Sunflower plants showing symptoms of Verticillium wilt infection caused by Verticillium dahliae in the field. H. F. Schwartz, Colorado State University, Bugwood.org
Sapwood of a maple tree showing symptoms of verticillium wilt (Verticillium sp.). W. Jacobi, Colorado State University, Bugwood.org
Common hop plants showing foliar symptoms of Verticillium wilt caused by Verticillium albo-atrum on a field in Oregon, USA. D. Gent, USDA Agricultural Research Service, Bugwood.org
Flagging of leaves is a non-distinctive symptom of Verticillium wilt. G. Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Whole-tree symptoms of verticillium wilt in sweet cherry. H.J. Larsen, Bugwood.org
Verticillium wilt is a fungal disease of over 300 host plants, including a wide range of garden and greenhouse crops in addition to woody ornamentals, most noticeably elms, magnolias, maples, redbud, and viburnums. (See the following list for a more complete list of susceptible plants.) Caused by the soil-borne pathogens Verticillium dahliae and V. albo-atrum, these wilts are prevalent throughout the tropical and temperate regions of the world. They exist in the soil primarily as mycelia that infect belowground plant tissue. High summer temperatures tend to halt the development of the disease. Groups of plants resistant to verticillium wilt include gymnosperms, monocots, members of the rose family, oaks, dogwoods, willows, rhododendrons, azaleas, and others.
Symptoms and Diagnosis
Symptoms of verticillium wilt vary somewhat in different host species and also within species due to varying environmental conditions. These might include sudden wilting of small branches, yellowing of foliage, stunting of growth, and premature defoliation. Sapwood of infected branches typically has olive-green to black streaks. Vascular tissue appears as a dark ring in cross sections or pin-point dark spots. The initial symptoms may occur on only one branch or may involve the entire plant. Oddly, following the initial symptoms, there may be no sign of the disease for several years, even though the infection continues to reduce plant vigor.
Verticillium species are opportunistic fungi that persist in the soil as saprophytes. The organism overwinters as mycelia or microsclerotia a dark, condensed mass of mycelium that collectively acts as a propagule, which germinates under favorable conditions. Infection begins in the root area where the resting hyphae of Verticillium germinate and penetrate feeder roots. The fungus also can enter wounds in the root area. The mycelium of the fungus may grow slowly. The disease spreads within the plant by mycelium or spores called microconidia that travel in xylem vessels to other parts of the plant. Where the spores lodge, new hyphae grow and increase the infection. The infected plant tissue becomes necrotic (dead) because the vascular tissue is clogged with mycelium, conidia, and by-products of fungal metabolism. As a result, water flow is restricted, and the plant wilts. The necrotic tissue is what causes the dark streaks that are symptomatic of this wilt disease. The entire plant may die quickly or may die section by section over many years.
Integrated Pest Management Strategies
1. Sanitation. Remove affected annuals and perennials or prune damaged areas of trees and shrubs. Pruning disease-damaged branches and foliage plus increasing the vigor of trees and shrubs may help to keep symptoms checked. Be sure to sterilize pruners between cuts.
2. Plant resistant or tolerant species. This is the best way to manage this disease. Fungicides are not generally effective or practical.
Following is a list of susceptible and resistant plants taken from "Plants Resistant and Susceptible to Verticillium Wilt", Leaflet 2703, University of California.
PLANTS SUSCEPTIBLE TO VERTICILLIUM WILT
PLANTS RESISTANT OR IMMUNE TO VERTICILLIUM WILT
Both of the recommended IPM strategies are strictly organic approaches.