Contents Fungus Diseases - - - ------ - - ---- - - ~ - -- -- - ----- --- - --- ----- -- - 3 MOSAIC AND OTHER VIRUSES Iris Severe Mosaic (Yellow or Latent Virus Diseases ----------------------------------------------- 3 Mosaic or Gray Disease) -------------------------- 17 Nematode Diseases ----------------------------------------- 3 Bacterial Diseases ------------------------------------------ 3 MISCELLANEOUS DISEASES AND CONDITIONS Physiological Diseases ------------------------------------ 3 Nematode (Eel worm) --------------------------------- 19 General Control Program ------ -- ------ ---- ------------- 4 Black Storage Molds ------------------------------------ 20 Black Tip _______ _____ ____ ___ _____ ------------------------------ 21 MAJOR DISEASES Botrytis Blight ---------------- --------------------- ----- -------- 21 Blindness (Three Leaf) 4 Suggestions to the Grower ____ ____ ___ ___ _______ __ _ 5 Gray Bulb Rot ----------------------------------------------- 21 Suggestions to the Forcer -------------------------- 6 Pythium Root Rot ----------------------------- ------------ 21 Summary of Forcing Problems _____ ______ ___ ____ _ 7 Rhizoctonia Neck and Bulb Rot _____ ___ ____ _____ __ _ 21 Blasting ------------------------------------------------------- __ 7 Rust -------------------------------------------------------------- __ 22 Bacterial Blight ___ ___ _- ------------------------------------ 8 Sunburn -------------------------------------------------------- 2 2 Black Slime (Black Rot) --------------------- -------- 9 Blue Mold ----------------- ---------- --------------------------- 10 Topple ( Sugarstem) ------------------------------------- 22 Crown Rot (Southern Wilt) ________________________ 11 Other Diseases ----------------------------------------- ----- 22 Fusarium Basal Rot -------------------------------------- 13 Key to Narcissus Diseases ---------------------------- 2 3 Fire (Leaf Spot) ------------------------------------------ 14 Ink Spot -------------------------------------------------------- 16 Much of the information in this bulletin resulted from cooperative research with many individuals, particularly Vernon L. Miller and Neil W. Stuart with the assistance of Worth Vassey. Financial support was provided by the Washington State Bulb Com­ mission, Northwest Bulb Growers Association, and many chemical companies. We are indebted to these and to the following for their review of all or parts of the manuscript: Drs. T. C. Allen, A. A. Brunt, L. W. Moore, D. Price, A. R. Rees, and N. W . Stuart. Issued by Washington State University Cooperative Extension, J. 0. Young, Director, and the U.S. Department of Agriculture, in furtherance of the Acts of May 8 and June 30, 1914. Extension programs are available to everyone without discrimination. Published June 1979. 2 DISEASES OF BULBOUS IRIS CHARLES J. GouLD, Plant Pathologist, Emeritus, and RALPH S. BYTHER, Extension Plant Pathologist, Western Washington Research and Extension Center. The mild, moist climate in western Washing­ like growths within a bulb or diseased plant tissue, ton is ideally suited for bulb production. Wash­ or as hardened lumps of fungus tissue in the soil ington is the leading producer of bulbous iris in where the bulb grew. These masses of weather­ the United States. Millions of bulbs are harvested resisting fungus tissue are usually rounded and annually to be used for the production of cut hard and are called sclerotia. flowers. These flowers are produced by either forcing them in greenhouses or field planting in Virus Diseases coastal areas. Twenty to thirty million are also shipped annually to England and Europe where Virus particles are so small that they can be they are used for early forcing because of their seen only with an electron microscope. When they high quality. Production is confined mostly to gain access to a plant, they usually penetrate into popular greenhouse forcing varieties, particular1 y every part except the seed. Therefore, whenever Ideal, \XT edgewood, and Blue Ribbon. a daughter bulb is taken from a virus-infected Although Washington iris are recognized as plant, it is usually also infected. Certain insects, some of the world's best, they are not without such as aphids, which feed on plant sap, can spread their disease problems. This bulletin describes most some virus diseases. of these problems and outlines control measures that can be used to minimize losses. Nematode Diseases For additional information see Diseases of Nematodes are very small thread-like worms. Bulbs by W. C. Moore, Ministry of Agriculture Many commonly live in the soil without doing and Fish, London 1949 (a revision is in press). any harm, while others attack various plants. The Another reference is Ziekten en Afwijkingen