Extension Bulletin No. 424 June 1950 Diseases of Bulbous Iris Extension Service Institute of Agricultural Sciences The State College of Washington Pullman. Washington Key to Iris Diseases Look first at this key to identify tentatively any particular iris disease, and then to the page indicated for a more complete description. More than one disease may occur on plants and bulbs at the same time, in which case the symptoms may overlap. LEAF SYMPTOMS DISEASE PAGE Outer leaves mottled, young inner ones not mottled .... Winter Injury 6 All leayes mottled, most evident on inner ones ............ Mosaic ...................... 6 Leaf tips black ...................................................................... Black Tip ................ 9 Gray or grayish-brown spots with a dark border.. ........ Lea£ Spot (Fire) ... .lO Black spots ............................................................................ Ink Spot .................. 12 Dark green or brown spots ................................................ Bacterial Blight ...... 14 Leaves yellowed .......... Usually one of the bulb rots. See below. FLOWER SYMPTOMS Discolored streaks in flowers, may be .. .......................... Mosaic ...................... 6 Flowers lacking (in blooming-sized stock) .................... Blindness .. ... .. ......... 15 Flowers present but prematurely dead or wilted .......... Blasting ............. ....... 17 BULB SYMPTOMS Tan sunken areas with brown sclerotia often present..Crown Rot .............. 18 Base shrunken, often covered by white or pinkish mold .... Basal Rot ................22 Basal plate gray, often surrounded by a dark crevice; dark streaks in fleshy scales sometimes present.. ...... Nematode ............... .24 Tan sunken areas with blue mold often in center.. ...... Blue Mold ............. .27 Black irregular patches on husks ...................................... Ink Spot .................. 12 Minor diseases ............................................................................................................ 30 Unless otherwise indicated, the photographs are of the ,-ariety W edgewood. 2 Diseases of Bulbous Iris By CHARLES J. GOULD* Associate Plant Pathologist Western Washington Experiment Station Because of the mild and moist climate in western Washing­ ton, this state has become the leading producer of bulbous iris. Production is confined largely to one variety, Wedgewood, which is popular for forcing in greenhouses. Although our iris are recognized as the world's best, they do have their troubles at times. Most of these troubles are caused by one or more of the following maladies: FUngi These are small, simple plants which lack the power plant (chlorophyll ) necessary for using sunlight to produce their own food. They must, therefore, feed on dead or living plants or animals. Fungi are often spread by small, seed-like bodies called spores. And they may live from one season to the next either on the inside or outside of plants as hardened masses of fungus tissue called sclerotia. Many, but not all, of our bulb parasites form these sclerotia. Viruses Viruses are protein-like particles, so mall we cannot see them with an ordinary microscope. They resemble fungi in that * The auth01· wishes to express hi s sincer e appr eciation to the following fo t· their h elp: Dr. F olke J ohn son, Dr. L eo Campbell, Dr. E. P. Breakey, and Mr. vV. D . Courtney (U.S.D.A.) at the W estern Washington Experiment Station and Dr. G. A . Huber (formerly at the Western Washington Experiment Station); Dr. M. R Harris and Dr. G. \V. Fischer a t The State College of W ashing ton, Pullman; Dr. S. M. Dietz and Dr. F. P . McvVhot·ter a t Ot·egun State College, Col'Vallis; Dr. vVm . New­ ton, Mt-. R. J. H astings, and Mr. J. E . Bosher a t the D ominion Laboratory of Plant Pathology (Saanichton, B. C.); Dr. A. A. F oster, Dr. A . vV. Dimock, and Dr. L. F. R andolph at Cornell niversity ; Dr. eil W . Stuart, Dr. W. D. McClell an, and Dr. Philip Bt·ierley (U.S.D A. ) at the Plant Industrv Station, B eltsville, M a ryland; Dr. C. F. D oucette (U.S.D.A.) at the E ntomological L a bor ator y, Sumner , vVashin gton; Mr. J, S. \Vieman and associates ( Oregon State D epartment of Agriculture); Mr. G. \ V. Eade ( Washington State D epartment of Agt·iculture) and Mr. H. J_ R eynolds (form erl y w ith '\Ala hington State D epartment o f Agriculture); Iris gmwet·s-Francis Cher venka, R. G. Fryat·, C_ vV. Orton , H. D. Edmonson, E. C. Orton, Ben K a rsten, Jr., S. Van Lierop, and J ohn Onderwater. n,-_ G. A. Hubet· took the photograph of the bac terial disease illustrated on page 14. 