fungal diseases
- Subject: [sibrob] fungal diseases
- From: K* W* <k*@cornell.edu>
- Date: Thu, 04 Nov 2004 08:17:38 -0500
Bob & everyone,
Soil funguses which can cause root rots include pythium, rhizoctonia,
phytophthora, fusarium, and a few others. They are all naturally present
in most soils, and if you have a problem with one, it's pretty likely that
the others will be there too. Once they are established as a problem in
the garden, they are very difficult to eradicate, especially if you have to
grow your irises in the same ground year after year. Collectively they
are sometimes called water molds, because they can spread through ground
water or irrigation systems.
Pythium has been the biggest problem for me. I have learned to identify
it by the look of the roots of an infected plant. It causes the roots to
turn dark brown or black. The outer portion of the root will have a watery
look and will slough off, leaving just a thread.
I have achieved only partial control. It depends on using several
different fungicides in rotation; no single chemical controls all of these
species. You have to establish a schedule of treatment that starts early
and continues at three or four week intervals. You can't rely on just one
fungicide or you run the risk of developing a resistant strain.
You can sometimes cure an infected plant if you catch it early. Drench
the soil around the clump with fungicide. Alternatively, you can dig up
the plant, take a division if necessary, wash it in dilute Clorox or Triple
Action 20, pot it up, and drench with a systemic fungicide.
You have to balance the desire to save a plant with the need to yank
infected plants to keep the problem from spreading.
Some of the products I've found useful: Triple Action 20 is a contact
disinfectant also used in hospitals, commercial kitchens, et. (also known
as Consan). It's useful, but doesn't have any systemic action. There is
not much information in the scientific literature about this product, and I
get the feeling it's aimed at consumers, not agribusiness.
Banrot is very useful, because it combines two chemicals which control
different fungi. So it's a broad spectrum treatment. Subdue Maxx controls
pythium; it contains mefenoxam, which replaced metalaxyl which you will
still find a lot of references to. I have a lot of hope for a product
called Rootshield, also available as Plantshield. This is a biological
control based on a fungus which is antagonistic to the nasty ones
(Trichoderma harzianum). It's a preventive not a cure, but it can be
applied at the same time as many systemic fungicides. I got some late in
this last season and can't say yet how effective it will be, but all
reports say it's great.
If you do an internet search on "pythium-ornamental-control" you will
turn up lots of information on these fungi. Look for the university
extension web sites, and some of the manufacturers' web sites. What you
won't find is any reference to Siberian or other irises. I don't know if
it's just under reported, or if I'm cursed by the fates. It has added a
lot of expenses to my hobby, and it definitely takes a lot of the fun out
of growing irises.
Bob, I remember you writing in TSI that some of the Japanese 6-fall clones
are susceptible to botrytis. Can you elaborate on that disease?
Ken
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