Sunday, 19 October 2014

Control of woolly aphid

Scrubbing  Cathi’s trunk

Being Cathi’s head gardener it is quite a responsibility and I now find I have fallen down on the job!

My normal attitude to pest and disease in the ornamental garden is not to see pathogens and expect natural predators and parasites to keep them under control. Where possible I choose resistant varieties and grow my plants well and in doing so, do not predispose them to sickness. It does not always work out that way!

I noticed this disgusting mess on Cathi’s lovely pendulous crab apple. It must have taken woolly aphid several years to create this state of affairs. Cathi get a new gardener! She had not noticed either! None gardeners don’t see these things until it’s too late.
Very messy trunk  If you brush against it the crushed aphid stains are like cochineal! 
If not brought under control, woolly aphid brings death and destruction. It usually starts with mild stress on a susceptible variety. I remember a client whose apple tree had light infections on straggly weak shoots at the base of the trunk. The tree probably suffered from drought and the base was heavily shaded. I would annually prune the woolly aphid away and the trees continued to thrive. 

From such little beginnings, if no action is taken, over the years the infection spreads to eventually take over the tree. Each year any initial stress is magnified as the aphid sucks life from the plant. Infection accelerates and the tree becomes a write off and eventually dead! It is very difficult to spray a large tree and I know no suitable pesticides available to amateurs that will penetrate the woolly protection and control the affliction. Systemic insecticides do not work well in woody plants so no help there either.

Aphids covered with waxy protective wool. A problem on bark and buds  it can also fly to leaves and fruit.
My initial reaction was that we would lose Cathi’s tree. On closer inspection it appeared that infection was still mainly at the base of the trunk and any woolly aphid aphid higher up in the tree was at the tips of the branches. The next day I returned with my secateurs, loppers and saw. I also brought a scrubber and a secret chemical weapon!

Kitchen scrubber, sponge, soapy water and whimsical weapon.

The basis of control is to cut out severely infected woody branches and scrub the bark clean. There is no guarantee that you will catch all the aphid and prevent its return but if you remove potential re-infection it will be relatively easy to manage next year. Although physical control might be sufficient a little chemical help does not go amiss. A nice soapy mixture will do. Soap will aid penetration of the waxy wool and gum up the insect’s spiracles. As I selected a kitchen scrubber and squirted hand soap into a bowl of water, my eye caught Brenda’s ‘Ecover’, the squirty sweet smelling liquid cleaner she uses to clean round the kitchen. I looked at its analysis, 10% alcohol - they will die die happy - <5% none ionic surfactants, citric and  lactic acid and perfume. It’s made with ‘natural materials, why it is virtually organic!
There was no way such a mixture would be harmful to bark and even if I were to spray the odd leaf they were already going senescent and it would do them no harm.
The work of pruning and scrubbing and squirting took no more than an hour. How I did it  in mid September is shown in the pictures.

Oh what a mess. If you carelessly brush past the trunk you cover yourself with sticky pink goo!
Woolly aphid stimulates the production of swellings and galls. My first job was to severely prune away a few lower branches, twigs and galls

I wonder if I did not clear this debris away whether the aphid would produce winged generations to fly back into the tree. Better take no chances. I think Cathi’s hens had a feast before I moved them!

Fortunately much of the canopy was not infected. 

The scale-like nymphs can crawl to fruits and leaves. 
On Cathi’s tree most of this type of infection was near the tips of the branches and the twigs could easily be pruned away.  In some cases on lower branches I gave them a squirt of Ecover and where accessible gently scrubbed them!

After scrubbing the abnormal galls stimulated to grow by the aphid can be clearly seen. I hesitated whether to completely cut them away or just leave them. I compromised and just cut out the big ones!

Now a nicer looking tree. The adjacent red weeping malus is completely uninfected, and is evidently more resistant.
The cuts will NOT be painted.  I grimace  at the very thought!

