Disease
Phytophthora cactorum has a long history in the plant pathological literature, having first been described as Peronospora cactorum - a pathogen of cacti - in 1870 (Lebert and Cohn, 1870) and subsequently found worldwide on well over 250 plant species in at least 150 genera (Erwin and Ribeiro 1996). With respect to forest trees, the potential for P. cactorum to cause disease across a wide range of hosts became evident early on inasmuch as Hartig (1876, 1880, 1882, 1883) recovered the organism from dying seedlings in the genera Abies, Acacia, Acer, Cedrus, Fagus, Larix, Picea, Pinus, and Robinia. Since then, P. cactorum has been reported from 21 additional genera of forest trees and from some commercially important roseaceous fruit trees.
In most cases, P. cactorum has been associated with crown and collar rot or damping off of forest tree seedlings and as such has been inadvertently managed in nurseries as part of routine maintenance programs to minimize damping off. However, certain strains have occasionally been associated with more serious problems such as lethal root collar cankers on Abies, Acer, and Cornus.
Of particular concern and deserving of considerably more attention than is currently available are the frequent reports of debilitating - if not lethal - bleeding stem cankers on deciduous trees worldwide. Affected genera include Acer, Aesculus, Betula, Castanea,Fagus, Juglans, Liquidamber, Ostrya, Quercus, Salix, Tilia, and Ulmus. Virtually every one of these reports has been generated by someone who was sufficiently concerned about the propensity of field symptoms to feel compelled to bring it to the scientific community. More often than not, the concerning episode seems to resolve itself without further human intervention and the report becomes little more than an addition to the historical record. But one has to wonder why these events come and go as they do and whether some other Phytophthora-caused diseases of grave concern were once written off or ignored because they, too, disappeared as quickly as they appeared.
Bleeding cankers on European beech (Fagus sylvatica) caused by P. cactorum
Despite the broad geographic distribution and diverse host range of P. cactorum, it seems to be causing little concerning damage to forest trees. One recent report from the Czech Republic (Cerny et al. 2009) indicates that an "outbreak" there caused noticeable damage to Fagus, Betula and (for the first time ever?) Populus, but there has been no follow-up to suggest that the incidence of the disease continued to attract attention. Perhaps one reason for the sporadic reports of these diseases in forest communities is that the little evidence available suggests that there is considerable host specificity among strains of the pathogen. In the most recent study of this phenomenon, strains from strawberries had no effect on birch trees from which a "birch strain" of P. cactorum had been isolated. Similarly, the "birch strain" failed to cause disease on strawberries. Given the heterogeneous nature of many forest communities and the presumably limited ability of predominantly soil-borne pathogens like P. cactorum to move readily to new hosts, evolution of such host specificity may be the norm and, as such, minimize intra-host spread.
The table of susceptible hosts below includes only the most ecologically and economically important host species.
| Host Latin Name | Host Common Name | Symptoms | Habitat | Region | |
|---|---|---|---|---|---|
| Abies amabilis | Pacific silver fir | Seedling disease | Forest tree nurseries | USA | |
| Abies balsamea | Balsam fir | Collar rot | Forest tree nurseries | USA | |
| Abies fraseri | Fraser fir | Seedling disease | Forest tree nurseries, Plantations | USA | |
| Acacia spp. | Acacia | Seedling disease | Ornamental Nursery | Germany | |
| Acer macrophyllum | Bigleaf maple | Canker | Forest, Residential | USA | |
| Acer palmatum | Japanese maple | Europe | |||
| Acer platanoides | Norway maple | Canker, Seedling disease | Ornamental Nursery, Residential | England, Europe, USA | |
| Acer pseudoplatanus | Sycamore maple | Canker | Forest, Residential | France, Germany, USA | |
| Acer rubrum | Red maple | Canker, Leaf necrosis | Residential | USA | |
| Acer saccharum | Sugar maple | Canker | Forest | USA | |
| Acer tatricum | Tartarian Maple | Leaf necrosis | |||
| Aesculus hippocastanum | Horse chestnut | Canker | Residential | Europe, USA | |
| Alnus oregona | Red alder | Canker | Forest | USA | |
| Arbutus menziesii | Madrone | Canker | Forest, Residential | USA - Pacific Northwest | |
| Castanea sativa | European chestnut | Canker | Forest | Italy | |
| Cedrus deodara | Seedling disease | Ornamental Nursery | Germany, USA, USSR | ||
| Cornus florida | Flowering dogwood | Collar rot | Ornamental Nursery, Residential | USA | |
| Cornus nuttallii | Dogwood | Canker | Forest, Residential | USA | |
| Fagus grandifolia | American beech | Seedling disease | Ornamental Nursery | Czech Republic, Denmark, England, Germany, Poland, Switzerland, USA | |
| Fagus sylvatica | European Beech | Canker | Czech Republic, Germany, Italy, USA | ||
| Fraxinus spp. | Ash | Seedling disease | Forest tree nurseries, Ornamental Nursery | England | |
| Juglans californica | Southern California black walnut | Collar rot | USA, USA - California | ||
| Juglans hindsii | Northern California black walnut | Collar rot | USA, USA - California | ||
| Juglans nigra | Black walnut | Seedling disease | Ornamental Nursery | Italy, USA | |
| Juglans pyriformis | Mexican walnut | Canker | Residential | USA | |
| Juglans regia | English walnut | Canker | Residential | Italy, USA | |
| Larix decidua | European larch | Seedling disease | Forest tree nurseries | England, Germany, Netherlands | |
| Liquidambar styraciflua | Sweetgum | Canker | Forest | USA | |
| Liriodendron tulipifera | Tulip tree | Canker | Forest | USA, USA - Southeast | |
| Picea abies | Norway spruce | Seedling disease | Forest tree nurseries, Ornamental Nursery | Germany, Italy | |
| Pinus coulteri | Big cone pine | Seedling disease | Native plant nursery | USA | |
| Pinus mugo | Swiss mountain pine | Seedling disease | Ornamental Nursery | USA | |
| Pinus muricata | Bishop pine | Seedling disease | Forest tree nurseries | USA | |
| Pinus nigra | Black pine | Seedling disease | Ornamental Nursery | Canada, USA | |
| Pinus ponderosa | Ponderosa pine | Seedling disease | Forest tree nurseries | Germany, USA | |
| Pinus resinosa | Red pine | Seedling disease | Forest tree nurseries | USA | |
| Pinus strobus | Eastern white pine | Seedling disease | Forest tree nurseries | Germany, USA | |
| Pinus sylvestris | Scots pine | Seedling disease | Forest tree nurseries | Bulgaria, Sweden, United Kingdom | |
| Platanus occidentalis | American sycamore, Sycamore | Seedling disease | Ornamental Nursery | Europe | |
| Populus alba | White poplar | Canker | Residential | Czech Republic | |
| Prunus avium | Wild plum | Canker | Forest | USA | |
| Pseudotsuga menziesii | Douglas-fir | Root rot | Forest tree nurseries | USA - Pacific Northwest | |
| Quercus lobata | California white oak | Seedling disease | Native plant nursery | USA | |
| Robinia pseudoacacia | Black locust | Seedling disease | Ornamental Nursery | Germany | |
| Salix scoulerana | Mountain willow | Canker | Wildland | USA | |
| Tilia sp. | Linden | Canker | Residential | USA | |
| Tsuga orientalis | Cypress | Canker | Residential | USA | |
| Ulmus spp. | Elm | Canker | Forest, Residential | USA |