Sudden Oak Death and Ramorum Blight

Although hosts of P. ramorum show a range of foliar symptoms, the disease is generally characterized by irregular, necrotic leaf lesions, instead of distinct leaf spots. Leaf infections can spread to the petiole and twigs, but diseased leaves often fall off before the lesion reaches the petiole. Sometimes infections occur initially on stems or develop into stems, where associated leaves eventually wilt, become necrotic, and die. On some hosts, a fairly distinct dark line can mark the advance of leaf infection, especially in cool and wet conditions.

Symptoms are most often found on leaf tissue where free water remains for long periods: leaf tips, leaves hanging down or located deep within the canopy, leaves near or touching the soil, or leaves that overlap or are cupped. Root infections are possible on some hosts, but the roots often appear more or less healthy. Root rot symptoms, typical of infections by other Phytophthora species, are not seen. The importance of these cryptic root infections in the spread of P. ramorum in the nursery trade is not yet clear.

While not commonly seen on nursery hosts, bark cankers on the trunks of woodland trees, such as oak and tanoak, are also associated with this disease. Cankers have red-brown to black discoloration, dark black to reddish colored sap, and often develop 3 to 6 feet above the ground, although they can be higher or lower. Symptoms caused by fertilizer burn, chemical injury, drought injury, freeze damage, sunburn, and root damage can sometimes look like P. ramorum foliar infection. However, these abiotic injuries are often found distributed over the entire plant, while lesions caused by P. ramorum are often only found initially on a few leaves or on one portion of the plant. There are many damaging species of Phytophthora other than ramorum, so diagnosis by a qualified lab is extremely important.

In the mid to late 1990s, portions of woodlands in Marin, Santa Cruz, and Monterey counties changed dramatically. Tree crowns turned brown within a few weeks, giving the impression of instantaneous mortality. Since then more than a million tanoaks, (Notholithocarpus [=Lithocarpus] densiflorus), California black oaks (Quercus kelloggii), coast live oaks (Quercus agrifolia), and other oak species have died as a result of P. ramorum infection in northern and central California. The pathogen also causes mortality in forested regions of southern Oregon. Additionally, it can be found in streams as well as the natural waterways of Washington.

A distinct European lineage of P. ramorum is causing similar diseases on ornamentals in some nurseries, botanic gardens, and landscapes in Europe. It has been infecting Vaccinium spp. in the heathlands of Scotland and Wales. Oak, and especially Japanese larch plantations, have been heavily impacted in the United Kingdom.

Camellias, rhododendrons, viburnums, Pieris spp., and other popular ornamental plants are susceptible to P. ramorum infection, and the pathogen can move long distances through shipments of infected nursery stock. Federal and state quarantines are in effect that require nursery inspections. If the pathogen is found, affected nursery stock must be destroyed. For a current host list (PDF) and additional regulatory information, refer to the U.S. Department of Agriculture Animal and Plant Health Inspection Service's website on P. ramorum.

Phytophthora ramorum, while having many features in common with fungal organisms, is not a true fungus. Phytophthora species are Oomycetes or "water molds" and require a moist environment to actively grow and reproduce. Phytophthora ramorum produces several reproductive structures important for pathogen spread and survival, including sporangia, zoospores, and chlamydospores. Sporangia give rise to the zoospores, which can swim in water. Chlamydospores are resting spores that help the pathogen survive extreme temperatures, dryness, and other harsh conditions. Phytophthora ramorum can grow within a temperature range of 36° to 80° F with an optimum temperature of 68° F.

Spores can form on leaf surfaces of susceptible leaves and twigs following prolonged wetting. They are moved from plant to plant via windblown or splashed rain or by direct contact with infected leaves. Spores produced on infected plants can move to healthy plants in water runoff created by rain or sprinkler irrigation water. In California forests the pathogen sporulates prolifically on California bay laurel trees (Umbellularia californica), which serve as reservoirs for inoculum. Infected California bay laurel can also be an important source of inoculum when near the nursery stock.

There are federal and state quarantines that require inspections for nursery stock in regulated California counties. If the pathogen is detected, the affected plants are destroyed and the pathogen is eradicated.

Disease symptoms may take weeks to several months to develop and become apparent, so infected plants may appear healthy at first. Fungicides that have activity on Phytophthora might prevent new infections and therefore interfere with detection of this pathogen; it is best not to apply fungicides while evaluating the disease status.