Five weeks later in October the crab apples are colouring up nicely and will last beyond Christmas

A half-hearted disclaimer
I do not consider the serendipity factor of my use of Ecover on a whim as essential to the successful control of woolly aphid! My thought was that it’s wetting power might aid penetration of the soap. I admit I got carried away merely squirting with alcohol rich ecover on a few of the higher infected branches. I made little attempt a few days later to find the aphids I sprayed. Although no aphid is apparent now, for all I know there may be some very hilarious aphids hicoughing away in happy hibernation. At least when I had finished my hands have never been cleaner! Perhaps there is scope for a project next year.

My holiday snaps
The very next day we went Italy to visit my son Tim who lives in Vico Equense on the Sorrento peninsula. What did we see but citrus trees infected with woolly aphid. Most of the orange tree roadside planting was in good shape but at the end  of the row it was so shady that every time I went back to take a picture, my camera flash activated. Such shade does not make for a happy tree, even shade tolerant citrus.

A holiday snap to text to my friends?

Obviously the aphid had been scrubbed away by Parks and Recreation. The galls are similar to those on the apple

A very unhappy tree in a dark garden in Sorrento

And a note about ipomoea

I recently wrote about the control of bindweed. I jested about killing bindweed in Folkestone and going home to my Morning Glory!

In Italy I was reminded what a weed ipomoea can be. Although only an annual see what it can do in a warm climate!

I found some real bindweed. In Italy in September it has almost died back from summer heat.  It would appear that it has a native predator and is not the same scourge as in England? 
I did not bother texting this picture home!

Thursday, 9 October 2014

Book Review. Gardening Myths and Misconceptions by Charles Dowding.

A book about myths written by a no dig (organic) gardener is too good an opportunity to miss! Peter brought it round, he had devoured it that morning. He confessed a little disappointment. I think he expected a little more depth - pun intended. Perhaps there is too much muck and magic in there and too many assumptions of unverified facts.
For me it is charming collection of old gardening lore and analysis of modern misconceptions. Charles has a fund of sound gardening knowledge that spills out of the pages. 

Poppy devoured the book too

Charles Dowding is a very different gardener to me. He grows organic vegetables on a semi-commercial basis. When I see pictures of his magnificent healthy high yielding vegetables and fruit on his blog I wonder how I have the cheek to venture any opinion at all.They are so beautifully grown and displayed. He meticulously records details of all his ongoing trials that test no dig principles. My own use of glyphosate, inorganic fertilisers and in his own words, synthetic chemicals would be anathema to Charles. (I could never understand why synthetic  materials such as plastics were acceptable and synthesized fertilisers and pesticides were not - I  promise this will my only dig at organic gardening today).
What I most admire about Charles Dowding apart from his fine standard of horticulture is his questioning mind. All of his opinions have been tested over the years when he has challenged orthodox practice. He passes on the results of years of successes and failures. I would like to think that I am the same although we have not always come to the same conclusions. I am 100% in agreement with all the benefits of no-dig he writes about in his books and on his blog.
En passant, I noticed on his blog his intriguing source of seed and potting compost. It has all the advantages of peat  - because it is peat - but without perceived doubts about coming from an ethical source. I have no personal experience of this product. 

 Is this the best thing since sliced bread? I have no idea.
Like most organic gardeners Charles imports into his garden horse and cow manure from external sources. Although my former boss P.K.Willmott described stable manure as the very finest bulky organic material you can add to the soil - it had an x-factor  he could not explain - I never use it. This is somewhat eccentric of me as I agree that it is wonderful stuff. I used to have half formed thoughts of being self sufficient in organic matter and on my Bolton Percy allotment sought to demonstrate how un-dug soil becomes black with organic matter if one merely recycles the products of in situ photosynthesis. Mr Dowding recycles his weed and debris via composting, whereas I do it directly by leaving fresh organic matter on the surface. He does not consider this appropriate in UK conditions. When it comes to tidiness and other practical considerations he is right. When it comes to  benefit to the soil, plant health and his fear of slugs I disagree.
I have two problems that stop me using farmyard manure. My local source is free manure at the garden gate in a near village and I fear it may be contaminated with aminopyralid herbicide. Sue Garrett blogs about this scourge. Charles mentions a simple seed germination test which I imagine involves sowing any quick growing seed, but only when the manure has been ‘made’ and is no longer fresh.
Unfortunately my local manure is full of weed seed. This would never do when I have gone to such lengths to prevent my own weeds seeding.

