Knowledge of host ranges is critical to making the right plant health diagnosis, as well as selection of the right plant for the right place.
Understanding the host ranges of pathogens that cause infectious plant diseases and insect and mite pests that infest and damage ornamental plants is important in plant health management, in evaluating the potential risks of a particular pest or pathogen, and in communicating with our clientele and customers. After all, the host is one of the three sides of the Disease and Pest Triangles (along with the pest or pathogen and the environment conducive to disease or infestation).
Will the crown gall bacterium spread from euonymus to rose, causing crown gall? Yes. Will the rose black spot fungus spread to crabapple? No. Will the apple scab fungus spread from crabapple to rose? No. Will emerald ash borer infest and damage ash? Yes. Will it infest maple? No. What about Asian longhorned beetle; will it infest and damage both ash and maple? Yes. How about cross-infection and host ranges with more closely related plants? Does impatiens downy mildew occur on bedding impatiens (Impatiens walleriana)? It does, but it does not occur on New Guinea impatiens (Impatiens hawkeri).
All of this matters, for example, when designing a landscape. If the customer is a dedicated rosarian, then certain euonymus that are either present or contemplated are problematic. This is because crown gall is a serious problem for roses, and there are no viable bactericide options and few management options other than not planting other susceptible hosts and the removal of affected plants.
Let’s take a look at a few key aspects of host range.
Host ranges are often narrow
Most plant pathogens have narrow host ranges. As noted, the downy mildew of impatiens water mold pathogen (Plasmopara obducens) occurs on Impatiens walleriana, the common bedding plant impatiens and some wild impatiens species, but not on other plant genera, and not on New Guinea impatiens (Impatiens hawkeri). The rose black spot fungus (Diplocarpon rosae) occurs only on certain roses in the genus Rosa. The plum black knot fungus (Dibotryon morbosum) infects only plants in the genus Prunus, such as cherry, almond, peach and, of course … plum. Bacterial fireblight (Erwinia amylovora) occurs only on genera in the rose family (Rosaceae), such as apple/crabapple (Malus), pear (Pyrus) and Sorbus (mountainash).
Likewise, many insect pests have highly limited host ranges. The devastating emerald ash borer (Agrilus planipennis) infestation in North America at least occurs only on ashes (Fraxinus). The bronze birch borer insect (Agrilus anxius) occurs only on birches (Betula). Hemlock woolly adelgids occur only on hemlocks (Tsuga), viburnum leaf beetles damage only Viburnum species, and European pine sawflies thrive only on certain Pinus taxa.
On the other hand…
Host ranges may be wide
In sharp contrast to the fraxinophilic emerald ash borer that we have gotten to know all too well, another borer in the news—the Asian longhorned beetle (Anoplophora glabripennis)—has a wide host range. Some very good hosts include maples, horsechestnuts, elms and willows, while additional hosts can be birches, planetrees, poplars, goldenraintrees and many others. It’s a fact that makes it even scarier to contemplate if we do not eradicate those infestations we know about. As another example, Japanese beetles (Popillia japonica) feed on many hosts, from turfgrass (as larval grubworms) to the wider training table for adults that includes everything from roses to lindens, dawn redwoods to primroses.
Crablandia is in full bloom at the Secrest Arboretum in Wooster, Ohio. It is here that OSU Extension performs comprehensive crabapple evaluations, scouting for signs of infestation and disease throughout the more than 75 crabapple taxa planted at the site.
Photo courtesy of Ken Chamberlain, OARDC
Similarly, there are pathogens with wide host ranges. The Verticillium fungus infects many plants, both woody and herbaceous, from Acer (maple) to Viburnum, from Impatiens walleriana to fruits and vegetables such as brambles and tomatoes. Botrytis cinerea and its cousins infect almost everything from redwoods in propagation to roses and petunias in the landscape. The bacterial crown gall pathogen Agrobacterium tumefaciens has a host range of at least 600 plant species in 142 genera and more than 90 families, including notably susceptible evergreen euonymus and roses. The debilitating effects of crown galls on the vascular health of roses is usually what makes rose gardens fade over time, and it is not easy to do anything about it other than put up with unhealthy roses or start over. Sadly, bactericides for crown gall are not effective.
Crown gall can spread from euonymous plants to roses and has an extremely wide host range. Knowing this helps the landscape professional make the right decisions when selecting plants for a project.
