Topic: Potato Blight

[gavin]

An e-mail service to warn you of when the weather is just right to expect blight in your postcode area!  http://www.potatocrop.com/.  I've "used" it - well, got the e-mails - for a couple of years now.

It's really for the "potato professional", but there is a category when you register for "Other" - I reckon that just about covers me ;D

All best, Gavin

[Ceri]

great site to wander through - thanks Gavin.  You can put in your pot variety and it tells you its rating against all sorts of things.

Gavin, can you tell me what a 'Smith Period' or 'Near Smith Period' is - I presume it is a blight danger - but what does it define?  couldn't find an answer on the potato site - presumably its for grown ups who know these things!

[Ceri]

sorry, found it now - for those who need this information desperately it is:

"A full Smith Period has occurred when:  
At least two consecutive days where min temperature is 10ºC or above and on each day at least 11 hours when the relative humidity is greater than 90%.  A Near Miss occurs when:
One or both of the above two consecutive days has only 10 hours when the relative humidity is greater than 90%."
And
"Smith Period conditions are conducive for sporulation of the potato blight pathogen on lesions - leaf wetness is also necessary for infection to occur. If Smith Periods occur at frequent and regular (7-10 day) intervals, there is a greater chance of blight development. This is because the generation time i.e. from original infection through to lesion development to subsequent sporulation is around 7-10 days. Smith Periods at these intervals would prove ideal for the blight pathogen."

[Multiveg]

Just having a look through the site myself. However, under insect pests, it lists slugs  :o
Last year, there was a full smith period for 3 days in a row at the end of May. And a few in july and august... Trying to be organic means that I only have bordeaux mixture for use against blight.
3 varieties of spuds have 9 and 9 on the tuber and foliar blight scores. They are very new varieties which I got at Ryton's potato day.

[The gardener]

I have always had this thought about 'potato blight'

Perhaps I can get a few opinions on it.

I have always been aware that blight has been brought about by 'humidity' and this is why I never water my potatoes.

Think of it in this manner;

1) Watering is replicating rain!

Rain occurs when air pressures are low, yet one waters when it is dry, i.e. when air pressures are high.

So I suppose it could be said that this method is the reverse of nature.

2) If we water i.e. when there has been a dry spell, and quite likely a warm spell, doesn't the water on hitting the 'warm' surfaces, cause them to cool?

If this is the case, isn't this very likely to cause 'condensation' which in turn raises the 'humidity' which in turn is conducive to creating conditions suited to the formation of 'blight'.

This is just a thought, your opinions might be interesting.

And before any one asks how do I keep my potatoes growing in 'moist' conditions..........its all in the pre-winter preparation, i.e. I dig in loads of farm yard manure, which soaks up the winter rains.

[Wicker]

Definitely inclined to agree with you Gardener about holding back on the watering but afraid don't have as much restraint as you - get nervous about shrivelled up crop!!

So now and again just aim hose along the "valley" between the rows and not above the leaves, don't know if that makes any difference to the condensation but we have only once been troubled with blight and that was quite a few years ago.

[philcooper]

As Ceri states the vulnerable peiods are when there's a combination of high humidity and warmth, so watering in warm weather could make the situation worse.

When you consider how deep the tubers are then it takes an awful lot of water to soak down that far so a small amount of water on the surface, as with most plants, does more harm than good.

Logically answers are therefore to make sure the roots are kept moist by:

incorporating lots of organic matter at (or before) planting time,

mulching to keep any moisture in the soil

and possibly using soaker hoses below the surface

Phil

PS the potato crop service is good but I supplement it with listening to local radio gardening programmes which give warning of blight in the area, it tends to move north and east from the south-west from June onwards. At that point, and when the potato crop warnings start, I spray fortnightly with Bordeaux - it doesn't seem to stop it but it slows it down and gives the tubers a bit longer to bulk up

[tim]

Do agree with the lack of point in a drop or two of water.

For a start, all water will run off into the furrow, below most of the roots. To soak sideways into the ridges, I reckon at least 4 gal/sqyd, equal to 1" of rain. I reckon?? = Tim

[philcooper]

Tim,

I'm not sure, I do recall seeing figures showing how much rainfall was required to penetrate 1" and it was frightening, using your figure of 1" rain = 4 gall per sq yd, it would be a huge amount just for a small amount of spuds

Phil

[kenkew]

My subsoil is only about a spit deep, the top soil is clay which I have 'improved' in the short time I've had this plot, but due to the water retention capabilities of subsoil and clay, no way will I water my spuds, if they get thirsty they can go a-hunting.

[john_miller]

  Just a suggestion Phil, but if blight has a 7-10 day life cycle would you not get more effective control if you increased the frequency that you spray Bordeaux mixture? I spray on a 7 day rotation and get excellent control until late in the season.
 I would go with Wicker about watering.I find it hard to believe that the application of water to a small area of potatoes, even in the quantity that Tim suggests, would make a significant difference to the overall humidity of the atomsphere. Most atomspheric moisture comes from bodies of water- here our peak humidity occurs at the time of year when the ground is at it's driest, even past the point of drought. If your crops are dry, water! Does anyone apply this method to tomatoes and, if so, do they have problems with BER or fruit split?
 I would also point out that even if water is applied that results in wetting the foliage and cooling it off to the point of causing condensation it would only be temporary and not long enough to allow infection (due to evapo-transpiration leaves are naturally cooler than the atomsphere, when wetted this stops until the leaf dries and starts to warm up again), especially if watering is done, as is commonly suggested, in the morning. It is condensate on the leaves overnight that allows the blight spores to germinate and establish. This film of moisture will stay on the leaves for a prolonged period.
 
 

[philcooper]

John,

The reason I go with 14 days is that use of any spray is not encouraged in organic gardening and the copper provides a protective coating which lasts around 14 days (in the absence of rain)

On the humidity bit, the watering will produce higher than usual humidity around the leaves, not, as you say, in the atmosphere overall. Does this not produce the blight conducive conditions around the plant?

[john_miller]

  Blight, like many other fungi, are not dependent upon humidity for germination. What they need for germination  is an extended period of time of continuous moisture on the leaf surface. Humidity being high sets up the neccessary conditions to cause condensation overnight as the air cools and is no longer able to hold as much moisture in suspension. This results in the leaf surfaces being wet enough long enough(it just requires a few hours) for the spores to germinate. It just seems to me that the small amount of water added during watering really isn't going to add much to the total atomspheric moisture. Certainly not enough to not water if the plants need it- watering in the morning would certainly be advised though so that you don't end up with soaked leaves overnight.
  A thought about getting water to where potatoes need it. Would it be practicable to bury these nearly ubiquitous soda bottles (or something similar) in the ridge as earthing up starts? Water could then be applied down through the bottle.

[tim]

My little bit about 1" of rain was the 'collected amount', rather than the penetration depth. I still believe you need at least that to get to any depth. Know nothing about blight - except that we spray, but sometimes get it. = Tim