The book
Written for a wide gardening public there are over a hundred headings each containing either major misconceptions or groups of related myths and unsound practices. Some gardening lore is quite trivial and is often repeated by those with no fundamental knowledge. It must be quite daunting for new gardeners to sort out the wheat from the chaff. 

Even beetroot can be transplanted. Best when several seedlings are planted together from a small pot or module.
The book not only is valuable to any questioning beginner but tackles concepts fascinating to an advanced gardener. I looked for myths that I have covered myself on this blog, found most of them and that we were in broad agreement. 

This achimenes has water scorch, a very real affliction but not caused by droplets of water concentrating sunshine. It’s not even caused by sunshine at all. The fact that water scorch occurs on gesneriads does NOT mean that you cannot spray water over most of your plants.

Charles correctly states that it is a myth that you can only water plants in the evening. When I noticed my kohleria was wilting this morning I watered it immediately.
I wondered if the book might suggest myths I might write about in future and found questions relating to row and greenhouse orientation, crocking and staking trees. I was pleased to find our agreement on many technical issues such as the gardening press's confusion with compaction and firm settled soil and indeed the general erroneous conception that 'fluffy soil' is a good thing! 

Greenhouses can be orientated in any direction. Much more important is the absence of shade from buildings and trees. nb my hedge is on the north west side and casts little shade.
Mr Dowding is up to date with modern happenings such as the discovery of glomalin in 1997 that transformed our understanding of the nature of the world's organic matter. I agree with his doubts about the fashionable adding of mycorrhiza from a packet when it is best to leave things to nature. Although mycorrhiza are fundamental to certain plant's survival in the wild I rather doubt that in his own highly fertile vegetable garden, mycorrhiza are of any significance at all - any more than in mine.

Charles has important and in some cases novel things to say about about sowing dates. He flirts with astrology to of course dismiss it. He has done trials with correlating planting dates with phases of the moon and reluctantly - I think - dismisses them too. I could be persuaded that the moon does effect plant growth - just perhaps - but not in any predictable way. In my view the major factor that thwarts success of a recommended sowing date is the vagary of the weather.   

It is not necessary to transplant leeks and insert them deeply to blanch them - but it is not wrong to do so. 
Where Charles and I would appear as one is that he contends that if you grow plants well - in his case organically - that they will succumb to far less pest and disease. He quite rightly does not claim complete success against virulent pathogens or in difficult seasons.

I was interested to read about Dowding's compost tea which he recommends as a soil conditioner. He also suspends decaying organic matter over water to catch its soluble content to make liquid feed. Not very different to my own methods when I leave fresh organic matter to decay at the soil surface in preference to composting. 
Why should I imagine that 'bacterial washings' from decaying vegetation should benefit my soil? I have observed nice crumbly soil directly below fresh surface decaying vegetation and I once read the tale, but I do not remember where. Now that's how myths are created!

And a final thought
Peter Williams made a perceptive observation when he commented that myths often arise when authoritative figures make dogmatic statements. I would back Charles’ authority against most TV gardening gurus, but even he can be wrong!

My previous book reviews 

To read my own take on myths put 'myth' in the search box at the bottom of this site.
..whoops this morning it did not work! I tried ‘Roger Brook myths’ on the main google search site and got a much more thorough record than I have ever seen before. I looked out this post where I reviewed some of my own myths.

Sunday, 28 September 2014

How to grow phlox

Colours of phlox

Phlox paniculata is one of my very favourite herbaceous perennials. Perhaps out of fashion it gets little acclaim  and some think it common. 

Choose you variety and you have almost all the colours of the rainbow. You can have the Union Jack, happily still flying in red, white and blue. Flower colours are strong and vibrant and I  can think of no plant with more intense exuberant white flowers.  Just google ‘phlox images’ to see the huge range of strong colours.

