As we move into late July and August, I typically see an increase in sample submissions for vascular wilt testing. Vascular wilts are diseases where the pathogen (typically fungal or bacterial) invades the water-conducting tissue (called the xylem) inside a plant and leads to blockage of this tissue. The blockage prevents water from moving from the roots to above-ground parts of the plant. Without water, the plants wilt and typically eventually die.
In late July of 2017, I completed what I affectionately refer to as the “Triple Crown” of woody ornamental vascular wilts, confirming diagnoses of Dutch elm disease, oak wilt and Verticillium wilt all on the same day.
Dutch elm disease. Remember the days when streets were shaded by cathedral canopies of American elms? Nope, I don’t either. At 55, I arrived at the tail end of the period when virtually every street in the US was lined with American elms and when Dutch elm disease (DED) was in its heyday of killing pretty much every American elm in sight. DED is a prime example of what can happen when a particular type of plant is grown in monoculture (i.e., in large numbers in close proximity to the exclusion of other plants) and a non-native (i.e., invasive) pathogen is introduced.
Two fungal pathogens (Ophiostoma ulmi and Ophiostoma novo-ulmi) cause Dutch elm disease and are thought to be of Asian origin. They appear to have been introduced into US through Europe via imported wood during the late 1920’s to early 1930’s ( ulmi) and the 1940’s (O. novo-ulmi). These fungi were subsequently spread by elm bark beetles (both imported European and native North American species) which introduced the fungi into (very susceptible) American elms as they tunneled into the trees to lay eggs. To make matters worse, elms along American streets were root grafted (i.e., their roots were fused together), so the DED fungi, once introduced into an area by bark beetles, were able to rapidly move from tree to tree underground via these grafts. Thus the disease decimated street after street of American elms across the US.
Elms (including true American elms) still exist in urban landscapes. Some are “escapes” (American elms that are susceptible to DED, but in some way have avoided infection), some are true American elms that have been bred for resistance, and others are hybrids (usually American elms crossed with Asiatic elm species) again bred for DED resistance. For large, susceptible American elms, routine (about every other year) fungicide injections can be used to manage DED. Keep in mind however, that no management strategy is perfect and even resistant and treated elms can succumb to DED.
Oak wilt. I often think of oak wilt as the “Dutch elm disease of oaks” because there are many similarities between the two diseases. The oak wilt pathogen (the fungus Ceratocystis fagacearum) is often initially introduced into an area via insect (several types of sap beetles can be involved). These insects are attracted to wounded trees and, if they carry the oak wilt pathogen, can drop spores of the fungus off as they fed on sap oozing from wounds. Because of sap beetle transmission, I typically recommend pruning oaks only when they are dormant. In particular, the “January thaw” period in mid-winter is a good time to prune because it is warm enough so that you will not freeze to death while pruning, but not warm enough that the oak wilt fungus and sap beetles will be active. If for some reason you need to prune an oak during the growing season, you should IMMEDIATELY paint over pruning wounds to make them unattractive to sap beetles (which can visit wounds within 10 minutes of their formation). You can use a commercial pruning paint if you like, but latex paint (left over from painting the rooms of your house) will work as well.
Once the oak wilt fungus colonizes a tree, branches will begin to wilt and the tree will eventually die. In wooded areas, the oak wilt fungus can move from oak tree to oak tree via root grafts, causing major tree loss. Management of oak wilt in woodlots and forested areas typically involves establishing a perimeter around infected trees, trenching around this perimeter to sever root grafts and then removing oak trees within the trenched area. In landscape settings, single, healthy specimen oaks can be treated with fungicide injections (typically made every two years) to help prevent infection.
Making oak wilt management a challenge is the fact that some people diagnose oak wilt based on visual symptoms. I do not agree with this method of assessment, and personally will only diagnose oak wilt if I can grow the oak wilt fungus from symptomatic oak branches or trunk tissue, or detect Ceratocystis fagacearum DNA in this tissue. The danger of visual diagnosis is that there are other diseases and insect problems that can cause branch dieback symptoms that somewhat mimic those of oak wilt. In particular, I worry about misdiagnosis of Armillaria root disease as oak wilt, because trenching (advocated for oak wilt management) wounds roots and wounded roots are a primary entry point for Armillaria (the fungus that causes Armillaria root disease). Another problem that mimics oak wilt is damage due to two-lined chestnut borer, an insect pest that tends to attack oak trees that are under stress. Fungicide injections for oak wilt management are a waste of time, effort and money if the real problem is an insect pest such as two-lined chestnut borer. Proper diagnosis is the first step in developing a successful disease and insect management strategy for oak wilt (or any disease for that matter).
Verticillium wilt. Verticillium wilt is disease that can affect a wide range of woody ornamentals including, but not limited to, maples (particularly Japanese and Korean maple), ash, redbud, magnolia, and smokebush/smoketree. The disease can also cause problems in vegetables (e.g., potato, tomato, pepper, vine crops and especially eggplant) as well as herbaceous ornamentals (I diagnosed Verticillium wilt in purple coneflower just recently). Verticillium (typically Verticillium dahliae) is soilborne and can be introduced into a location via contaminated soil, mulch (be cautious of using mulch composed of chipped up street trees that might have died from Verticillium wilt) or even leaves that have fallen from infected trees and been blown into an area. The fungus infects through roots, colonizing and blocking the xylem, resulting in branch dieback. In particularly susceptible trees (e.g., Japanese maples) and vegetables (e.g., eggplant), death can follow very rapidly. Proper diagnosis of Verticillium wilt is important because if Verticillium is present at a location, use of Verticillium immune or resistant plants is the best method to prevent problems in the future. That said, over the past three years, the PDDC has documented previously unreported hosts for Verticillium including seven-son flower, wafer ash, buttonbush and Eastern leatherwood.
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