Disease Cycle and Epidemiology

Disease cycle for P. ramorum in the forest (left; click to enlarge), and for P. ramorum in the nursery (right; click to enlarge).

Because SOD and ramorum blight are diseases that have emerged recently, many details of the disease cycles are not yet understood. It appears that Phytophthora ramorum shares many characteristics of other aerially dispersed Phytophthora species. After infection of the host tissue, sporangia are produced and dispersed to new host material. There, the sporangia either germinate or release motile zoospores. A new infection event occurs, and the asexual cycle is repeated.

Ramorum blight is a polycyclic disease (has more than one disease cycle per growing season) on foliar hosts, producing many sporangia during wet conditions. Chlamydospores are produced in abundance by this pathogen and likely serve as a reservoir of inoculum during conditions unfavorable for disease development. SOD hosts are often considered "dead end" hosts because no sporangia are produced and mortality occurs as a result of canker formation or colonization of sapwood. However, sporulation has been observed on leaves and petioles of one canker host, tanoak. Sexual reproduction has not been observed in nature.

P. ramorum has a large host range that includes more than 100 species in over 40 genera. APHIS (the USDA Animal and Plant Health Inspection Service) maintains up-to-date lists of proven hosts and associated plants (those plants for which Koch’s postulates have not yet been confirmed) for P. ramorum. Not all species in a particular genus are susceptible, and within a given species a gradient of susceptibility among individuals is often evident. The factors that cause these differing responses by closely related species are not known. P. ramorum has been found in the nursery, the forest, and the urban-wildland interface. In 2006, P. ramorum was detected in 62 nursery-related sites in 11 states. Outside of nurseries, in 2007, ramorum blight affected coastal mixed-evergreen forests in 14 counties in California and in part of one county in southwest Oregon (Figure 21). It occurs discontinuously, not necessarily affecting all potential hosts. P. ramorum is believed to be exotic to both North America and Europe.

Figure 21

Genotyping of P. ramorum isolates shows that there are three clonal lineages: EU1, NA1 and NA2 (Table 1). These distinct genetic lineages suggest that strains were likely introduced separately to Europe and North America from P. ramorum’s unknown geographic origin. The finding of both EU1 (mating type A1) and NA1 (mating type A2) isolates from the same nursery in California in 2007 introduces the possibility that sexual reproduction could occur as it does in its place of origin. This underscores the need to prevent movement of nursery stock infected with P. ramorum.

Table 1. The three clonal lineages of Phytophthora ramorum in relation to distribution and mating type.

In Europe, ramorum blight was first observed on rhododendron and viburnum in the early 1990s and by 2007 had spread throughout nurseries and retail centers in 16 countries. In addition, the pathogen has been detected in gardens, parks, and woodlands in eight countries. In a few gardens and parklands in the U.K. and the Netherlands, beech and oak trees have begun to show symptoms of disease. In most cases, the trees are adjacent to infected rhododendrons.

As with other aerial Phytophthoras, P. ramorum is likely transmitted by rain splash, air currents, and wind-driven rain, but the specific details of dispersal and infection are not fully known. Sporangia are believed to be moved by air currents following wet weather. They likely initiate leaf and twig infections high in the forest canopy. Following rain or fog, more sporangia are produced, which can be blown or splashed to other leaves. They may also run down the bole with rainwater, initiating cankers on susceptible hosts. Infections often spread from taller trees to understory trees and shrubs beneath them. Root infection has been observed on young tanoak seedlings in heavily infested forests, but the epidemiological significance of this is not known, since abundant aerial inoculum was also present at these sites. There is a high risk of propagule transmission by humans who move infested plant material, logs, and soil. In the forest setting, viable propagules can get lodged on the shoes of hikers or on bicycle tires, but it is not known if these soilborne propagules can initiate disease. P. ramorum can be recovered from streams in infested areas, but it does not appear that this has led to new infections in the forest. While these and other potential bird, mammal, and insect vectors have not been ruled out, the pattern of disease occurrence in the forest is most consistent with windborne dispersal of inoculum.

In the forest setting, P. ramorum follows a classic dispersal gradient. Most inoculum remains within 5-10 m of the host, and the number of propagules decreases with distance from site of formation. Yet, it is possible that a few propagules are dispersed long distances. Caducous sporangia carried above the canopy during a storm event might initiate an infection far from their origin. This is a likely scenario of what occurred in Curry County, Oregon, as this site is 400 miles from the nearest known infested forest site.

P. ramorum does not sporulate on all hosts. Reproductive propagules have not been recovered from at least one of the species most devastated by SOD, coast live oak. However, sporangia are produced readily on the foliage of California bay laurel, which is commonly found in association with coast live oak and tanoak in infested forests in California. Ramorum blight is not lethal on bay laurel. It is thought that sporangia and zoospores produced on bay laurel can readily infect coast live oak and tanoak in the forest. The ecology of chlamydospores is poorly understood. Chlamydospores are produced in or on leaves of some hosts. When infested leaves fall to the ground or chlamydospores are shed, it is hypothesized that chlamydospores may lie dormant during the dry summer, then eventually germinate and produce more sporangia when the winter rains arrive, initiating new infections.

In horticultural nurseries, infections have been found up to 0.5 m from the closest infected plant, but infections due to long distance wind dispersal have not yet been noted. Irrigation water can be an inoculum source for nurseries using contaminated recycled irrigation water or water from infested streams. Rhododendrons and some other nursery plants can become infected through their roots if inoculum is present in the potting mix.

Long-distance dispersal of P. ramorum is facilitated by shipments of infected nursery plants across the country. For example, in 2004, a large wholesale nursery in southern California shipped 1.6 million camellias throughout the U.S. (Figure 22, 23). The nursery was later found to be infested with P. ramorum (Figure 24). Although APHIS checked each of the nurseries that had received plants and destroyed the plants that were infected, many had already been sold. Despite fears that these infected camellias might spread the disease to surrounding forests, there is no evidence yet that P. ramorum has become established in forests outside of California and Oregon. P. ramorum has since been detected in several other west coast nurseries (see Diagnosis and Monitoring), which is why plant material must be certified as free of P. ramorum before host plants are shipped from these west coast states.

Figure 22

Figure 23

Figure 24

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by The American Phytopathological Society