CULT:Southern Blight and Leaf Spot

From: <HIPSource@aol.com>

In a message dated 98-08-17 00:29:54 EDT, you write:

<< I turned on the Telly, to watch my favourite gardening show 
 "Gardening Australia". The segment showing was about leaf spot, and I 
 was greeted with a close up of a petri dish, filled with white fluff and 
 dark brown spores - the mustard seed fungus.  >>

Not to contradict here, but I have never seen Southern Blight causing
'spotting' as we generally think of it on irises. Columbines yes, irises no.
Here the leaves turn yellow and fall over. This blight attacks the base of the
fans at the soil line and moves inwards, severely weakening or killing the
plant or making it vulnerable to other fungi including Erwinia carotovora,
which causes that soft rhizome rot of the loathsome stench. Which is not to
say that other leaf symptoms may not occur at some point, but, in general, the
two iris leaf spots we usually see here are a bacterial one, and a fungal one
caused by the pathogen Heterosporium gracile, which is also known as
Didymellina macrospora. I would assume that if someone had leaf spots and the
fans were not rotting or pulling apart at the bases, Southern Blight would not
be involved.

That said, the World of Irises, the classic book published by and available
from the American Iris Society which should be on everyone's bookshelf even if
it is getting a bit dated and is not what you might call a charming read,
notes that the fungus of Southern Blight is found living in the soils
worldwide in tropical and semi-tropical regions. In the USA, it is generally
thought not to flourish above the Mason Dixon line. 

Anner Whitehead, Virginia Zone 7
HIPSource@aol.com

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