Verticillium wilt: how to diagnose it, what to do about it
Verticillium fungi infect all kinds of plants, blocking their sap flow. This leads to overall wilting, yellowed leaves, dead twigs and branches, and poor crops yields. Those heirloom tomatoes you've fallen in love with? Quite prone to Verticillium. Those resistant new hybrid tomatoes? Losing their edge against new, tougher strains of Verticillium. There is no fungicide treatment available to home gardeners for this disease. Have I got your attention yet?
Verticillium wilt is a disease caused by one of two specific fungus in soil. Gardens across the US and Canada can harbor these fungi. They like conditions we'd probably describe as "very pleasant for our plants" but can withstand years of deprivation. Verticillium will survive adversity and become active when conditions favor it again. It flourishes in the warming soil temperatures of spring, enters a plant, and collapses its cells so that the first onset of hot weather brings "sudden" sickness.
Vegetables, annuals, perennials
Trees and shrubs
Sudden wilt on hot days
Wilting of leaves on just part of the plant, not the entire plant
Yellowing and dying lower leaves
Yellowing of leaves in a V shaped patch between veins
Light brown discoloration in plant stems
Dead twigs in a limited area of the tree/shrub
Branches with few and smaller than normal leaves
Bark splits, bark seperates from wood
Single branch, or entire specimen, dies over period of weeks
Discolored or scorched leaves
Brown color in a ring (cutting sick branch crossways) or streaks in thw ood (when sick branch is cut lengthwise)
* While these are typical of Verticillium infection, other conditions can cause similar symptoms. Look for and rule out other possible problems. Absolute diagnosis of Verticillium wilt needs laboratory analysis.
There are thousands of species of fungus in normal garden soil. The majority are benign or helpful to trees and flowers. Some are harmful to green plants. Verticillium dahliae and V. albo-atrum are known to infect many of the plants we find useful and pretty. The fungus invades the roots of plants. Then it clogs up the circulatory system of the plant. This "arboreal atherosclerosis" causes wilting and yellowing as the plant cannot deliver needed sap to all parts. The fungus favors mild soil temperatures around 70 degres, but the symptoms may appear more severe in hotter weather. That's when daytime heat extracts more moisture from the plants extremities than the clogged plant can provide.
Susceptible plants and trees
Unfortunately for us, Verticillium can infect a wide range of ornamental specimens and food plants. The majority of vegetables are infected to some degree. Among trees, maples and redbuds are generally very vulnerable to Verticillium. Many other trees can be infected. Quite a few favorite shrubs, from azalea to Viburnum, can suffer this ailment. Perennials and annuals can be stricken; many aster family plants are at risk from Verticillium, as are Dahlias, as you may have guessed from the species name of one of the offending organisms.
Is anything safe?
The situation sounds dire, and indeed there are lists of plants of all types that can fall prey to this wilt; see the links in Resources below. Fortunately, other plants shrug off Verticillium; see the same links for their lists of plants resistant or immune to this disease.
Monocots (plants in the "grass" family) do not suffer from Verticillium wilt. Monocots have generally elongated leaves with veins that run lengthwise on the leaf. This group includes loads of plants that may not instantly come to mind as grasslike. All kinds of Lilies are members of this tribe. Iris and Hostas are, too. In the vegetable garden, asparagus and any onion type crop (onions, chives, garlic, leeks) are monocots, and thus safe from Verticillium. In shrub and tree size specimens, the selection narrows greatly. Bamboos and palms are the large members of this group.
Common evergreens with needle like, evergreen leaves are safe here. They are "gymnosperms'" and members of this group do not suffer from Verticiloium wilt. Pines, junipers, and yews can be considered safe from Verticillium. And though a Ginkgo tree is deciduous and not "needle leafed," it is a gynosperm and thus also safe.
Garden practices to avert Verticillium
~ Crop rotation is a tried and true, traditional method of disease control. When plants appear to have Verticillium, be sure not to plant Verticillium prone plants in the same spot the following year. Tomatoes, peppers, eggplants, potatoes, strawberries and raspberries are all prone to similar Verticillium strains. Break the cycle of verticillium by not planting any of these crops in the same spot year to year. In the years between, if possible, plant grasses or grass family crops. Keep the area weeded so verticillium cannot harbor in alternate host plants. Asparagus and all alliums are resistant. Sweet potatos, beans and peas, lettuce, and carrots are less prone to this wilt.
~ Choose tomatoes of resistant hybrid varieties. Catalogs and labels should indicate resistance; a " VFN resistant" cultivar resists Verticillium, Fusarium, and nematodes.
~ Burn or destroy (do not compost) any clippings or leaves from sickened plants. When the entire vegetables or flowering plants are removed, include the rootball. Disinfect tools after using them on verticillium-affected plants.
~ Good care keeps plants and trees in tiptop shape. Then they are most able to resist infection. Attention to soil fertility and proper watering reduces the stress that makes plants most vulnerable. Use proper planting methods to prevent the root damage. Damaged roots are more easily infected.
~ Extent of infection on trees and shrubs is subject to several variables. Prune out affected branches from trees and shrubs. Limited branch loss on woody plants does not necessarily dictate removal of the entire specimen. When a woody plant must be removed due to Verticillium wilt, do replace with a resistant choice.
Resources: Publications with extensive lists of prone and resistant species
"Verticillium Wilt", Kemper Center for Home Gardening, Missouri Botanical Garden, http://ohioline.osu.edu/hyg-fact/3000/3122.html, accessed 8-12-2012
" PLANTS RESISTANT OR SUSCEPTIBLE TO VERTICILLIUM WILT", Publication 2703, University of California, Agriculture and Natural Resources, http://depts.washington.edu/hortlib/resources/ucdavis_verticillium.pdf accessed 8-12-2012