Cause In most container and bareroot nurseries, the genus Fusarium is ubiquitous in nursery soils, on seeds of several conifer species, and on healthy and diseased conifer seedlings, especially Douglas-fir, western white pine (Pinus monticola), and ponderosa pine. Fusarium commune, however, is the primary species of Fusarium associated with root disease in conifers. Root rot of young seedlings can be caused by Phytophthora or Pythium as well.
Diseases caused by these fungi are generally more serious in bareroot nurseries than in greenhouse container nurseries, where contamination of growing media, temperature, and irrigation are controlled more easily. However, these fungi can become a problem if growing media become contaminated or a pathogen is introduced on seeds or in irrigation water. These fungi are able to survive for various periods as resting spores or mycelium in soil and residual host tissues, and they can be seedborne. They also can persist in styroblocks.
Seedlings are infected when favorable soil moisture and temperature conditions and seed or root exudates are present. Fusarium species may be seedborne and can cause pre- and post-emergence damping-off as well as hypocotyl rot and root rot later in the growing season. Fusarium primarily affects 1+0 seedlings, while Phytophthora affects 1+0 and 2+0 seedlings; symptoms are most apparent in 2+0 seedlings.
The practice of tilling in "green manure" cover crops can aggravate disease problems by providing high levels of organic matter that result in increased pathogen levels. A bare fallow rotation instead of cover crop can significantly reduce pathogens and improve fumigation efficacy.
Symptoms Fusarium hypocotyl rot occurs as sporadic death of emerged seedlings in seedbeds, especially during warm weather. There is a characteristic browning of the stem just below the cotyledons. Fusarium root rot shows as sporadic, patchy death of seedlings in seedbeds. The disease is associated with warm weather and has been called "fourth of July disease." Affected seedlings are reddish brown and typically bent over in a "shepherd's crook." Roots are poorly developed.
Phytophthora root rot in 1+0 seedlings occurs as reddish-brown discolored roots. Aboveground symptoms occur in the spring of the second year and include patchy chlorosis, browning, and mortality in low, poorly drained beds. Roots initially are discolored and eventually rot off completely. A characteristic dark brown stain in the root cambium, often with a distinct margin between healthy tissue above and diseased tissue below, is diagnostic.
Pythium-infected seedlings have stem tissue that separates around the root collar where the epidermis sloughs away from the inner xylem. Fusarium-infected seedlings undergo a softening of the root collar tissue, and the trees fall over without separation of the stem tissues.
Cultural control
- Avoid poorly drained soil.
- Use properly conditioned seeds.
- Rotate planting beds with nonsusceptible hosts such as small grains to reduce fungal inoculum levels. Avoid legume cover crops. Keeping beds weed free and fallow with periodic tilling during interrotational seasons is even better.
- Do not reuse planting containers or thoroughly wash and disinfect them if they are reused. Sanitizing styroblocks by heat, fungicide, or biocide treatment helps control disease problems. Discard older styroblocks because after multiple uses sanitation efficacy is reduced.
- Use well water or filter and/or decontaminate surface water used for irrigation. Irrigate only enough to thoroughly wet the root zone. Water as needed based on water use of the seedlings.
- Plant when soil temperature is warm enough to promote rapid germination and emergence.
- Soil pH of between 5.2 and 5.7 is ideal for most Pacific Northwest conifers.
Chemical control
- Preplant fumigation is the most commonly used control method in conifer nurseries, but cultural practices can reduce the need for fumigation.
- Basamid; see label. 5-day reentry. Restricted-use pesticide.
- Telone II. Rates are based on planting depth and soil type. See label for details. 5-day reentry. Restricted-use pesticide.
- Telone C-17. Rates are based on planting depth and soil type. See label for details. 5-day reentry. Restricted-use pesticide.
- Seed treatment is not recommended, as it has not been very successful.
- Postplant fungicides for Phytophthora and Pythium. Rotate fungicides from different groups that have a different mode of action for resistance management.
- Aliette at 1.25 to 4 lb/100 gal water for conifers in nurseries. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Areca at 2.5 to 5 lb/100 gal water as a foliar application for conifers in nurseries. Group P7 fungicide. 12-hr reentry.
- Banrot 40 WP at 6 to 12 oz /100 gal water. For use in commercial greenhouses. Group 1 + 14 fungicide. 12-hr reentry.
- Empress at 1 to 3 fl oz/100 gal water can be used for seedlings grown in the greenhouse or outdoor nurseries. Group 11 fungicide. 12-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Registered for conifers in commercial nurseries, plantations, forests, landscapes, and parks including Christmas trees. Group P7 fungicide. 4-hr reentry.
- Mefenoxam 2 AQ at 1.23 pints/A as a broadcast spray over beds in at least 50 gal water at seeding or transplanting. Use at 2.45 pints/A as a broadcast spray for newly transplanted 2-year-old seedlings. Do not use as a curative treatment. Registered for conifers in nurseries and plantations including Christmas trees. Group 4 fungicide. 48-hr reentry.
- Subdue MAXX at 1.25 pints/A as a broadcast spray over beds in at least 50 gal water at seeding or transplanting. Use at 2.5 pints/A as a broadcast spray for newly transplanted 2-year-old seedlings. Do not use as a curative treatment. Registered for conifers in nurseries and plantations including Christmas trees. Group 4 fungicide. 48-hr reentry. No restrictions on reentry when used as a soil drench.
- Terrazole 35 WP at 3.5 to 10 oz/100 gal water as a soil drench only at time of seeding or transplanting. For use in commercial nurseries and greenhouses. Group 14 fungicide.12-hr reentry.
- Postplant fungicide drench for Fusarium.
- Cleary's 3336 EG at 8 to 16 oz/100 gal water. Registered for use in nurseries, greenhouse, shade or lath house, and landscapes. Group 1 fungicide. 12-hr reentry.
- Tourney EZ at 1 to 4 oz/100 gal water in a volume of 1 to 2 pints of solution per square foot of surface area. Group 3 fungicide. 12-hr reentry.
Biological control Several biocontrol formulations have been tested for efficacy in preventing or treating damping-off and root diseases in conifer nurseries, but so far none has proved effective.
References Hildebrand, D.M. and Stone, J.K. 2001. Field B Demonstration Comparison of Grass Cover Crop, Bare Fallow, and Dazomet Fumigation at J. Herbert Stone Nursery 1997-1999. FID Technical Report R6-01-01. USDA Forest Service, Forest Service Natural Resources, Portland, OR.
Kim, M.S., Stewart, J.E., Dumroese, R.K., and Klopfenstein, N.B. 2012. Occurrence of the root rot pathogen, Fusarium commune, in forest nurseries of the Midwestern and Western United States. Journal of Phytopathology 160:112-114.