The foliage and bracts might even be black or brown
Often scented, sometimes magnificently so, a fine feature of phlox is that  the flowers last for three months! Some perennial plants have the achilles heal that although magnificent in pictures they only last a few days. Not phlox. If you choose your range of cultivars carefully you can have them in flower from May to October.

A double flowered variety. I have forgotten the name. Visitors love it or hate it!

Cultural notes

Phlox like a water retentive soil and readily available water, right through the Summer. Although they do not like water-logged soil, parts of my garden are not very far from it. Some readers may recall that the bottom of my garden sometimes floods for short periods and I have created raised beds above the water and have eccentrically buried great wads of water retentive newspaper. I think my Phlox ‘Laura’ does its very best planted there.

Phlox ‘Laura’ is planted over more than a foot of newspaper!
In relation to Summer water supply, my zero cultivation has special value as roots love to come right up to the surface where they can benefit from light showers. Mulching helps  and has multiple benefits but is sometimes over-rated where a blanket of absorptive material intercepts light rain. The latter comment does not apply to a gravel mulch and I have a large clump of phlox in a gravel mulched and otherwise dry part of my garden. In this latter case the strength of transpiration is reduced after about one o’clock when the sun moves to the back of my house and my plant is in partial shade.

Phlox does not like the dehydrating conditions near hedges and under trees. Although in Bolton Percy cemetery I have had a fine clump of several square meters for about 25 years it is now starting to severely decline as adjacent trees have got bigger and take increasing amounts of water and light.

Now declined from former magnificence this plant has been in place in Bolton Percy cemetery for thirty years.
Needless to say if your borders are very weedy  and plants are crowded together there will be increased competition for water.
I do not irrigate any of my own phlox but of course in Summer drought it is generally worthwhile to heavily water a plant with several gallons. Water infrequently and never water established perennial plants ‘little and often’ - unless you grow them in a tub!

Phlox grows well in full sunshine provided it has freely available moisture. If not, it is best if they receive some shade for part of the day. It seems to be agreed that they are best if exposed to direct sunshine that accumulates to at least four hours on a sunny day. Less and they will grow with little vigour and be susceptible to mildew. A few of mine fail this test but  the shade is not heavy.

This phlox is at the base of the house and only receives direct light from 3pm

This phlox receives morning sun until 11am when it is shaded by the house. Only in high Summer does it it receive any more direct sunshine
Stocks of phlox, planting and propagation

Sorry about the pun but I could not resist it as so many none gardeners confuse stocks and phlox!

Phlox are very easily propagated by simple division. With a sharp spade, just chop and replant, perhaps a little deeper than before. From early September right through until early May will be fine to propagate and/or move them. It would be silly to divide in full Summer but if you are desperate and moving house just cut back the top and move them! Plants from the garden centre unless soft from a tunnel can be planted anytime but if you do so in Summer remember to water.

Division. A sharp chop from a spade might break your wrist. Perhaps better to undercut gently. Although division will be easier if you lift the whole plant, unless you are a nurseryman or moving house I do not recommend it.
Like many herbaceous perennials your plants will improve from year to year if you leave them alone. The second year plants will be much stronger than the first year when plants are establishing. We were very disappointed when we moved to our sandy soil - sandy soils are notorious for lacking nutrients - that our phlox were very slow to get going. It was year three before we were proud of them. I now wish I had given them some NPK fertiliser.

I read that in some cases phlox need to be in place for up to five years before they achieve their full grandeur. I am inclined to agree, especially with variegated varieties. Well established clumps will have greater stored food reserves and I suspect a stronger root system to go down deep and fine water.

Phlox ‘Norah Leigh’ improved every year
I am not one to believe strictures to replant herbaceous borders every three to five years. Gardeners who need to do so must have made a very bad fist of it first time round! I do not buy into the myth that large clumps  of phlox become hollow and diseased in the middle.

In the old days every article one read about phlox emphasised you could propagate them by root cuttings to eliminate stem eelworm. Forget all that nonsense - albeit the fact is a true one. I have never suffered this pest in fifty years and if I did I would burn my plants.

