Is this potato blight?

[pieman]

Hi folks.

Noticed quite a few yellow leaves on my potatoes the other day, and saw the same on a few adjacent plots as well. This picture shows a leaf that doesn't seem as yellow as most of them, but I'm rather worried it's something nasty. Any ideas?



Thanks

David

[pieman]

[Tee Gee]

doesn't seem as yellow as most of them,

The above statement suggests your potatoes are earlies that are dying back (yellowing) and if so this is not blight (in my opinion)

Am I correct in thinking they are earlies?

[Kepouros]

To be absolutely sure in diagnosing potato blight (either early or late) it is essential to see the underside of the leaf, and it would be helpful if you could show a photo of this (preferrably top and underside of the same leaf).  However, even without this I`m pretty sure that what you have is not blight, but is probably a physiological disorder caused by either soil or weather conditions.

If you have a lot of really yellow leaves on your plants, with brown markings interspersed between the leaf veins, then you may well have a magnesium deficiency, although this could quite easily be a purely temporary one caused by either heavy rain or abnormally dry soil conditions rather than a general soil deficiency.

You will probably be advised by others to reach for the epsom salts immediately, and spray generously, but I would suggest that unless the plants are badly affected you wait a while, keeping an eye on the plants to see whether the problem gets worse or stabilises itself before doing so.

[davyw1]

have a look at the photo on this link

http://www.allotments4all.co.uk/smf/index.php/topic,52605.0.html

[pieman]

Thanks everybody, those pics of a magnesium deficiency are exactly it.

As suggested, I'll leave using salts for a bit - made sure I watered them properly and will keep an eye on them.

This really is a very handy place. Thanks again.


David

PS: just noticed that apart from the link davyw1 gave me, shirlton's got a similar post with the same problem. Perhaps I'll look around the other topics a bit more next time!

[Unwashed]

With magnesium deficiency the leaf dies between the veins, but that's not happening with your leaf.

[pieman]

I see what you mean, but it's a bit hard to tell on that rather poor sample I used for the photo. I'll have another look today and perhaps post another picture.

D

[Unwashed]

My guess is that it's blight - see those concentric rings in the brown bits - but I'm not sure.

[Kepouros]

I`m afraid that I have to disagree strongly with Unwashed on two grounds:-

1. According to the Potato Council Reports, the only cases of Blight so far are in Northern Ireland and South West Cornwall.

2. The markings on the leaf are not indicative of blight, but of drying necrosis, were they blight they would not appear dry and blistering, and with the extent of the patches the whole leaf would be severely chlorotic.

The leaf shows green veins with interveinal chlorosis, and severe interveinal necrosis - typical of magnesium deficiency.

[pieman]

Thanks again everyone for your comments.

Hopefully these pics will make it clearer:

Top of Desiree leaf:



Underside of same leaf:



Bad case on the neighbouring plot:

[OllieC]

They look the same as my swift & rocket every year...

There's a kit advertised in my magazine, claiming to tell you for sure if it's blight... about £6 from Harrods Horticulture. Not sure if it actually works though!

[Kepouros]

I still think it`s not blight.  If all those dark brown markings had been blight lesions most of the foliage would be dropping off by now.   I`ll stick with magnesium deficiency

[Toadspawn]

I would agree that it is NOT potato late blight. It could be a nutrient deficiency/imbalance or just general dying off which is common in early varieties at this time of the year. Potato early blight caused by a completely different fungus shows similar symptoms but it is usually not worth trying to control it..