spring with the straw as it will provide a good soil cover. Then till it into the soil if it is a small amount but not if it is a thick layer - I'd compost that."
Jeremy Dore on Friday 4 September 2009
"@Doreen, yes, I agree about it being too risky to home-compost diseased plants. I guess I should have mentioned that it's fine to send diseased plant material to a municipal recycling scheme as there they get the compost really hot enough to kill everything harmful. However, I woudl only do this if it doesn't mean leaving the composted material around waiting for it to be collected as spores and viruses can easily spread from old plant material to good plants."
Jeremy Dore on Friday 4 September 2009
"As long as you can raise the temperature of your finished compost to above 120 degrees F for a few hours, any late blight spores will be killed. I place mine in black plastic pots enclosed in clear plastic bags on a hot sunny day. Geri, the disease that most often takes the lowest tomato leaves is early blight. It usually stops a little less than halfway up the plants. By this time of years, many plants show several diseases at once, but it's no big deal because the crop is about done anyway. You can till in your old mulch or compost it, but do cover the soil with something during the winter months. Good luck! "
Barbara Pleasant on Saturday 5 September 2009
"Thanks to all! My plants do not exhibit the browning of the stems, just some die-back of the lower leaves, and the fruit is still ripening normally. By the way, if you haven't grown Striped Romans, give them a try. What a tasty and attractive tomato!"
Geri on Saturday 5 September 2009
"I ought to mention that while my general principle is to removed diseased plants as soon as they are seen, there is sometimes a case for leaving them. As Barbara pointed out, when a harvest is nearly ready it can be best to leave the plant to finish producing. With my badly-affected tomato plants they were too far gone for this but I now have a little blight on some other plants. However, the fruit look good and are still ripening, so I will leave them for a couple of weeks and perhaps ripen the last part of the harvest on a windowsill indoors."
Jeremy Dore on Sunday 6 September 2009
"Hi Jeremy Thanks again for your useful blogs. I too have trouble with tomatoe blight. It has affected my plants for the last 3 years - particularly the larger varieties. I even tried growing some in a growbag last year but to no avail. The spores would appear to be airborne? certainly the problem is endemic on our allotment site. I am also experiencing a problem with ?aphid? pests on my blackcurrant bushes. The leaves have all curled. Should I be worried? These bushes have so far yielded huge quatities of blackcurrants and I should hate to lose them. My redcurrant bushes seem to be unaffected. Any suggestions please? Ann"
Ann Pannett on Wednesday 9 September 2009
"@Ann, Yes, the spores of blight are airbourne and spread during warm damp weather. If you type 'blight' into the search box on our site you will find we have an article all about this. As for the blackcurrants, aphids do migrate back to them in Autumn, although they rarely affect the fruiting ability - see http://www.rothamsted.ac.uk/insect-survey/STHyperomyzus_lactucae.php for details. I would recommend squashing them by hand (or some people prefer to use a vacuum cleaner!) and then encouraging beneficial insects to keep their numbers down in spring - see our GrowGuide article on Natural Pest Control."
Jeremy Dore on Thursday 10 September 2009
"Hi Jeremy, I have had tomato blight in my polytunnel this year. I have removed all the diseased plant material but do the spores survive in the soil? The bed is too big to replace the soil and I want to grow tomatoes again next year."
Carol Chamberlin on Friday 18 September 2009
"In theory, late blight spores don't overwinter in soil unless two types A1 and A2 mate, which is said to be very rare in the UK. Instead, spores usually spread through the remnants of infected plants or potato tubers which is why the clean-up operation after an infection is so important, along with pulling up and destroying any potato and tomato seedlings that appear where a blight-infected crop was grown last year. A good period of freezing temperatures is also said to help rid an area of blight spores, although in a polytunnel this is less likely to happen. So the best advice is to ensure that every last bit of infected plant material is removed and rotate your crop to a new area each year. Replacing the soil is not necessary unless you keep getting repeat infections in which case I would add an inch or more of fresh sterilised compost (this is what I do in my greenhouse where I grow tomatoes every year in the same bed)."
Jeremy Dore on Friday 18 September 2009
"Hi Jeremy, great site. I planted about 800' of potatoes which succumbed to late blight 2009. I removed the plants and disposed of them off site. There are still many tubers in the ground, some of which I have eaten this spring. My plan is to dig any that sprout and dispose of them, and then cover crop the area for this year and the winter. I will probably not grow potatoes there for many years. I plan to not grow any solanacea anywhere near this plot for the next couple seasons. does this sound like it would be eventually successful? Colin Whitefield, Maine"
colin on Friday 19 March 2010
"Hi Colin, Yes, digging up any that sprout will be key to getting rid of the spores. Remember not to put them into any normal 'cool' composting piles too - unless the temperature is really high right through a compost heap then plant diseases can survive."
Jeremy Dore on Friday 19 March 2010
"I may have missed any references on the site to the Asian Ladybirds (called Ladybugs in the U.S.). Over a year ago a National Geographic edition featured under "Bug Shot" a profile on the Asian or harlequin ladybug, which it said had first infected Britain from mainland Europe in 2004 It also included a photo comparing a helpful, indigenous ladybug (ladybird)and two different "guises" of the "Tiny Troublemaker." How rampant has the problem become, or is there good news in my not finding reference to the culprit on your site? "
Clarice McKenney on Tuesday 27 April 2010
"Clarice, there is an interesting map charting the spread of the asian ladybird on this web page: http://newsandviews.ceh.ac.uk/topics/harlequin_ladybird. Although it is bad news for native ladybird species and they can be a nuisance in houses the asian (or harlequin) ladybird still feeds off aphids which is good news for gardeners."
Jeremy Dore on Friday 30 April 2010
"I was struggling with wasps eating my fences and making my life a nightmare at my allotment. I used http://www.pestoclear.co.uk to get rid of them. "
Matthew on Saturday 5 March 2011

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