I noticed on the net many seedsman offer seed strains. I have no experience of growing them and they might be quite wonderful. I am however doubtful about growing seed raised cultivars of hardy perennials as explained here

Phlox Powdery Mildew

Although phlox has its fair share of pest and diseases, my own have only a few minor blemishes and I never spray them. Mildew is the most reported disease and some varieties are promoted as resistant. Not only are there differences in varietal susceptibility there would seem to be fungus strains of varying virulence. Carried by air borne spores, powdery mildews are quite host-specific and by way of example rose, apple and phlox mildews are all different.

According to the UK gardening press it has been the year of the mildews. The disease has been rather over-hyped, and I looked around my garden to find some and failed. My friend Iris remarked she had suffered from mildew badly this year. Was it that her garden is heavily shaded and her plants rather crowded, or was it that having seen it on the tele she noticed it for the first time? I suspect I am myself similarly blind when plants such as pulmonarias show mildew when they die down. It is just part of the natural cycle.     
Mildews are a classic example of the principle that when plants are well grown they are less likely to suffer from pest and disease. Predisposing factors are too little direct sunshine and lack of moisture. Although phlox will grow well under trees the shade must not be heavy.

I managed to find some powdery mildew in fairly heavy shade on the village plot

An unknown affliction. Is it a an opportunistic pathogen, result of a damaged stem or my own careless spray? I don’t really care and in seconds pruned the damage away.
This variegated phlox is reverting.The dark green shots should be pruned away
link to reverting

Does phlox paniculata need staking?

In my view no. But then I do go rather over the top on this issue! I think for many gardeners a crutch is more of a mental one for themselves rather than a support for their plants. When in a gale their plants are severely damaged some gardeners comfort themselves that they did their best as they set about undoing the damage and re-staking, whereas I just stroll round my borders, observe that a few stalks are broken or out of place and cut them away.
I fully understand that in some gardens the beds are too narrow or the plants are feeble and perhaps crowded; plants might be drawn because they are at the back of a border, have not grown sturdily in an open position or are subject to swirling winds between buildings and do need staking. And perhaps if you face Atlantic winds on the Isle of Mann you will be lucky to grow any tall herbaceous perennials at all. Even in my own garden Brenda insists on staking delphiniums and putting a string around a few of the taller herbaceous plants. If you do feel your phlox might need support you might be better growing the more compact varieties although my tall ones such as wonderful white Phlox 'Mount Fuji' stand sturdy like soldiers. Another strategy is to give your phlox the ‘Chelsea chop’ to ensure compactness - I can barely bring myself to use that phrase.

Growing old gracefully
Other phlox species.

There are many other wonderful phlox and I have grown a few of them. Often with limited success and sometimes total failure. Many can be described as a ‘good nurseryman’s plant '. They look wonderful, die on you and you buy another!
This is not true of them all and in my own garden Phlox subulata thrives.

Phlox subulata so welcome in May

Phlox subulata makes excellent ground cover. Here I have plunged winter-tender pittosporum. The few yellow patches are where I have sprayed out weed.
Some further thoughts on herbaceous border maintenance can be found here

Tuesday, 16 September 2014

Crop Rotations discussed

Are formal rotations necessary?

If anything would seem to be a fundamental part of horticulture, it is crop rotation. It is therefore with some hesitation that I challenge the value of the traditional three and four year formal rotation. If you read the popular gardening press it appears to be mandatory  for anyone who wishes to grow vegetables. Whenever I pick up a beginners gardening book there are complicated diagrams of how to precisely manage your plot!
I wonder how many allotment growers actually formally rotate. Personally I have always paid lip service to the notion of not following one crop with another of the same kind and more specifically not growing in succession plants from the same family. For example if I wanted to grow turnips, they would not immediately precede or follow cabbages. Actually if I was to be really truthful I don’t like turnips and delicious swedes (which we call turnips in the north) are best grown by a farmer!
As the years pass I find myself less and less a slave to the idea of rotation. I grow numerous brassicas and although I vaguely try not to follow one with another, I frequently fail. I practice a lot of inter-cropping and frequently one crop immediately follows another and for example I might even sow my broad beans under the canopy of not yet completely consumed sprouts (a rather unorthodox practice). I frequently plant in double rows or blocks and my layouts are more designed to efficiently use my enviro-mesh to keep off the pigeons, discourage the mice or keep the carrot fly away. If my only choice of planting site is near my wretched cherry tree when it is really dry in summer or planting one brassica after another the latter choice wins!