For most nurseries, the foremost objective of pest management programs is to prevent the introduction of the pathogen into the nursery via infected plant material. This can be partly accomplished by careful inspection of all incoming host propagative material and stock.

• Monitor new outside source stock at least biweekly in summer and at least weekly during rainy periods when environmental conditions are highly conducive to pathogen infection and development.

For nurseries surrounded by native host trees and shrubs and in the vicinity where P. ramorum is found, monitor areas surrounding the nursery, especially wet areas, near puddles, or rain runoff zones. It is very important to detect the pathogen early, while it is still at very low levels.

• Periodically inspect nearby native hosts for disease symptoms. Infected California bay laurel trees near the perimeter of nurseries may produce inoculum that can spread and cause infection of nearby host plants, so removal of these trees may be warranted.
• Consider building berms to prevent water and soil movement into production areas from hillsides surrounding the nursery that may contain infected hosts.
• Irrigation water pumped from streams and ponds in areas of infected native hosts may be contaminated with P. ramorum. Consider having this water periodically tested to detect P. ramorum. If it is found to be present, use alternative irrigation sources, such as well water or water disinfection treatments.

For more information on developing a detection and monitoring program, see Nursery Guide for Diseases Caused by Phytophthora ramorum on Ornamentals: Diagnosis and Management, UC ANR Publication 8156.

Cultural practices that can be useful to reduce disease risk:

• Avoid irrigation practices that leave foliage wet for prolonged periods. If sprinklers are used, irrigate in the morning to allow for thorough and quick drying of foliage.
• Monitor and maintain irrigation systems to insure the most uniform application of water to the crop. Correct low spots, areas of poor drainage, and clogged or leaking irrigation heads.
• Monitor irrigation-water sources, other than well water, for P. ramorum. Use disinfection systems if using recycled water.
• Wounded leaves (even tiny wounds or scratches) are much more susceptible to infection. Avoid handling host plants if they might be wounded when environmental conditions favor disease.
• Avoid splashing water from soil to foliage, as well as from foliage to soil or container soil. Use raised benches, gravel, or other means to elevate susceptible plants above soil. Transplants, even on gravel beds, appear to be very susceptible to disease because of the close proximity of foliage to soil, runoff water, or rain splash. Raised benches may be warranted for transplants.
• Remove plants or plant parts that are suffering from poor vigor, disorders, or other serious problems from production areas and destroy them. A small number of plants or plant parts could be bagged and disposed. If a cull pile is needed temporarily, cover it with a clear polyethylene sheet until the culls can be destroyed or composted.
• Propagate cuttings only from disease-free hosts.
• Use only new or disinfested containers and soil. Place potting soil piles as far from infected native hosts or cull piles as possible and covered with clear polyethylene sheeting. Do not mix potting soil components on bare soil.

Fungicides used to protect nursery stock from P. ramorum function as preventive treatments only. Currently, even the most active fungicides do not stop the development of P. ramorum once foliar lesions are present. The use of Phytophthora- specific fungicides may be warranted for high risk situations, such as where a nursery is exposed to local inoculum sources from surrounding infected native hosts or where P. ramorum has been detected previously in the nursery. Fungicides should only be used after other management strategies and preventive steps have been fully implemented.

When applying fungicides, good coverage over the foliage is important; add a wetting agent to prevent significant run-off and loss of fungicides on the hard-to-wet leaves of certain plant species. Apply treatments before environmental conditions favor pathogen infection; for example, spraying before a period of rainy weather will allow water to linger on leaf surfaces for many hours.

Some fungicides applied to the foliage move into leaves and are not washed off by rain or sprinkler irrigation, while others provide a protective layer of chemical on the leaf surface. Some can be applied to the soil, where they are absorbed and moved upward to the leaves, to protect them from infection. Some have residual activity that can last for several weeks after they are applied. Read fungicide labels and technical information provided by the fungicide manufacturer to learn how the fungicide can be used most effectively. See PHYTOPHTHORA ROOT AND CROWN ROTS and management in this section.

Resistance management

To help reduce the potential for the development of resistance by P. ramorum to fungicides, alternate or tank mix fungicides with different mode-of-action group numbers. Fungicides active on P. ramorum may already be used in the nursery to control other foliar or soil-inhabiting Phytophthora species or related pathogens (such as downy mildews), and their use should be considered in planning the overall fungicide resistance management program.