Downy Mildews

Plant diseases
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Chris Yeates
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Downy Mildews

Post by Chris Yeates » Fri May 29, 2015 9:01 pm

I wonder if Adam could change the sub-forum to Rusts, Smuts and Mildews?
This would then include the powdery mildews which are ascomycete fungi, and downy mildews - which are not fungi at all (septa rare or absent, cell walls composed of cellulose rather than chitin etc.). In fact it is now thought that downies are most closely to certain groups of brown algae, but analogous to the myxo's mycologists study them so that's fine with me.
As with many of these parasites a great advantage is that if you have a decent botanical knowledge you can dive straight in and make useful discoveries. Most downy mildews are restricted to a plant genus, in a few instances to a family and in a growing area molecular and other studies are showing that quite a number are species specific.
These organisms are extremely under-recorded all the more reason to look out for them. So keep an eye out for yellowed areas of otherwise healthy leaves, if a DM is present the sporangia almost invariably will appear on the corresponding underside of the leaf. Under a lens they appear like tiny trees - one just has to beware aphid damage which can look similar, a handlens is essential.
For starters I would recommend checking any suspiciously pallid-looking shoots of goosegrass Galium aparine; its DM Peronospora aparines is abundant at the moment. Also yellow patches on greater plantain Plantago major on muddy tracks etc will more often than not produce Peronospora alta.
A couple of examples here: Peronospora arthurii while still with few records seems to be increasing on evening primroses Oenothera spp. Brian Spooner wrote an article about fungal parasites on Oenothera in Field Mycology in 2013: "Such leaves were first observed in Surrey in June 2012 and subsequently in South Somerset in July and S.W. Yorkshire in September". I have added further sites to that Yorkshire sighting - it's interesting how it appeared (or was discovered) in widely separated sites around the same time.
Perart0b2.jpg
Peronospora dipsaci is striking with the sporangia often covering almost all of the large teasel leaves; this image shows the forked sporangiophore "trees" quite well. Recently added to the Yorkshire list there are relatively few records all (?) from the southern half of England: https://data.nbn.org.uk/Taxa/NBNSYS0000015414
Perdip0a2.jpg
Finally Peronospora myosotidis; a recent collection inspired me to write this post. Obvious in the field, with abundant sporangia and also lots of the thick-walled reticulate oospores (resting spores); one does not often come across them in such quantity.
Permyo0b2.jpg
enough (more than enough I hear you say) for now

Chris
"You must know it's right, the spore is on the wind tonight"
Steely Dan - "Rose Darling"

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adampembs
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Re: Downy Mildews

Post by adampembs » Fri May 29, 2015 9:07 pm

Hi Chris, I requested an ID for something which people said was a mildew. Its in my garden on Borage, I can look under the scope if I know what to look for :-)
http://fungi.org.uk/viewtopic.php?f=4&t=5

wrt change of name, it seem right, although the asco mildews wouldn't know whether to go here or to the asco forum. :)
Adam Pollard
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