bij bulb nematode and root-lesion nematodes which Bolkewassen, Deel 2: Amaryllidaceae Iridaceae, affect iris are too small to be seen without a micro­ e.a., 1978, Laboratory for Flower Bulb Culture, scope. Nematodes survive from season to season Lisse, Netherlands. It includes iris in addition to as eggs. They also can survive for several years ·certain other bulbs. Information on insects and as worms in bulbs or in infected plant tissue. Any culture is contained in the Handbook on Bulb movement of soil or infected plant parts can be Growing and Forcing, edited by C. J. Gould in responsible for the spread of nematodes. Farm 1957. It is available from Xerox University Mic­ equipment, irrigation, flooding, and planting In­ rofilms, 300 North Zeeb Road, Ann Arbor, fected bulbs are common means of spread. Michigan 48106. 13acterial l)iseases Fungus Diseases Bacteria are simple, microscopic, usually single­ Many important diseases of iris are caused by celled plants that depend upon other plant or fungi. Fungi are small and rather simple plants animal sources for their food. Most are beneficial unable to produce their own food because they and only occasionally do they cause trouble in iris. do not have the green coloring matter ( chloro­ Spread and survival depend on the presence of in­ phyll). Rather than manufacture their own food, fected plants or plant tissues, since the bacteria they feed on living or dead plants and animals, can not survive long in soil. resulting in disease or decay of the tissues they attack. Fungi produce very small seed-like bodies called spores. These microscopic spores can be Physiological Diseases spread by splashing rain, by wind, or carried along All problems which are not caused by para­ with the movement of soil or infected plant debris. sitic organisms are considered in this group. Bulb­ Fungi survive from one season to the next ous iris are quite susceptible to unfavorable weath­ either as dormant spores in the soil, or as thread- er. Frost may injure their leaves, the sun may burn 3 the bulbs, and undesirable temperatures may pre­ kills nematodes, insects, and many fungi, including vent flowers from forming, or ruin them after crown rot and black slime. Therefore a regular they have formed. Because of this, they are cor­ HWF treatment of planting stock is recommend­ reedy considered one of the most difficult of all ed. Certain varieties may be injured by this treat­ bulb crops to grow commercially. ment, thus information on how each variety re­ acts is necessary before large-scale treatment is attempted. A General Control Program Recommendations to control disease and in­ Specific fungicides are not listed in this pub­ sect pests of bulbous crops are revised annually lication because materials recommended constantly on the basis of current research results. These are change. Specific recommendations are contained published by Washington State University as EM in EM 4314, which is revised annually or as need­ 4314 for iris diseases, EM 4313 for narcissus dis­ ed. Suggestions for a general control program are: eases, and EM 4315 for tulip diseases. They are 1. Always plant well-graded stock of known available from your local County Extension Agent disease-free ancestry. Establish a mosaic-free in Washington. and nematode-free foundation stock. 2. Either discard or plant moderately and se­ verely diseased stocks separately. Give these diseased plantings special attention, such as intensive roguing, spraying, etc. Dig and treat them before the major digging commences. Blindness (Three-Leaf) 3. Plant on heavy (silt loam or heavier) well­ Symptoms drained soil on which iris have not previously Blindness is the failure of flowers to develop been grown for 3 or 4 years. A void tighter from normally flowering-sized bulbs. Only three sandy soils. Ridge the rows. leaves are produced instead of the usual five. 4. Spray with a fungicide if leaf spot is a prob­ This condition is sometimes confused with blast­ lem (see EM 4314). ing, in which case buds form but sooner or later 5. Rogue all mosaic-infected plants. Rogue on die (see p. 7 ) . the basis of symptoms on central leaves of new growth ear I y in the season and on flower Importance and Host Range symptoms at the time of blooming. Blindness is found throughout the world. It 6. Remove crown rot-infected plants. Also re­ primarily occurs in forcing greenhouses and is sel­ move the infested soil or sterilize the infested dom seen in the field. It is most prevalent in area. forced bulbs which have been produced the sea­ son before under cool, cloudy conditions, or not 7. Dig nematode-infected stocks 1 or 2 weeks given the proper temperature treatments, or pre­ earlier than normal and treat as recommend­ cooled too early. ed in EM 4314. 8. A