 I had a wonderful early crop of broad beans, but now soil under the cherry tree is severely dehydrated!

I have only just discovered wonderful enviro-mesh
Rotations as an advance in agricultural history

Rotation is a respected agricultural tradition. We all remember ‘Turnip’ Townsend’s four course rotation from our history lesson at school. It was a major leap forward in farming three hundred years ago.The four year rotation continued to be used for a very long time. No longer relevant in modern agriculture, much longer rotations that last many years still have a place and farmers who insist on mono-cropping have their own special problems.

The four course rotation was an important  innovation as intensive agriculture started to develop. Before that land was often left fallow from one crop to another. 
It was long before fertilisers had been invented. I imagine it likely that although artisans appreciated the value of animal manures on their own plots they were not initially much used in the new technology of farming. What an advance when nitrogen fixing clover was introduced and both turnips and clover brought browsing cattle and perhaps rootling pigs onto the land to add droppings and urine. 

Clover, peas and beans in a rotation all facilitated bacteria to fix atmospheric nitrogen and enabled this most valuable of nutrients to enter the ground in an available form.

Weed control was primitive and barely existed. Some crops such as peas were considered  ‘dirty’ and allow prolific growth of weeds, other crops like potatoes could be used as cleaning crops because of their strong weed smothering growth.

A rotation of crops probably helped to efficiently recycle organic debris and nutrients. 

Different crops have different nutrient requirements and might make varying nutritional demands of the land. In those days there was great merit in following nitrogen fixing legumes with nitrogen hungry brassicas. It makes a huge difference to the yield of brassicas to have an ample supply of nitrogen.

Some soils over the years can become acid and adding lime can sometimes increase yield. It was sensible in olden times that where lime was needed, it was best to apply it immediately before brassicas. Soil alkalinity reduces the severity of clubroot, a severe scourge of these plants.

If there was (and still is) a single major advantage of rotation it is reducing the infection and build up of soil borne pest and disease.

So why is a formal rotation irrelevant now?

A very significant disadvantage is that it commits the gardener to growing more of a particular crop than he needs. Worse it encourages an annual cycle with what many of we minimum cultivators regard as the decadent practice of leaving an empty, fallow, dug-over  plot to be leached, denuded and eroded by wind and rain throughout the Winter. Best for both the kitchen needs and soil fertility to grow vegetables in every season. 

Vegetables such as overwintered leeks, sprouting broccoli and sprouts will be in the wrong place when a new cycle of rotation commences.

On a similar theme, most modern vegetable growers grow more than one crop in a year. Many fashionable vegetable crops such a ‘salad leaves’ need to be resown several times over to achieve continuity. Many gardeners these days wish to explore the possibilities of inter-cropping, companion planting and green manuring. It’s very inconvenient and technically limiting to insist on growing similar plants together.

No rotations are needed for perennial vegetables such as asparagus and rhubarb. Not the best example of intercropping here
Some traditional reasons for rotation are just out of date! Good gardeners whatever their precise methods maintain good weed control all the year round. Many of us try to practice zero tolerance of weeds setting seed. None diggers refuse to bring ancient buried weed seed to the surface! My own peas and beans are just as clean as my potatoes! As a user of glyphosate I have no perennial weed to smother!

The balanced take-up of nutrients where various crops have different requirements now fails to impress. Organic gardeners, especially the none digging kind, maintain high level of soil organic matter which provides an excellent store of nitrogen and all necessary plant nutrients. Growers like myself do the same and are also prepared to supplement plant nutrient requirements with fertilisers. In whatever order we grow our plants they will not go short of nutrients and leaching of nutrients will be no less or greater than if we were to formally rotate.

