BASIC CONTROL OF DISEASE
By Richard Ernst
Recently, I read some bogus information on diseases, mainly assorted rots, in the AIS bulletin. In an attempt to correct the misinformation, I wrote the correct information and forwarded it to the Editor. It was printed, but without introduction or explanation. So the article didn’t come across very well. Let’s begin again.
I answer somewhere between 100 and 200 letters with questions about rot every year. There are several types, and their prevalence is mainly determined by climate.
Bacterial Soft Rot. This is easily recognized because of its foul odor. Fans fall over and the affected plant parts are soft and mushy. You can treat this rot with Streptomycin, or remove as much rotting material as possible by hand and dust the wound with sulphur or chlorine. This treats the symptoms but not the problem. Bacterial Soft Rot begins when you have too much water, fertilizer (nitrogen) or heat without air circulation, or a combination of these. Too much water is sometimes expressed as inadequate drainage. If you put a head of lettuce in a plastic bag and lay it in the sun, you will create Bacterial Soft Rot. The iris respond the same way.
Prevention is much easier than attempting to cure disease. Give the plants good drainage, and don’t over water. Go lightly on the nitrogen, including manures. Be careful how much water the iris get immediately after fertilizing. Use a fertilizer that contains sulphur, and don’t give any nitrogen except in the springtime.
Be careful not to plant too deeply, as the rhizome can’t breath, and avoid mulch materials that contain green manures, like grass clippings. Have you ever piled up grass clippings? When you pull the pile apart, it’s a steaming, stinking mess. That’s Bacterial Soft Rot, and you don’t want it on the iris. Use a mulch material like chopped straw that provides air and loft. It’s also a brown manure.
Bacterial Soft Rot is more prevalent in climates that are wet and humid or at rainy times of the year, like spring here in Oregon. It is also a companion disease, developing in injuries caused by other diseases. Removal of leaves by tearing or pulling instead of cutting with a sharp knife or shears can create a point of entry and development of soft rot.
Mustard Seed Mold (Sclerotium rolfsii). This is also referred to as Mustard Seed Fungus, Crown Rot, and Southern Blight. The peculiar part about this disease is its persistence. It is soil borne, and over 300 other plants can be host carriers, from beets to wheat. The list is extensive.
You can recognize Mustard Seed Mold usually only after it has progressed to a reproductive stage, releasing “fruiting bodies” or spore sacks that resemble Mustard Seeds with a reddish color. These will be found around the “crown” or top of the rhizome, and around the soil particles near the plant. Eventually, the fan falls over and bacterial rot sets in, making it worse. Without the bacterial rot, the rhizome developes a chalky white look, flaking and cracking like a bad skin disease, and root systems will have a white or greyish webbing like spider-web or cotton strung through them. Treatment is to remove the plant and burn. But if the plant isn’t too far gone, a soak in Terrachlor (PCNB) will kill the fungus, and save the plant. Since Mustard Seed Mold is soil-borne, Terrachlor must be drenched into the ground where the fungus was found before replanting.
The fungus can persist in the soil for four or five years, maybe longer, and when you replant iris there, it will infect them. Sterilization by composting or burning will, of course, eradicate the fungus, but composting may not reach adequate temperatures for total eradication, and burning may not go deep enough into the soil. For this disease, the chemical control seems best.
The disease earned its nickname Southern Blight because of its prevalence in a geographical belt that runs from Tennessee to Texas, but it can be a problem in any climate, from Maine to Southern California.
Botrytis Convoluta. This rot is also a fungus, but unlike Mustard Seed Mold, it is found only on bearded iris. Very little is known about it. In some ways it resembles Botrytis Cinerea, harmless to iris and may appear as a simple grey mold. But extensive research in the 1960’s yielded no conclusive evidence of a link between the two. More likely was the theory that it was the “imperfect state” of another fungus, which remains unidentified.
Botrytis Convoluta can be recognized by a dry, corky, weightless appearance of the rhizome. Often this occurs in early spring when the plants are emerging. For this reason, freezing and thawing temperature variance has been linked to development of the fungus. Accompanying the destruction of the plant is the formation of small black sclerotia, looking like little black brains, although they can appear before the plant’s destruction as well. Typically, in late spring, a grey mold appears, then the Sclerotia form spring through summer, and during freezing and thawing periods through the winter the fungus moves through the plant and causes destruction of the rhizome. Amazingly, often the new increases will show no signs of infection, but do not think this is resistance to the disease. Those new increases, left untreated, may succumb to infection the following season.
Past chemical treatment included Benlate, no longer registered for iris. The approved chemical is Bayleton, and it seems quite effective.
Fungal Leaf Spot is the last of the common diseases I’ll try to cover here. Bacterial Leaf Spot is not so common. Fungal Leaf Spot begins with spots on the foliage resembling wet dots, about 1/8 inch to 1/4 inch in diameter. As the spot ages, the cells die, and the spot turns yellow, then brown. At the same time, the spots spread. They are unsightly, and render leaf surfaces incapable of photosynthesis, so growth slows and the plant weakens.
Leaf Spot is almost never seen in arid climates both warm and cold, like Arizona and Colorado. But in the Northwest it can be a real headache, and the Eastern seaboard shares in this malady.
Treatment is preventative, not curative. Bayleton, Cleary 3336, Daconil and copper-based fungicides are all effective, but zinc-based fungicides, or others like captan have not done what the manufacturer claimed. Spraying must begin early, before spots appear, for the best control.
Fungal Leaf Spot runs a two year cycle, so if you had it last season, you’ll have it this season. Start your spray program early with a copper, then rotate the Bayleton and Daconil about every two weeks, weather permitting, until the days are warm and dry. The sun is nature’s cure, but if you live in an area with rainy summers, the leaf spot can persist. Clean your garden regularly, removing dried leaves and old bloomstalk parts, where spores can overwinter. Keep aphids, thrips and whitefly in check, because they can spread the spores. Spray in early morning so the morning sun dries the foliage. Divide your plants every three or four years. The denser the foliage, the worse the leaf spot. Allow good air circulation and always morning sun.
In the next issue, I hope to cover a few bugs, like aphids, thrips and borers. Certainly the diseases I’ve mentioned are not all the diseases that can attack your iris, but they are the most common, and are controlled without much difficulty. Just be sure of what you have before you treat it.
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