Ordinarily, the dry South Eastern Columbia Basin of Washington/NE Oregon is an ideal place to grow iris. Fairly fertile soil, just adequate rainfall, and little to no leaf spot or soft rot. **BUT** there is the spectre of Botrytis Convoluta that can make annoying to occasionally serious inroads to plantings and iris stock.
Of course, the beastie has a infuriating predilection for the central fan followed by the odorless collapse of the central rhizome. A gray fuzzy mold will subsequently appear. The problems start in mid October and last thru late March to early April, and by then warmer and drier weather seems to suppress the problem. There seems to be a little uncertainty about the exact nature of the pathogen, some saying its a juvenile or latent form of another fungus, but whatever it is or what form it takes, it is a juggernaut that I need to sally forth and combat. So here is what I've learned over the last 15 years of growing iris in this region.
1) Solarization is not a long term solution -- maybe a season but the fungal spore will reinoculate the iris rhizome over time.
2) A soil mulch in winter to prevent freezing and thawing of the rhizome can help SOME, but that method, suggested by Richard Ernst, now deceased, of Cooleys Iris is very impractical for large to very large plantings like mine.
3) Absolute drainage is essential in the beds as well as exposure of the rhizome to summer heat and sun. We don't get quite as hot as the Southwest or desert regions.
4) Foliage trimming after Halloween to 6" and removal of dead foliage seems very beneficial to allow air circulation into the clump and to reduce foliar hiding places for the fungus
5) Bed rotation seems to keep my clumps from getting attacked as badly, but this lasts only one season.
6) Badly infected clumps ive tossed, bagged and/or incinerated. along with the soil, and replaced it with fresh topsoil in the spot where I removed the clump. This is only a stopgap measure and will last only one season.
7) Susceptibility varies widely from variety to variety with the blues, purples and white varieties being somewhat less affected than the plicatas, luminatas, pinks and browns, and cultivars from warmer regions but color and resistance to botrytis is very weak association, but I thought I'd mention it.
8) All new arrivals are planted in sterilized pots (clean with bleach solution) and planted in a steam sterilized commercial potting mix. They spend the first winter in these pots then are planted in the beds the following spring. This prevents some varieties (especially my 50-60$ ones) from succumbing the first winter. The Aussies and Californians seem to be the most susceptible, but all iris from all hybridizers have been affected at one time or another.
9) As loath as I am to use chemicals, there seems to be no way around it. So here is what im experimenting with --
*** disclaimer -- im not making any recommendations on products here, just experimenting and sharing what ive found out. Your risk is your own if you try the products ive mentioned ***
10) Im using an alternating spraying program beginning in early October every two week using Consan 20 as a spray to wet the lower bases of the fans and rhizomes, followed by Greencure Fungicide -- a potassium bicarbonate based remedy. Alternating applications is essential. This pathogen is notorious for building a resistance rapidly to a single remedy.
11) I take meticulous notes on which varieties are hit hardest and make sure they get treated
This is the first year I've implemented this program, and ive seen a 20-30% reduction in the incidence of botrytis occurrences. It may not seem like much but I see this system becoming more effective especially on replanted stock with i'll be doing a lot of this upcoming year. Established clumps seemed to be less impacted by the above treatment regimen, but the new plantings fared much better as far as incidences of botrytis infections. That, I consider important. I plan on switching to more powerful remedies such as Strike 50%WP and Clearys 3336 in the future.
I know this is a long fraught post, but if anyone finds my experiences usual, I hope it helps. My research is still ongoing