Grow Your Own Magazine

I have blight on my tomatoes.

I know that many of you have had it on your potatoes already, so you have my greatest sympathies.

Thing is, I am trying to make sure that I understand the way that blight is perpetuated year on year.

My reading so far, has revealed that the fungus is able to over-winter on any infected plant tissue - i.e. tomato fruits, tomato leaves, tomato seeds (or potatoes and tubers). Most information blames the continued spread almost entirely on potatoes, I see.

Am I right in thinking, therefore, that the fungus does not actually stay in the soil itself without the presence of some already infected plant material?

I have read some advice to make sure that compost and soil not be used again for tomatoes which have suffered from blight, but surely this is only applicable if you leave plant material in it? It is advised that you don't grow tomatoes in the same place again - but I see that is more about general nutrient availability and disease build-up prevention, rather than blight persistence in the same place.

Does "infected plant material" include roots...?

Anyone with more technical understanding about blight able to advise?

Thanks

Good questions Cutecumber, and all stuff I was wondering myself. There seems to be conflicting advice too, I've read on one site that you can compost blighty material and on another that you should burn it... The biggest no-no seems be to leave 'volunteer' potato plants if they spring up in the ground or compost heap (I wonder if they're particularly susceptible or something?). Apart from that, I haven't found any definitive answers...

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There are, at least, two types of compost heap - one that gets hot and will kill the weeds/seeds/spores and one that is cool that takes time to rot.
As for volunteer potatoes - well, the ones that have sprung up in my garden near compost heap look a lot healthier than some on the plot!!

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re volunteer potatoes - the one growing out of my cold heap is the only potato on my plot that didn't get blight!

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Yep, my compost potatoes were the best of the lot, and earlier too. I have just re-planted 3 of the best Pentland Javelin in the hope of new spuds for Xmas

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I don't compost diseased plants, they are dried out and put on the bonfire. I don't think there should be a problem with "volunteer" spuds. They, like the weeds, seem to do better and be healthier than the seeds we sow and cosset.

Well I am convinced that it is not a good idea to compost infected potato tubers and tomatoes on a standard cool garden heap. But that's as far as I've got!

It has been suggested, however, that it is much more likely that plants are infected from incoming spores in the air than from anything in the soil underneath them.

Can apparently healthy plant material harbour the spores, I wonder?

Would you always know by looking at it, that a seed or tuber was a carrier?

As I understand blight spors are is carried by the rain and wind currents. I have read lots of advice about how to protect tomatoes in the open. Out here it is recommended that the ground where the tomatoes are to be planted is watered well with Bordeaux mixture before planting and the plants sprayed to prevent the spores gaining a hold on the leaves. The same way as potatoes are sprayed. Another suggestion is that the ground is covered in a straw mulch to stop rain splashes on to the plants. A roof of some kind over the plants to stop the rain fallng on them, not practical with spuds.

This year the tomatoes have been planted in the open with a straw mulch and individual perforated plastic sleeves over them and so far they are blight free I didn't water the ground with Bordeaux mixture.The Potatoes sucumed in June to blight The sleeves have other problems but I can cope with them if it keeps the blight off.

Nothing Tomato or potato related is put on the compost heaps that will be used on the Veggies but I do put them on what I call the Rubbush Compost which is destined eventually for the flowerbeds.

Hope this helps

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I can give you a bit more info Cutecumber (and others) but don't really want to blind you with science.

Basically, the causal pathogen of (late) potato blight is phytopthora infestans, this is in the fungal group of 'oomycetes' i have no idea what the name implies itself but another fungal disease - downy mildew (of brassicas, lettuce and onions) also occurs in this group.

