Know thy hosts

When managing fungal and other plant diseases, an important consideration is the disease triangle — the fact that for infectious diseases to occur there must be a virulent pathogen, an environment conducive to disease and a susceptible host plant. This third leg of the disease triangle — host range of a plant pathogen — is important not only for your disease management strategies, but also for communicating with your customers.

Will the rose black spot fungus that infects roses spread to and infect rhododendrons or for that matter to any plants other than roses? No. Does the particular powdery mildew fungus that infects roses also occur on beebalm? No. Does the downy mildew organism on viburnums spread to impatiens? No. Does the new invasive downy mildew on impatiens spread to other plant genera? No. These pathogens have narrow host ranges.

The fungal pathogen Apiognomonia veneta causes sycamore anthracnose, resulting in twig cankers and leaf blight. It has a narrow host range, occurring only on the genus Platanus. American sycamore/planetree (Platanus occidentalis) is far more susceptible to this fungus than is oriental planetree (P. orientalis) and as it turns out the hybrid between these two species, London planetree (P. x acerifolia) is intermediate in susceptibility to sycamore anthracnose.

Understand host ranges
On the other hand, there are some plant pathogens that have broad host ranges. The Verticillium fungus occurs on hundreds of host plants, from maple to magnolia to impatiens. The Botryis gray mold fungus occurs on everything from petunias to pine seedlings in propagation.

Cedar rust fungi have an unusual host range, in that they have both an incredibly wide host range, but also a relatively narrow host range. How so? The cedar apple rust fungus and its cousins, the cedar hawthorn rust and cedar quince rust fungi, cycle back and forth between junipers (Eastern red cedar) and plants in the rose family (Rosaceae), such as crabapples and hawthorns.

Junipers are gymnosperms, seed plants that do not produce true flowers and fruits while crabapples and other members of the Rosaceae are angiosperms, flowering plants with seeds totally enclosed by fruits — very different plants from an evolutionary perspective. Yet the cedar rust fungi cycle back and forth between these two distantly related plant groups, but only a narrow range within these groups. Cedar rust fungi does not affect any other gymnosperms, such as arborvitae and pines, not any other angiosperms, such as oaks or maples, but only between junipers and certain genera in the Rosaceae.

Top: Rose black spot symptoms and lesions. Bottom: Cedar apple rust gall on juniper. Millions of microscopic spores spread to the rosaceous hosts (crabapples, hawthorns).

Host range knowledge helps teach us a great deal about plant relationships. As horticulturists, we need to know how closely plants are related to each other. A genus is a group of related species: The plum black knot fungus occurs only on the genus Prunus, which includes plums, cherries, flowering almond and others. Bacterial fireblight has a limited host range but it is a little broader host range than these other two examples, occurring only on plants in the rose family (Rosaceae). A plant family, by definition, is a group of related genera, and fireblight is common on related genera such as Malus (apple), Pyrus (pear), Pyracantha (firethorn) and Sorbus (mountainash).

Knowing this is very helpful diagnostically. If a plant is not in the Rosaceae family, then bacterial fireblight is not one of the possible causes of problems. Knowing that mountainash (Sorbus) is not related to true ashes (Fraxinus) also tells you not to worry about fireblight on the true ashes (such as green ash and white ash), which are in the olive family (Oleaceae). Knowledge of plant relationships also helps you recognize that conversely emerald ash borer is not a problem on mountainash (Sorbus) because emerald ash borer only occurs on true ashes in the genus Fraxinus in the family Oleaceae. Host range knowledge matters for insect management as well as for plant disease pathogens.

Keep it in the family
Finally, let’s consider several plant diseases recently in the news — boxwood blight and downy mildew of impatiens. Boxwood blight is caused by a new invasive fungus from Europe, Cylindrocladium pseudonaviculatum, and is now of concern in U.S. nurseries and landscapes. Boxwood species are in the genus Buxus, ranging from Buxus sempervirens (common boxwood) to Buxus microphylla (littleleaf boxwood). Does boxwood blight spread to any other genera common in U.S. landscapes? It does — the genus Pachysandra. Say what? Yes, Pachysandra is indeed in the Buxaceae family, Buxus and Pachysandra are related genera. The pathogen is not, however, known to occur on genera in other plant families.

Photomicrograph of Plasmopara walleriana fungus.

Downy mildew of impatiens is caused by Plasmopara walleriana, a serious new greenhouse and potentially landscape pathogen that is a news item this year. Will this downy mildew organism spread to other host plants? So far, it appears to be limited to the genus Impatiens in the Balsaminaceae family. Does the host range of this downy mildew species extend to other greenhouse and landscape herbaceous plants plant families? It does not. It is not like the impatiens necrotic spot virus pathogen that was so devastating years ago because of its multitude of host plants in different plant genera and families.

Host range matters. Growers, know thy plants and the host ranges of their common diseases.

Jim Chatfield is associate professor, Ohio State University Extension, Department of Plant Pathology, Department of Horticulture and Crop Science; chatfield.1@osu.edu.