Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.
Reprinted with permission from Branching Out, An IPM Newsletter for Trees and Shrubs, Vol 14, No.7 http://branchingout.cornell.edu/. This article was originally printed in the July 13, 2007 issue of the Landscape Alert.
Verticillium wilt (120,121) is caused by a soil-borne fungus, Verticillium dahliaethat commonly attacks maples, especially Norway maple, as well as about45 other woody plants in the Northeast. Verticillium does not affectyews and conifers. V. dahliae survives in the soil as clusters ofthick-walled cells known as “microsclerotia.” Microsclerotia areextremely resistant to drought and cold and can survive for a decade ormore as dormant propagules just waiting for exudates from a suitablehost to wake them up. Then they germinate much like a normal fungusspore and infect plants through the roots. Once in the vascular system,the fungus grows up into the stem and branches, plugging the vessels asit goes and eventually causing leaf wilt and branch death.
Symptoms
Symptoms of verticillium wilt can be confusing because they are sovariable. They include marginal scorch and complete wilting of leaves on individual branches in the crowns of potential hosts. Symptoms canoccur at any time of the year but often show up when hot, dry weatherbegins.
Sometimes a single branch or the foliage on one side of a tree will die. Treescan go through years where no symptoms are present and then the symptoms show up again several years later. Some trees can struggle along foryears, while others may die soon after symptoms appear. In addition towilting, other symptoms may include: small leaves, stunting shootgrowth, poor radial growth, sparse foliage and abnormally large seedcrops. (view photos below)
There are many other factors that can cause leaf wilt and branch dieback. Ingeneral, adverse site conditions and environmental stress such asgirdling root injury, drought, flooding, compaction, deicing salts orgas leaks can cause symptoms similar to verticillium. In addition,canker-causing fungi that grow through the bark and cambium to almostgirdle a branch can cause wilting. Bark beetles, wood boring insects orsapsuckers can attack selected branches in a tree and cause enoughgirdling to result in branch failure as well. The question of whetherthe cause is verticillium or something else is best answered by havingthe sample tested.
Vascular streaking
One reasonably reliable diagnostic feature is that sapwood plugged byverticillium may be stained green (in maples) or brown (in otherspecies). However, this discoloration is not always easy to find because it may occur up to several feet below the point where leaves areactually wilting. If you suspect verticillium, begin looking for thestain in the outer sapwood of wilting branches, but if it’s not there,continue removing small pieces of bark to expose sapwood closer to themain stem. You may have to examine portions of the tree all the way back to the junction of the suspect branch with the main stem. The vasculardiscoloration can vary in prominence depending on tree species andsometimes isn’t visible at all, especially on young twigs.
Diagnosis
Conclusive diagnosis of verticillium requires laboratory culture of thepathogen from symptomatic wood. Other injuries to wood can also causevascular discoloration. Don’t be too hasty to make the conclusion thatverticillium is the cause, because if it isn’t, decisions aboutmanagement and selection of replacement trees will be quite different.The Plant Disease Diagnostic Lab at Cornell, as well as diagnosticlaboratories at other state universities, are able to provide thisservice - sometimes for a modest fee. Fortunately, verticillium doesn’tcolonize its hosts with the speed that some other pathogens, like Dutchelm disease, do. There is time to do the culture, get the results andthen plan a course of action. The sample you send to the lab shouldpreferably be a 1 inch or larger diameter symptomatic branch (but NOTDEAD) that is 8-12 inches long.
Life cycle
V. dahliae can persist in the soil as resting structures called“microsclerotia” for many years without a host. When roots of apotential host are placed near the fungus, exudates from those rootsstimulate the microsclerotia to germinate, and the fungus attempts toinvade the plant. Wounded roots are especially likely to be colonized.
Once infection has occurred, verticillium grows upward within the xylemvessels (the vascular tissue that transports water) of its host, causing the plugging that eventually leads to wilt. It apparently does not grow well from one growth ring to the next, so new vascular tissue,including that surrounding previously infected wood, must usually becolonized by new root infections each year.
The microsclerotia, however, do function as large, durable spores and canbe spread in contaminated soil, on infected roots, or even in someinfected fruit.
Management
There is no fungicide treatment available to control verticillium wilt.However, some other measures may be taken to prolong the life and toimprove the aesthetic value of an infected tree. Management of thisdisease includes proper pruning, watering and fertilizing. If so littleof the crown is affected that branch removal will still leave anacceptable specimen then prune symptomatic branches back to theirjunction with the next largest asymptomatic branch. Severely infectedtrees, which would be unacceptable as landscape specimens aftersymptomatic branches are pruned, might just as well be removedcompletely.
Water during dry periods, especially if they occur in summer or fall.Fertilize if needed with a low nitrogen, high potassium fertilizer.Excessive fertilization apparently increases problems with this disease.
Donot replant in the same site with a verticillum-susceptible speciesbecause the fungus can survive in the soil for years. Instead, chooseresistant or immune trees such as birch, ginkgo, sweetgum, mulberry,willow, hornbeam, hawthorn, honeylocust, crabapple, London plane, oak or mountain ash. Also, where maples are desired, be aware that red andsugar maples are more resistant than silver or Norway maples. Of theNorway maples, ‘Jade Glen’ and ‘Parkway’ have shown more resistance than other cultivars of the species.
There is no guaranteed way to get rid of the fungus once the soil has beencolonized. Where the disease has been confirmed and the tree has beenkilled, replacement with a verticillium-resistant tree may be the onlyfeasible option.
Verticillium spread by woodchips?
Colleagues at the University of Wisconsin have looked at potentialhazards associated with use of wood chip mulch from trees infected withVerticillium dahliae. They discovered that the pathogen will survive inchips for up to one year, albeit at relatively low levels. Furthermore,13 months after Amur maple, green ash, and redbud saplings were plantedin a potting mix amended with chips from a verticillium-infected tree,up to 28 percent of the trees became diseased. While there are stillconsiderable differences between the “real world” application of woodchip mulch and the parameters used in these experiments, the threat ofthe chips is of sufficient concern that arborists are advised not toincorporate debris from verticillium-infected trees into chip pilesplanned for landscape use.
Verticillium as a bioherbicide?
Our colleagues at Penn State have recently made the intriguing discovery that verticillium wilt can be a devastating disease on invasiveTree-of-Heaven (Ailanthus altissima) in forest stands and Christmas tree plantations. In fact, preliminary results suggest that the pathogen may be far superior to traditional herbicides for suppression of the“weeds”. So far, there appear to be no adverse effects on maples, oaks,cherries and other desirable species, but additional work is needed toconfirm that a purposeful introduction of verticillium would be safe for non-target organisms.