Photo courtesy of Jim Chatfield, OSU Extension
Natural selection matters
We talk a lot about plant selection: “Right plant for the right place” is an important mantra for our daily Zen. Host range plays a role here as well. In Ohio and other areas with moist springs and heavy apple scab pressure, it is important to select crabapples that bring good apple scab resistance along with desired horticultural characteristics of flower, fruit, form and foliage. But selection within a different time frame is also critical to our understanding of host range. Namely, the role of natural selection with regard to host range matters.
As Ohio State University entomologist Dan Herms points out, “No natural selection history pressure—no resistance.” Asian ashes that have coevolved with the emerald ash borer (EAB) have greater resistance to this insect than our North American ashes, which never encountered the pest until the past few decades. Thus our native ashes have not evolved through the cauldron of natural selection. Each mutation of Asian ash that provided, for example, plant chemical advantages over EAB made that ash a little more resistant to the effects of this insect. Multiply this over thousands and millions of years, and the result is more resistant ashes in Asia. If only we could wait that long. Hopefully, as horticulturists we can speed up the process through plant breeding efforts.
This story plays out again and again. With birch and the bronze birch borer, the story is the same—but in reverse. Bronze birch borer is a North American native insect. Our native birches, such as paper birch and gray birch, have relatively good resistance to bronze birch borer due to the eons of natural selection history. It is the European and Asian birches that have not faced this insect until planted here and that are more vulnerable because of their lack of natural selection history. Are they worried in Europe and Asia about the bronze birch borer as an invasive species to their birches? Absolutely.
This story unfolds in an interesting way with thousand cankers disease. The walnut twig beetle (Pityophthorous juglandis)/Geosmithia fungus combination is not officially classified as an invasive problem because the insect and beetle are technically native to the United States. This combination is not a big deal on Arizona’s native walnut, affecting stressed plants in the state but not appearing to cause much overall damage to healthy plants. That’s a typical bark beetle scenario. A problem, though, was noted early in the 21st century by Colorado State University entomologist Whitney Cranshaw and plant pathologist Ned Tisserat: Black walnuts were dying throughout the West.
Black walnuts are native to the eastern United States, but were planted out West. The problem is, black walnut had no natural selection history to this seemingly innocuous insect/fungus combination. No resistance. Devastation to black walnut ensued in the West, and now this new black walnut/thousand canker combination threatens the native range of black walnut here in Ohio and the East. The cause? Wood products imported from the West. From an invasives biology perspective, this, again, is the same old story: “No natural selection history—no resistance.”
The plum black knot fungus (Dibotryon morbosum) infects only plants in the genus Prunus, such as cherry, almond, peach, and, of course … plum.
Photo courtesy of Joe Boggs, OSU Extension
This natural selection history story, told over and over again, is neatly summarized by Mike Raupp and Paula Shrewsbury of the University of Maryland and Dan Herms of OSU in recent articles (see sidebar), in which they point out some of our most familiar and important pest/pathogen and host combinations. Among many examples, consider Dutch elm disease and North American elms; in reverse, our native pine needle scale and Eurasian pines. There are, of course, many other examples. This leads us to the obvious, although not often recognized, conclusion:
Natural selection history matters.
The rose black spot fungus (Diplocarpon rosae) occurs only on certain roses in the genus Rosa. It will not spread to crabapple.
Photo courtesy of Jim Chatfield, OSU Extension
Jim Chatfield is associate professor and extension specialist in Ohio State University’s Departments of Horticulture and Crop Science and Plant Pathology; he can be reached at firstname.lastname@example.org. Joe Boggs is assistant professor in Ohio State University’s Department of Entomology and commercial horticulture educator with OSU Extension, Hamilton County; he can be reached at email@example.com. Dan Herms is professor and chairperson of the Ohio State University Department of Entomology at the Ohio Agricultural Research and Development Center in Wooster; he can be reached at firstname.lastname@example.org. All are members of the Ohio State University Extension Nursery Landscape and Turf Team.
Want to read more?
- Check out more from Mike Raupp, Paula Shrewsbury and Dan Herms in these excellent references.
- Raupp, M.J., P.M. Shrewsbury, and D.A. Herms (2010) Ecology of herbivorous arthropods in urban landscapes. Annual Review of Entomology 55:19-38.
- Raupp, M.J., P.M. Shrewsbury, and D.A. Herms (2012) Disasters by design: outbreaks along urban gradients.
- P. Barbosa, D.K. Letourneau, and A. Agrawal (eds.) Insect Outbreaks Revisited. Wiley-Blackwell, Chichester, West Sussex, UK.