Cause Verticillium dahliae, a fungus that lives a long time in soil and can infect a wide range of hosts. Rootstocks such as Rosa odorata and 'Ragged Robin' are susceptible; R. multiflora and 'Dr. Huey' have more resistance. R. chinensis var. manetti is very resistant. Under favorable growing conditions plants may be able to tolerate infection.
The fungus grows into the xylem where it colonizes the plant through mycelial growth and conidial production. Fluid movement in the xylem passively transports the conidia. Once in the xylem, this fungus partially blocks water movement and produces toxins that result in wilt symptoms. Wilting occurs under periods of water stress such as mid-summer heat and drought. After diseased plant parts die, microsclerotia form and live several years in soil. Many weeds are susceptible and can help the fungus survive and disperse.
Symptoms At first, leaves near the growing point of young canes wilt, and lower leaves yellow. Sometimes mature leaves become necrotic between veins while the veins remain green. Defoliation progresses from the base of canes to the tip. Permanent wilt, defoliation, and death also may occur. If only a few canes are infected, they may grow normally next season or die back. Symptoms generally are more severe in the greenhouse than in the field. The characteristic vascular discoloration in other plants is not evident in rose.
Sampling Send soil samples to any of various private and public laboratories to assay for Verticillium propagules. Nurseries may wish to test individual core samples to determine the distribution in a particular field. The presence of any microsclerotia in the soil should be interpreted as a potential disease risk.
- Use resistant rootstocks.
- Avoid planting in old vegetable gardens.
- In the greenhouse, use steam-sterilized soil or use a sterile soilless potting mix.
- Remove and destroy symptomatic or dead branches preferably before leaves fall and thus before new inoculum gets incorporated into the ground.
- Clean pruning equipment after use.
- Mulch with conifer-based products as conifers are resistant to infection. General yard waste mulch could contain infected plants and thus inoculum of the fungus.
Reference Hammett, K.R.W. 1971. Symptom differences between rose wilt virus and verticillium wilt of roses. Plant Disease Reporter, 55:916-920.