3 they cannot make their own food but must rely upon living plants or animals for it. Infected plants usually contain the virus particles in all their parts. Thus, whenever a "daughter" bulb is taken from an infected plant, the virus or "disease" travels along. Nematodes These are very small, thread-like worms. Many types com­ monly live in the soil without doing any harm, while others parasitize various plants. The nematode (or eelworm, as it is sometimes called) attacking iris is too small to see without a microscope. Physiological Disturbances A ll troubles which cannot be traced to some parasite are considered in this group. Bulbous iris are quite susceptible to bad weather. Frost may injure their leaves, the sun may burn the bulbs, and undesirable temperatures may prevent flowers from forming, or ruin them after they have formed. Because of this, they are correctly considered one of the most difficult of all bulb crops to grow commercially. Sanitation Burn or bury all diseased plants, bulbs, and infested debris well away from all bulb fields and sheds. If you use a pit, sprinkle discarded plants with lime and cover with dirt at the end of each day's operations. 4 AGeneral Control Program 1. Always plant well-graded stock of known disease-free ancestry. Establish a mosaic-free and eelworm-free fo un­ dation stock. 2. P lant on heavy, well-drained soil on which iris have not previously been grown for 3 or 4 years. Ridge the rows. 3. Spray with homemade Bordeaux if leaf spot is trouble­ some. A dd Penetrol as a wetting and sticking agent. 4. Rogue all mosaic-infected plants. R ogue on the basis of symptoms as found on central leaves of new growth early in the season and on the basis of flower symptoms at the time of blooming. 5. Remove crown rot-infected plants and infested soil or sterilize the infested area. 6. Dig nematode-infected stocks 1 or 2 weeks earlier than normal. 7. · Avoid bruising in all operations. 8. Store bulbs under dry, airy conditions. 9. Discard all infected and moldy bulbs after digging and again before planting. 10. Treat nematode-infected stocks in a hot water-formalin bath for 3 hours. Cool rapidly after treating. Treat after the bulbs are cured, but before A ugust 15 in normal years in the Puyallup Valley. 11. Cure salable stocks with heat soon after digging. 12. Treat phnting sto: k with Tersan oon after cleaning. Dry or plant immediately . TERSAN TREATMENT Increa ed yields by dipping iris bulbs in Tersan have been obtained in several experimental tests at the Western Wash­ ington Experiment Station, and in some large-scale commercial trials by local growers. The experimental treatments showed that both healthy and diseased (crown rot and basal rot) stocks of Wedgewood and DeWitt varieties increased in yield. The "why and wherefore" of these increases are not yet known, but occasionally have been as high as 25 per cent over the untreated sets. The planting stock should be dipped for 5 to 15 minutes, as soon as cleaned, in a suspension of 1 pound Tersan in 9 gal­ lons of water. Either plant immediately or dry rapidly . to pre­ vent the roots sprouting. Tersan is safe on bulbs, but · ·it may irritate the skin of certain persons. Handle according to the manufacturer's recommendations. 5 t~osaic c Fig. 1. MOSAIC. A-Healthy plant. B-Infected plant. C-Different types of leaf symptoms. SYMPTOMS Leaves and stems. Plants stunted, sometimes severely; stems often weak; yellow or yellowish streaks against nor­ mal color or patches of normal green surrounded by yellow background. Symptoms strongest in bud sheaths and lower part of young leaves growing vigorously under cool condi­ tions, becoming less noticeable as temperature rises and the plants mature. Healthy plants of certain varieties, especially \ Vedgewood, normally show a slight mottling which be­ comes more evident with the maturing of older leaves. Winter injury and sometimes drought cause a regular pattern of flecks or squarish markings of white or pale green, usually confined to older leaves. The mottling from mosaic does not usually follow a regubr pattern and is most evident on the youngest leaves. Flowers. Dark teardrop markings on white, blue, and lav­ ender fl owers; clear feather-like markings on yellow fl owers. The ribbon-like markings, sometimes present on flowers, are apparently not virus-caused. On bud sheaths, bluish-green blotches on a pale green background, or, less often, yellowish 6 streaks. Not all infected plants show both leaf and flower markings. Mosaic symptoms develop more strongly in the green­ house than outdoors. Cut flowers from greenhouse-grown, mosaic-infected plants are poor in quality and do not last long. Bulbs. Often fewer in number and smaller in size than normal. IMPORTANCE World-wide; major, especially on many Dutch, Spanish, and Tingi tana types, including the variety W edgewood; spread by aphids in field and perhaps in storage; not spread by picking flowers. CONTROL Mosaic control in iris involves a special planting program for two reasons : first, diseased plants may not show symp­ tom's the season they are infected; and, second, mildly in­ fected plants are difficult to identify and eliminate. Emphasis should be placed upon establishing and maintaining a di­ sease-free foundation stock from which all planting stock can be selected.