Cause Rhizoctonia tuliparum, a fungus that survives in the soil for at least 3 to 5 years as sclerotia. After 10 years there may still be 10% viable sclerotia. Sclerotia germinate and infect bulbs after planting or in the very early spring. Can be transported on diseased bulbs. Volunteer plants from previous bulb crops may help fungus survive from year to year. Primarily a problem on tulip and iris but daffodil, crocus, gladiolus, and lily are also possible hosts.
Symptoms Bulbs rot from the top down. The fungus forms felty masses between the scales. Bulbs and roots turn grayish in color and progressively dry and shrivel at the site of infection. Brown to black, flattened sclerotia (1 to 10 mm in diameter) can be found on bulbs and in the surrounding soil. Infected bulbs may not show any growth in the spring. If plant does grow, it is usually slow to develop and withers and dies before flowering. The disease may occur in patches in the field.
- Sort out and destroy infected bulbs.
- If possible remove diseased plants and soil with sclerotia from affected area.
- Use long rotations before replanting tulips again on the same ground. Remove volunteer plants from previous bulb crops. Avoid other hosts such as lilies and onions.
- Deep plowing and shallow planting will also help.
- In display gardens, make sure to use soil that is free of the pathogen to cover the bulbs.
- Use well-drained soilless potting mixes in the greenhouse.
Chemical control Several different options can be used.
- Emblem at 1 to 2 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
- Empress at 1 to 3 fl oz/100 gal water. Group 11 fungicide. 12-hr reentry.
- Medallion WDG was effective on the cultivar Pink Impression when used at 2.5 to 5 oz/1000 ft of row/10 gal water used in the furrow at planting. Group 12 fungicide. 12-hr reentry.
- Moncut 70-DF at 1.63 to 6.53 lb/A as in in-furrow spray directed over the tops of planted bulbs prior to hilling. Do not use spray adjuvants. Washington only (SLN WA-150011). Group 7 fungicide. 12-hr reentry.
- Terraclor 400 at 3 pints/3.2 gal water. Before planting, soak bulbs for 5 min. Add 1% sticker and maintain good agitation. Group 14 fungicide. 12-hr reentry.
- Preplant soil fumigation with metam-sodium.
- Spirato GHN at 2 to 4 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
Reference McGovern, R.J., and Elmer, W.H. 2017. Diseases of Tulip. In McGovern, R.J., and Elmer, W.H. (eds.) Handbook of Florists' Crops Diseases. Springer Int.