The cycle in a rotation defined as ‘roots’ is fundamentally flawed and still supports the ‘baby-talk’ that roots require extra phosphate. There are so many different families of plants that develop fleshy roots (or swollen stem bases such as kohl rabi and some fennels).  Carrots, swedes, parsnips, beetroot all belong to very different families with distinct nutrient requirements. Turnips and swedes are also brassicas and are both susceptible to clubroot.

Does it count as a root or can you grow fennel anywhere? er, not in the lawn!
In fact many of the constituents of the formal vegetable groupings are rather a rag bag of disparate types. Even the RHS on its website admits that for many of the extended range of vegetables we grow these days it does not really matter about rotating. Plants such as sweet corn, cucurbits, french beans, runner beans, salads (a huge group) and peppers can by common consent be grown anywhere. When I think about that list it includes almost all of the vegetables I grow!  

My biggest objection to formal rotations is that they often fail in their main purpose of reducing pest and disease. In very small gardens the different sections are just too close together to properly isolate them. Rotations are too short to prevent clubroot which can survive in the soil for as much as twenty years. I know many gardens where clubroot has efficiently been dispersed across the whole plot on soil attached to plant roots - not necessarily brassicas - as they have been rotated around.

At least the crops are isolated here!
Club root
The only way this fungus disease will infect your crop is from soil. On dirty spades, boots and principally plants. There is no other way it will enter your system. Sad to say many gardeners inherit a vegetable garden completely contaminated and unless their name is Methuselah they will never see the end of it! It used to be such common practice to buy a few brassica plants at the market and - bingo, you had introduced this disease.
If you are lucky enough to be club root free, always propagate your own plants and do so in uncontaminated compost. In the past I have had some near misses when so called friends have insisted they give me a wonderful new brassica variety. After they have gone they go on the fire!. They must find me extremely incompetent when on their next visit their plants are not to be seen!
My allotment in Bolton Percy was clubroot free for the twenty years I cropped it. I cannot say the same of the adjacent plot that I took over from another gardener. Fortunately he did not have the plot very long and he had not had time to rotate clubroot around!. I knew the exact location of the infected soil and never grew brassicas there. It might have helped that I don’t dig and don’t do any cultivations that involve shifting soil. I have no idea whether there was any degree of spread from my numerous worms! I supposed it helped that the plot was long and thin! The fact that I did not have a compost heap and weeds were left to die in situ was not insignificant. If you move contaminated compost around you might very well spread this debilitating disease.

Can soil on a trowel spread clubroot around?
The good news is that you can get a very good brassica crop on infected soil if you use lime and raise the soil pH to 7.5. It’s not that brassicas really like to be so alkaline it’s just that club root hates it! This involves doing the complete opposite of rotation as pH 7.5 will be far too high for any crop that follows. By mono-cropping you might still enjoy all those lovely Winter greens! 
I use lime in the form of dolomitic limestone which provides both calcium and magnesium

Nice cauliflowers even with clubroot if pH is high - but the soil must be fertile

Closing thoughts
I have run out of time! I wanted to explore each of the following.

1.Mono-cropping of tomatoes.
 Little and Large of tomatoes Albenga (Mole seeds) and Sweet Million (Marshalls). Both are more pest and disease resistant than similar Marmande and Gardener’s Delight. Too much of a challenge for even these fine varieties as the soil has not been changed for five years!
But badly grown tomatoes taste very nice

2.My peppers now looking good resown and planted in the same soil for four years now.

3.Considerations when replanting  perennial crops such as raspberries, strawberries and blackcurrants.

My Autumn Bliss have been in place seven years now. When I replace them I will choose a new site

4.Specific replant disease.
5.Whether there is a need for rotation in the ornamental garden.
6.How some different plants do specially well and others particularly badly, when one follows another.
7.Whether garden and natural ecological successions are inhibited or promoted by pest and disease. 
8.Gardens I know where it is now impossible to grow delphiniums and trilliums because of soil contamination. 
9.Not to mention brother-in-law Dave who has white rot contaminated soil and has had to stop growing onions.

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