[Ma and Pa Snip]

One of the things I have previously read in magazine articles is that its a good idea to keep the floor of a greenhouse damp as red spider mite doesn't like humidity.

Now here's the but..

I've read on some of the replies here to tomato blight questions that humidity is a cause of tomato blight.

Soooooooooooooooo,

are the risks of red spider mite and tomato blight something that takes place at different times of the year ?

[arugula]

What you want to grow should have a bearing on whether you want to encourage dry or damp conditions...

[DD.]

Unless you have live plant material in your greenhouse over winter on which the blight pathogen will live, e.g. potatoes, tomatoes, then what you do in your greenhouse won't have any bearing on whether you get blight in there or not.

Blight is airborne and can easily drift in from the outside.

To propagate blight needs a certain spell of temperature and humidity, known as a Smith Period.

Learn all about blight here:

http://chat.allotment-garden.org/index.php?topic=56717.0

[mumofstig]

Maybe this will confuse matters further, sorry if it does.................
but I've never seen red spider mite in my greenhouse (maybe I'm lucky, or blind) but keeping the greenhouse damp can definitely lead to plants getting Botrytis (grey mould) which my toms had badly last year.

I found this which I think is interesting, about spraying plants with PLJ, although I presume this would not be legal under the EU regs

http://www.nvsuk.org.uk/medwyn-williams-vegetable-grow-show-285.html

[DD.]

Medwyn would never do anything illegal.

He must have accidentally spilled some and noticed its effects.

[Ma and Pa Snip]

Thanks DD, I had read your link a few days ago. Whilst it gave good tips with regard to blight unfortunately it didn't ease my concerns about the two different issues I now question.

I do grows toms in both the greenhouse and the outdoor beds, and do understand that blight is an airborne spore. We have suufered from it in the past and infected parts of plants have been carefully disposed of, as has the entire plant once we have picked any fruit.

I am just querying the two problems because of the advice I had seen written elsewhere about 'damping down' being effective against red spider mite.

Since we are completely unable to use any form of chemical spray anywhere within our growing areas we have tended to dampen down the greenhouse as a matter of course in the hope of preventing, rather than having to cure, a mite problem.

My confusion is therefore based on wondering if the likelihood  of a blight problem could be enhanced by our actions of damping down increasing the humidity in the greenhouse when the 'smith period' bringing in airborn spores is in place.
Could we infact create our own mini smith period, the spores that growers get have to start somewhere I guess.

Unless that is, the potential time of year of a 'Smith period' is different to the time of year one might experience the onset  of red spider mite. In which case I could damp down, or not, as per the risk time.

****************

Hmmm
Have just read MoS's reply about Botrytis. We not had this during veg growing periods thankfully.

So maybe my thinking  that by damping down we may prevent one potential problem in fact may cause more, or bigger, problems than it solves.

[DD.]

the spores that growers get have to start somewhere I guess.

Blight is usually overwintered on things like growers' "outgrade" piles, (i.e. not good enough for market), so unless you're keeping duff spuds in your greenhouse over-winter, it's purely coming in from outside.

[Kristen]

Maybe this will confuse matters further, sorry if it does.................
but I've never seen red spider mite in my greenhouse

I've had it on Cucumbers and Aubergines - didn't seem to bother the Tomatoes right alongside them. I put trays of water down around the Cucumbers to increase humidity - which made a significant difference - reducing the amount of red spider activity and allowing the Cucumbers to carry on until the end of the season.

Damping down the greenhouse floor often used to help reduce the temperature in Summer too ...

The humidity will help Blight, but mostly it starts after rain, and it is the foliage getting wet that helps it infect the plants, so watering the soil, and not getting the foliage wet, will also help and I think (inside a greenhouse) is a useful preventative measure (don't let water splash onto the leaves either)