This group of funguses are also known as 'water moulds', and the ability for the spores to be 'motile' or have the ability to move in water is often an important transmission mechanism. In practical terms this means that rain splash, dew, irrigation/watering will aid the transmission of the blight spores into the leaf tissue. the spores are also carreid in the air and this is often how they come to be in contact with the leaves of susceptible crops (they won't infect those crops unless the conditions are right however)

A Smith Period is a modelling method used to help identify 'risk' periods for farmers growing potatoes, it doesn't say that there are blight spores about, just that if there are, there is a likelihood of them infecting plant tissue. both humidity and warm temperatures are required for a Smith Period - look at the British Potato Councils website for more info on Smith Periods: http://www.potato.org.uk/department/...ledge_transfer

In terms of preventing disease - leaving dead, dying, diseased foliage or tubers out in the open is likely to encourage sporolation of the fungus and release of spores into the air, likewise some volunteer potatoes that may have been infected with blight, but perhaps not showed symptoms at time of harvest may harbour the disease ready for sporulation under appropriate conditions for future crops.

The potato is a member of the solanacae family and there are many other crops and weeds from this family that are also susceptible to the fungus, including tomatoes (as already mentioned), peppers, chillies, and weeds such as deadly nightshade. These may all play a part in disease transmission, although peppers and chillies are not generally toubled by blight.

Most of us have seen 'foliage blight' where the leaves and sometimes stems of the plants go black and slimy, if these are cut off it is less likely to transfer to the tubers, but in exceptionally wet periods it is possible for the fungal spores to 'swim' down the plants into the soil and infect the tubers, this obviously also affects the roots and root hairs of the plants. hence not re-using the same soil/compost for other plants of the same family that year (rotation) however there's no reason not to use ot for plants from a different family (with compost you mgith want to add it as a soil conditioner to flower beds, or maybe bulk up some sandy compost for carrots where fertility needs to be low?).

As someone mentioned - a hot compost is more likely to kill off the spores than a cold compost, if you want to compost the infected foliage. Personally, i've given all mine to the local council as i find it difficult to get a hot compost heap and don't want to risk it festering in a cold one.

Seed tubers: its important to use certified disease free tubers, not just from the point of view of blight, but there are a host of other diseases that you wouldn't want popping up in your potato crop (like viruses). I don't know much about if the disease can be stored in a seed potato, but i assume that similar to tuber blight making potatoes unsuitable for storage, infected tubers destined for seed, would either not grow, or grow poorly, and possibly be a source of infection under appropriate conditions. It is therefore not a good idea to use supermarket spuds to grow your own crop of potatoes. i know it is done, but its not what what you would call 'best practice'!

Erm, can't think of anything else at the moment - feel free to fling more questions my way. I should point out that although i am well aware of the transmission routes, and the various conotrol methods (a reply for another day methinks!) my potatoes still got blight this year and i'm very cross with myself for not spraying earlier! knowledge is not always the key to success!

Anyway, hope that helps some of you. i'm off to bed now!

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Happy Gardening!

Thanks, Protea!

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You are a child of the universe,
no less than the trees and the stars;
you have a right to be here.
Max Ehrmann, Desiderata

blog: http://allyheebiejeebie.blogspot.com/ and my (basic!) page: http://www.allythegardener.co.uk/

anyone got any clear pictures of what blight infected leaves look like please,
potato and tomato.

Protea - that's great. That goes along with some more stuff I have been reading.

A few articles from the US describe how there can be a change in biology (can't remember the details!) allowing the fungus to breed sexually and thus persist and spread more - does that happen commonly or only under unusual conditions? Is that, in fact, what you are describing?

Headfry - there are various different indications, but I'll see what pictures I can find.

Hi,

I have posted these pictures before, but this is what my potatoes leaves looked like when they developed blight. There were brown spots on some leaves, others had turned completely brown and shrivelled. On the userside there was a white "fur".

Hope this helps,

Clareg

Attached Thumbnails

     

That right hand picture shows it really well - it is similar on both tomatoes and potatoes.

On the stem, blight produces dark brown patches. They gradually become more numerous and join together. Fruit show a strange "bruising" which quickly goes brown.

Is there always the "fur" on the underside of the leaf?

No, I've never had it.

I think the fur comes after a while, and if it is really damp.

Jennie, all my outdoor toms got early blight this year but there was no fur on the leaves.

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Rather than fur, I would say that there is a sort of mould that shows when the temperature and dampness in the air are 'right' (or wrong moreso - since they spelled the end of all my outdoor toms and my spuds) but I remember seeing the 'fur' on mine too - can't see it when the leaves are dry and can't see it when they are sopping either.

Wahey - clear as mud!!

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