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Since Wednesday, May 10, 2006 it has rained almost non-stop here through today, May 15. The forecast is for some rainfall at least through tomorrow, May 16. The fields, gardens and roadside drains are totally saturated with water! Iris growers know all too well the conditions which cause diseases such as bacterial soft rot (Erwinia carotovora), leaf blight (Xanthomonas tardicresens, a bacterial pathogen) and severe outbreaks of crown rot (Sclerotium rolfsii) which is also known as 'mustard seed fungus' and southern blight. Continued rainfall, high humidity and saturated soil are optimal conditions for all of these diseases in the eastern part of the U. S.: And even the northwest is not immune from crown rot and leaf spot.
To make matters worse, severe thunderstorms with high winds flattened many TB and IB spikes which were heavy with buds, or in full bloom last Thursday, May 11. In some cases the tender stem tops were simply snapped off, while the clumps in full bloom were blown over. This occurs at least once in a season here, and in spite of these problems, the irises generally look good, with the promise of a fairly decent season.
As disappointed as I am, it is not a time to relax, but to be on the watch for the above named problems mentioned in the first paragraph. I have found at least three cases of crown rot, and at least that many of what appears to be leaf blight or iris 'rust' (Puccinia iridis/Puccinia sessilis). Rust is a fungal problem, while leaf blight is bacterial. In all cases (strictly in my opinion) the offending problem needs attention NOW!
For crown rot I use a large table spoon to scrape away any visible rotted tissue from the rhizome neck, as well as any yellow, discolored leaves and other debris around the base of leaf fans. For what I suspect is leaf blight (large brownish lesions spreading down leaf margins) I use a pair of garden shears and a bucket of water diluted with household chlorine bleach; a personally favored formula is 4 oz. (1/2 cup) bleach to each gallon of water. Leaf blight is easily spread by cutting utensils, so I dip the scissors after each infected leaf is cut and placed into a separate container for removal and disposal from the garden. There is no known cure for leaf blight: The best practice is to diligently watch and remove any traces of infected leaves from the plants. In dry, sunny weather the condition ordinarily will disappear. For crown rot, I generously sprinkle the household cleaner 'Comet' over the cleaned rhizomes. In addition, I spray a good garden fungicide over the entire plant, especially around the rhizomes.
I am sure other midwest area gardeners have the same problems in prolonged rainy, humid weather. It is not an 'easy fix', but I have had some degree of success with the steps listed above. And I must add there is little we can do about the weather. I know there are others out there who must experience the same or similar problems. It is not a pursuit for the faint of heart.
Entries and Updates
May 16, 2006
This is a particularly nasty disease, which I believe to be Iris Bacterial Leaf Blight (Xanthomonas tardicrescens). It can also affect Japanese & Siberian irises as well. It first appears as water soaked spots which quickly turn brown. The spot lengthens downward along the edges of the leaf. Nearby spots may join together. There is no known cure; the best control is garden hygiene. All cut leaves infected with this pathogen must be kept quarentined, and disposed of OUT of iris plantings. I like to seal them in a plastic trash bag and removed for either burning, or with household trash removal.
May 16, 2006
This picture is of a stem of 'Gala Greetings' which had fallen over from the rhizome. It had begun to rot at the crown of the rz, and resulted in the death of the entire fan. It is recognized by the sickly yellow color of foliage, a very foul odor and mushy rotted rz tissue at the neck of the rhizome. It is called by many Bacterial Soft Rot (Erwinia carotovora), and in some areas might be confused with Crown Rot (Sclerotium rolfsii): Neither of these are welcome in any iris planting! In both cases, affected tissue must be removed out and away from the garden. I dig out all rotted tissue, leaving the affected area exposed, and treat with a generous sprinkling of either Comet and /or garden sulphur. In addition, I spray the healthy foliage with a good garden fungicide such as Daconil. In this picture I had removed the damaged foliage to the compost heap for "picture taking", and to get it away from the clump.
May 16, 2006
Another view of Leaf Blight showing more detail. Note the greasy, watery spots which have not yet turned brown.
Feb 22, 2008
This update is to post a picture of Daconil that I use for help in controlling fungal problems. I realize other gardeners have their own solutions & devices for these problems, and I say whatever works, use it! I am often asked about certain problems; and all I can offer is my own attempted solutions which may or may not work for everyone.
With that said, I want to add my own thoughts of modern tall bearded irises. This is strictly my own line of thinking, but I feel hybridizers have gone the way of "Glitz & Glamour" rather than concentrate on disease resistance & hardiness. I have found there are some cultivars that simply will not grow & bloom here, and are more susceptible to rot & disease than others.
