Scolytus multistriatus - Vector of Dutch Elm Disease

Dutch elm disease is caused by a fungus in the genus Ophiostoma (formerly Ceratocystis) by the species ulmi or the more virulent strain novo-ulmi. The fungus is classified in the Kingdom: Fungi (which produce mycelium with no chloroplasts and the walls contain glycans and chitin, chitin being also found in insect cuticle). The fungus is further classified in the Phylum Ascomycota (ascomycetes, the sac fungi), most of which have a sexual stage and an asexual stage. In the sexual stage, the fungus produces ascospores, generally 8 to an ascus (which is like an elongated polyp containing the spores). In the asexual stage (replication of the self, i.e. no sexual pairing), the spores are called conidia on free hyphae or in fruiting structures called pycnidia or acervuli. The class Pyrenomycetes (Ascomycetes with perithecia) has the Order: Hypocereales which contains the Ophiostoma among others.

Dutch elm disease was described on elm in the Netherlands in 1921. The fungus has been spread around the world by Scolytus bark beetles in elm wood colonized by the beetles. In the United States, the disease was first detected in Ohio and a few other states on the east coast in the early 1930s and by 1973 it had reached to California. The disease kills branches and entire trees within a few weeks to years. Millions of trees were killed in the eastern United States within a decade in the 1950s to 1960s leaving whole town without the major shade tree (some places, like Moline, Illinois, had only elms!).

Symtoms: The first sign of Dutch elm disease appears as a sudden wilting of leaves in some branches, but sometimes the entire tree. Willed leaves curl and turn yellow, then brown and fall off the tree. Most branches then die. Often this process takes more than a year. Trees that are infected in the spring or early summer often die quickly, whereas those infected in late summer are less affected and can even recover until infected again in subsequent years. When the bark is peeled back there are brown streaks in the outer wood.

The more dangerous or virulent species O. novo-ulmi has been supplanting the predominant O. ulmi subspecies. In both, the fungal mycelium is creamy white, and inside the xylem vessels, the mycelium produces stalked coremia with many conidia (spores) in a glistening, whitish to yellowish droplet that sticks to bark beetles tunneling in the dead phloem. When the beetles mature they leave the brood tree and fly away to spread the fungus to new trees.

The fungus needs two different sexual strains for sexual reproduction, and since only one stain occurs over a wide geographic area, sexual reproduction rarely occurs. On the zone of contact between two sexual strains there is sexual recombination of genes. The geographic areas are smaller in Europe than in the USA so there is more sexual pairing in Europe. This is probably because the fungus is native and many more genetic variations have evolved but that only a few of these were spread to the USA and other areas of the world and thus the fewer strains spread out over the large areas so that sexual recombination is then rare. When sexual pairing occurs, the two mating types come in contact and produce perithecia. The perithecia are spherical and black, about 120 microns in diameter and have a long stalk of 300-400 microns. Inside the perithecium many asci develop but then disintegrate leaving the ascospores free in the perithecial cavity which are discharged through the stalk canal and also accumulate in a sticky droplet for dispersal by the bark beetles.

The fungus overwinters in the bark of dying or dead elm trees (American elm, European elm, and sometimes in the semi-resistant Siberian elm, and rarely in the Chinese elm). When the elm bark beetles, that prefer to colonized weaken, sick elms, develop and become adults, they pick up the spores and carry them to new trees. The adults of Scolytus multistriatus, the European elm bark beetle, fly to the crowns of elm trees and bore a small hole in the twig crotches (see picture at top). The sexes mate at this time and then the female may bore into the main trunk and lay eggs in her straight galleries. The fungus, especially in the spring, can be transported and grow down the larger xylem vessels into the main trunk of the tree, and later to all parts and kill the tree. The fungus can even spread to adjacent elms through root contact (where the roots can graft together). In this way, several elms that lining a street in the eastern USA were infected one after another). The elm bark beetle in warmer areas has two generations per year. It is interesting to note that the beetle and the fungus have evolved a symbiotic relationship that accounts for the prevalance of the two species. Without the fungus, the bark beetle can only colonize a few old or unhealthy trees and so there would be only a few, or no trees being killed locally. However, the fungus kills more trees providing more hosts for the beetle to colonize so it is a dominate, prevalent, or keystone species. The beetle, on the other hand, helps the fungus become wide spread and common by carrying it to new trees or new patches of trees. However, this symbiotic relationship can work "too well" and cause the fungus to kill all the elm trees, whereupon both the beetle (which only can eat elms) and the fungus can become locally extinct. Usually, they do not become extinct over all the planet and some new, less virulent strains or species of the fungus appear, that are able to live saprophytically or somehow survive without elm trees all the time, and that are not killing all the elm trees, only the weak ones. Then the host tree, bark beetle, and fungus can all coexist over evolutionary time. This is the idea that parasites or predators can never be too efficient as this will kill off their host and themselves. Of course this can happen, but if there are some that can survive for a while elsewhere and some hosts as well, they can immigrate into the area of extinction and be more in balance. The key is that less virulent genotypes of the predator or parasite must have some way of surviving, probably in less numbers, on other hosts or saprophytically in case a new virulent strain (which can't survive this way) comes into the area again and wipes out the host (but this only happens occasionally in evolutionary time maybe) so that usually the "more balanced" and coexisting way of nature is found. Well, this is a hot topic of debate in Ecology and my explanation is only approximate.

Control: There are no resistant clones of susceptible American elm species. Chinese elm is resistant but is not a good shade tree. Siberian elm is rather resistant, but has lots of twig fall. However, I think they should plant more varieties of trees in general so that no one disease can wipe them out since most diseases can kill only one species, or a few related species, of trees. This is also true of insect pests. It should never be a good idea to plant all one species, such as American elms, as seen from the cruel experiment earlier across the USA. Usually all that can be done is remove dying trees so that less infection is present, but this merely slows the epidemic spread down so that money can be obtained to remove the dead trees. Many laboratories across the USA (especially) and Europe tried to find solutions to this disease but largely failed. This effort was largely before the advent of molecular biology. Chemical sprays against the bark beetle are rather ineffective as the beetle breeds under the bark. Insecticides in an urban (city) environment are especially an idea of "last resort" since the public does not like to be sprayed with pesticides! Also, it is rare that enough bark beetles can be killed to stop the spread if the trees are susceptible. Pheromone traps (baited with 4-metyl-3-heptanol and multistriatin pheromone components) were used in New York and California, trapping many beetles, but never enough to stop the progression of the disease. Some systemic fungicides show promise but still have not solved the problem I think. Pseudomonas bacteria can attack the fungus, but are not effective enough. Also, some nonaggressive strains of the fungi of Ophiostoma or Verticillium which compete and probably are antagonistic to the Dutch elm fungus can also be useful, but apparently have not solved the problem. It seems that nature will take its course and all the susceptible elm tree genotypes will become extinct, replaced with less susceptible ones, and as well less virulent strains of fungus possibly, but that is a matter of debate. This example shows the instability of monocultures - which are inherently boring and unesthetic.

Selected scientific references and abstracts on Scolytus multistriatus, vector of the Dutch elm disease:
70K file


Image © 1996 by John A. Byers, Chemical Ecology.